- Email: [email protected]
Estimation of gaseous real-world traffic emissions downstream a motorway
ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 5665–5684 www.elsevier.com/locate/atmosenv
Estimation of gaseous real-world traffic emissions downstream a motorway M. Kohlera,, U. Corsmeiera, U. Vogtb, B. Vogela a
Institut fu¨r Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universita¨t Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany b Institut fu¨r Verfahrenstechnik und Dampfkesselwesen, Universita¨t Stuttgart, Stuttgart, Germany Received 22 March 2004; received in revised form 23 August 2004; accepted 2 September 2004
Abstract The consequences of air pollution scenarios caused by road traffic or the impact of exhaust gas reduction techniques are estimated by emission models. To ensure the quality of model results, it is necessary to evaluate the used emission factors under real-world conditions. Therefore, the Institut fu¨r Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe initiated the field campaign BAB II (BundesAutoBahn, Federal motorway). The campaign was conducted in May 2001 with the objective of measuring the traffic emissions at a motorway section and to compare them to modelled emissions. Based on experiences during a precursor campaign (BAB I, 1997), a symmetric experimental set-up was installed which allowed measurements up- and downwind of a motorway nearby Heidelberg, Germany. This paper focuses on the determination of source intensities and emission factors for CO and NOx, whereas other papers in this issue handle VOC and particulate matter. First the basic approach of BAB II measurements up- and downwind of a motorway was approved, showing that it is possible to detect the plume originating from traffic emissions. A case study during a traffic jam illustrates that driving patterns have a strong impact on the emissions and therefore a detailed traffic census is required to obtain reliable emission calculations. Two different strategies were used: (i) long-time measurements during the whole campaign to obtain vertical profiles each 30 min and (ii) measurements during eight special operation periods (SOP) in a higher temporal resolution of 5 min, using instrumentation in elevators. It could be shown that even at a distance of 60–80 m from the motorway the structure of the plume is still inhomogeneous, and concentration changes within short times. The inhomogeneity of the plume not only affects the temporal scale, the spatial scale is also influenced and frequently concentration maxima in higher altitudes are observed. Mean source intensities of 9.5 kg km1 h1 CO and 4.4 kg km1 h1 NOx on working days have been calculated. The ratio of CO/NOx was found to be 2.1 on working days and 4.5 on weekends and holidays. Emission factors of 2.62 g km1 veh1 CO and 1.08 g km1 veh1 NOx have been determined, which agrees well with results derived from similar studies. r 2005 Elsevier Ltd. All rights reserved. Keywords: Source intensity; CO; NOx; Traffic emissions; Emission calculation; Emission model; Plume structure
Corresponding author. Fax: +49 7247 82 4377.
E-mail address: [email protected] (M. Kohler). 1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.088
ARTICLE IN PRESS 5666
M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
1. Introduction 1.1. State of the art Traffic emissions are one of the major sources for anthropogenic air pollution. Carbon monoxide CO and nitrogen oxides NOx are, beside others, important components of these emissions, and therefore it is necessary to improve the knowledge about their quantity and distribution. The estimation of traffic emissions depends strongly on the quality of emission factors used in model calculations. Generally they are acquired by dynamometric test measurements and there is still the demand to compare these factors with those determined under ‘realworld conditions’. Therefore, a couple of field studies had been conducted to evaluate the emission factors for the most important compounds of road-traffic emissions. Most of them are tunnel studies, like those in the Gubrist Tunnel, Switzerland (Staehelin et al., 1995, 1997, 1998; John et al., 1999), the Fort Mc Henry Tunnel of Baltimore Harbor, Maryland, USA (Pierson et al., 1996), in the Taipei Tunnel, Taiwan (Hwa et al., 2002), the Tauern Tunnel in Austria (Schmid et al., 2001), the So¨derleds Tunnel at Stockholm, Sweden (Kristensson et al., 2004) or in the Kiesberg Tunnel at Wiesbaden, Germany (Kurtenbach et al., 2001). Tunnels are an almost closed system with a welldefined mass flow through the tunnel tube but have the disadvantage that the design of the tunnel influences the driving pattern and the airflow through the tunnel. Hucho (1981) and Peters (1990) found that depending on the geometry of vehicles and tunnels the air resistance number, cw, of a vehicle increases by entering a tunnel. The airflow through tunnels, especially through those with one-way tubes, is influenced by the piston effect of moving cars, which was found i.e. at the Gubrist Tunnel (John et al., 1999). This effect causes an artificial tail wind, which leads to a reduced engine power and as a consequence influences the traffic emissions. Another approach to evaluate emission factors is to carry out measurements perpendicular to an open road. The TU¨V Rheinland performed a field campaign at the A3 federal motorway near Siegburg, Germany, with two towers close to the road in up- and downwind directions (Leisen et al., 1992). The position close to the road (8 and 11 m) had the advantage that the vertical extension of the plume did not exceed 20 m, but the contribution of the turbulent flux to the mass flow is not negligible in this case and leads to an uncertainty in the results. This disadvantage could have been only handled by using analysers with high temporal resolution, which were not available. The Institut fu¨r Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe carried out another field campaign in 1997. It took place along the motorway A656 near Heidelberg, Germany
(Vogel et al., 2000). From 10 July to 30 July 1997, continuous ground-based measurements of the major compounds of traffic emissions were conducted. Additional vertical profiles of wind direction, wind speed, temperature and ozone were measured, using a captive balloon system during special operation periods. Parallel, a detailed traffic census was carried out by the Institut fu¨r Energiewirtschaft und Rationelle Energieanwendung (IER), Universita¨t Stuttgart. As a result, a good compliance was found between emissions given by the Umweltbundesamt (UBA) and measured in the field for NOx. In contrast, the CO emissions were found to be up to two times higher than the modelled CO emissions. Emanating from these results and the experiences made in the field campaign in 1997, a second field campaign with enhanced equipment and objectives was carried out in May 2001. 1.2. Method and assumptions The main feature of the study is to perform measurements up- and downwind of a motorway. In the upwind direction of the motorway the background concentration of the trace gases will be measured, while downwind the background concentration in addition to the emissions of the road traffic will be observed. This principle can be expressed by the following equation: Zh Qi ¼
v? ðzÞ ðci ðx2 ; zÞ ci ðx1 ; zÞÞ dz,
(1)
0
where Qi is the source strength, ci is the concentration of the substance i, v? is the wind velocity perpendicular to the road and h is the height of the traffic exhaust gas plume at the position x2. The position x1 is located on the upwind side, the position x2 on the downwind side of the road. To use Eq. (1), a few assumptions have to be made. First, no deposition and no chemical transformations may take place between x1 and x2. Second, v? only depends on the vertical coordinate, and third, the turbulent fluxes are negligible compared to advection. In addition stationary conditions are assumed. Under conditions meeting these assumptions, the source strength of the traffic emissions can be determined by measuring the variables v?(z), ci (x,z) and h. 1.3. Realization and experimental set-up As in 1997, the motorway A656 between Heidelberg and Mannheim, near Heidelberg-Wieblingen (at 10.5 km), was found to be ideal for the field campaign. The motorway runs from northwest (3151) to southeast (1351), which is perpendicular to the main wind direction
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
(Corsmeier et al., 2005a). The four lanes are on an approx. 1 m high, grass-covered embankment and are surrounded by agriculturally used flat terrain. At a distance of 500–1000 m northeast of the motorway there are six small farm houses; to the east a bridge crosses the motorway in a distance of 400 m from the measurement site. The field campaign started on 1 May 2001 and ended on 25 May 2001. The campaign was preceded and followed by a quality assurance programme (Vogt et al., 2005). Within the whole period, eight special operation periods (SOPs), with extended measurement activities were carried out (Corsmeier et al., 2005a). Essential parts of the measurement site were two towers, one on each side of the motorway, both of them 52 m high. They were established 60 m north and 80 m south of the motorway (Corsmeier et al., 2005a). For budget calculations it is necessary to cover the total vertical extension of the plume. On the other hand, the towers had to be built up at a distance far enough from the motorway that turbulence is reduced but not so far that the emissions are overly weakened by dilution. Therefore, the distance of the towers from the motorway had to be optimized to fulfil the guidelines. The symmetric set-up of the measurement site allowed analysing of both main wind directions with northeasterly and southwesterly winds. Meteorological measurements of wind speed, temperature and humidity were conducted at six heights (Fig. 1). The wind
5667
direction was measured at the top of the towers and additionally in four different heights on the towers. For the measurement of CO an Aero Laser AL 5001 and for NOx a Monitor Labs 9841 analyser was used on each side of the motorway. Sophisticated measuring systems were installed on each tower, which allowed measurements at different heights, using only one analyser for each gas. It consists of 10 inlets on each tower at heights of 8, 13, 18, 23, 28, 33, 38, 43, 48 and 51 m above ground level (agl). The inlets were connected through Teflon tubes with the analysers, which were based in a housing at the ground. Bypass pumps provided steady airflow through each tube. A system of valves redirected the airflow from the inlets to the analysers at a specific time schedule. At each height, three 1-min averages were taken, so in total one vertical profile was measured within 30 min. This set-up allowed continuous measurements over the whole time period of the campaign, interrupted only by regular calibration periods. In addition to the long-time measurements additional equipment was used during the eight SOPs. At the towers elevators were installed (Fig. 1), which were running continuously during SOPs, interrupted only for calibration purposes every few hours or in cases of gusty winds. The elevators were equipped with analysers for NO, NO2, O3, CO2, VOCs and particulate matter (Table 1). The results of NO, NO2, O3 and CO2 measurements are the subject of this paper, VOC and
height 51.5 m
elevator
Fig. 1. Sketch of the experimental set-up for gaseous compounds and meteorological measurements at the north tower: temperature T, wet bulb temperature Tf, wind velocity v and wind direction wd. Parameters followed by a quotation mark indicate turbulence measurements.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5668
Table 1 Components, measurement methods, manufacturer and type of analysers used in the elevators Component
Method
Manufacturer
Bendix NO Chemiluminescence with O3 NO2 Liquid chemiluminescence with Luminol—Luminox Scintrex UV absorption Thermo Electron, Horiba O3 O3 Solid chemiluminescence GEFAS CO2 IR absorption Fischer-Rosemount VOC Adsorption on charcoal, CS2 extraction and GC-FID analysis Dra¨ger PM particle number Optical particle counter Grimm PM particle number Electrical low pressure impactor—ELPI Dekati/TSI GmbH PM particle surface Diffusion charger Matter Engineering AG
particle measurements are analysed in other papers of this issue (Petrea et al., 2005; Imhof et al., 2005). The ascent speed of the elevator was about 9 m min1, which meant that an ascent from ground (5 m agl) to the top of the tower (50 m agl) took about 5 min. Thus in one hour 12 profiles (ascents and descents) were taken. The data were collected at a temporal resolution of 3 s, resulting in continuous vertical profiles. Traffic observations were carried out by the IER (Ku¨hlwein and Friedrich, 2005) with detailed speed measurements, license plate observations and traffic census. Analysis of license plate observations supported by the German Kraftfahrzeugbundesamt resulted in detailed information about the composition of the traffic fleet. The number of vehicles on the motorway could be obtained by an automatic traffic monitor in the neighbourhood of the measurement site.
1.4. Period of investigation During the entire investigation period, in total more than 1230 vertical profiles of each, NOx and CO, were measured. Additionally, the use of the elevators during the SOPs produced another 500 profiles. According to the assumptions stated in Section 1.2, limits were defined to filter the data. The profiles were aggregated to 1-hourly means. For further investigation, profiles were selected, where the wind direction was within a sector of 7451, perpendicular to the motorway. This means wind directions within the sectors 01 to 901 and 1801 to 2701 were valid. To select cases with a more homogeneous structure of the plume, it was postulated that the number of vehicles should exceed more than 1000 h1. This was fulfilled between 06:00 CEST and 24:00 CEST on working days and between 08:00 CEST and 24:00 CEST on weekends. Further analysis was restricted to clearly identifiable plumes. Therefore, a concentration difference, averaged over the heights less than 18 m, between up- and downwind side was required, which exceeded 6 ppb for
Type
LMA3, LMA4 TE 49C, APOA 360 OS-2-B BINOS NIOSH 1.108 ELPI DC
NOx and 20 ppb for CO. The number of valid profiles was increased by allowing data even when the wind direction was just beyond the defined wind direction sector or if single profiles just completed a contiguous time period. Applying these restrictions finally resulted in a total number of 280 profiles, out of 1230 from the long-time measurements, for further evaluation. Approximately 60% of the selected profiles were measured with prevailing northerly winds, 40% with southerly winds (Fig. 2). During working days the traffic density is nearly two thirds higher than on weekends. The highest numbers of vehicles were observed during the rush hours in the morning (07:00–08:00 CEST) and in the evening (17:00–18:00 CEST). On weekends and during holidays (1 and 25 May), only one peak of traffic density in the afternoon is observed. The fraction of heavy-duty vehicles (HDV) ranges from 5.5% on working days to 1.1% on Sundays and holidays (Ku¨hlwein and Friedrich, 2004). A particular traffic situation occurred between Saturday, 19 May at 17:00 CEST and Sunday, 20 May 17:00 CEST when the motorway was closed due to the demolition of a bridge nearby the site.
2. Results 2.1. Long-time measurements of NOx and CO As described in the section above it is expected, that in case of a wind direction perpendicular to the motorway, a plume, originating from the traffic on the motorway, will be found at the downwind side. To illustrate the validity of this assumption, a special situation during the campaign can be used. Between 19 May 17:00 CEST and 20 May 17:00 CEST a bridge nearby the measurement site was torn down and the motorway was closed for 24 h. With the closure the
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5669 360
6000
225 180
3000
135
2000
90 1000
45 0
0 1
2
3
4
5
6
7
8
9 360
6000
vehicles h-1
270 4000
225 180
3000
135
2000
90 1000
45 0
0 9
10
11
12
13
14
15
16
17 360
6000
315
5000
270
vehicles h-1
wind direction in degrees
315
5000
4000
225
3000
180 135
2000
90 1000
wind direction in degrees
vehicles h-1
270 4000
wind direction in degrees
315
5000
45
0
0 17
18
19
20
21
22
23
24
25
May 2001
Fig. 2. Wind direction and traffic density during the BAB II campaign. The solid line indicates the number of vehicles per hour on the motorway; dots indicate the wind direction at 51 m agl. Filled dots indicate profiles used for emission calculation. The light shaded areas mark sectors with a wind direction perpendicular to the motorway, dark shaded areas mark time periods with a well-mixed layer between 09:00 and 18:00 CEST.
number of vehicles decreases to zero (Fig. 3). The wind direction was northeast to north and therefore perpendicular to the motorway. As long as there was traffic on the motorway a plume was observed with concentration differences of 100 ppb CO and 10 ppb NOx (Fig. 3). After the road was closed, the same concentration level was monitored on both sides of the motorway. At the end of the closing period a similar process was observed.
One hour before the motorway was opened the concentrations of CO and NOx up- and downwind of the motorway were at the same level; 1 h after the opening a plume was found, which reached a height of approx. 23 m agl. A difference of 40 ppb CO and 10 ppb NOx, at 8 m agl, was observed. It is clearly evident that plumes downwind the motorway originate from traffic emission and can be used to calculate the source
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
3500
May 19/20, 2001
80 vehicles
wd
70
vehicles h-1
3000
wind direction in degrees
5670
60
2500
50
2000
40 1500
30
1000
20
500
10
0
0 18
00
06
12
18
time in CEST 60
60
height in m agl
May, 19 15:00
60 May, 20 17:00
May, 20 19:00
50
50
50
50
40
40
40
40
30
30
30
30
20
20
20
20
10
10
10
10
0
0 100
200
300
0 100
CO in ppb
200
300
0 100
60 May, 19 15:00
200
300
100
CO in ppb
CO in ppb
60
height in m agl
60
May, 19 18:00
60 May, 20 17:00
May, 20 19:00
50
50
50
40
40
40
40
30
30
30
30
20
20
20
20
10
10
10
10
0 0
20 NOX in ppb
40
0 0
20
40
NOX in ppb upwind
300
60
May, 19 18:00
50
0
200
CO in ppb
0 0
20
40
NOX in ppb
0
20
40
NOX in ppb
downwind
Fig. 3. Development of a plume along the motorway on 19 and 20 May, before and after the motorway was closed. Upper part: number of vehicles per hour and wind direction. Middle: CO concentration profiles before (left), during (middle left and middle right) and after (right) the closure of the motorway. Lower part: the same for NOx. Solid lines indicate downwind, dashed lines upwind measurements.
intensities from the measured concentration differences. The formation of the plume is also validated by the results of differential optical absorption spectroscopy
(DOAS) measurements, which were additionally carried out in the first few days of the BAB II campaign (v. Friedeburg et al., 2005; Pundt et al., 2005).
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
Fig. 4 shows the typical development of the plume during a day with a nearly constant wind direction perpendicular to the motorway. On Friday, 11 May 2001 from 10:00 CEST to 18:00 CEST northeasterly winds prevailed (Fig. 2). The number of vehicles on the motorway decreased from 3600 veh h1 at 10:00 CEST to 3200 veh h1 at 11:00 CEST. Afterwards the number of cars increased to 4800 veh h1 during the rush hour at 17:00 CEST (Fig. 2). The wind speed increased from less than 1 m s1 at 10:00 CEST in the morning to 3 m s1 at 8 m agl and 5 m s1 at 51 m agl. At 10:00 CEST large concentration differences were found (16 and 80 ppb, respectively), for both, NOx and CO, coupled with low wind speed. The plume reached a height of about 43 m. During the day the concentration differences varied within the plume from 4 to 16 ppb NOx and from 10 to 80 ppb CO. For NOx two maxima occurred at 12:00 CEST (16 ppb) and at 16:00 CEST (18 ppb), whereas at 13:00 CEST and 16:00 CEST maxima were found for CO (80 ppb). During the day the height of the plume was approx. 18–23 m.
2.2. Special operation periods—SOPs Eight special operation periods (SOPs) were conducted with enhanced instrumentation. For reasons of manpower, the use of additional equipment had to be restricted to time periods when an ideal wind direction perpendicular to the motorway was expected. The main objective of the project was to calculate the air pollutants emitted from the traffic on the motorway. Budget studies require information about vertical distribution of meteorological parameters and at least the vertical extension of the plume or, if available, its vertical profile. Therefore the measurements, which were carried out with the help of elevators up- and downwind of the motorway, were along with long-time measurements a decisive part of the measurement strategy. In Fig. 5a typical vertical profiles for an upwind– downwind situation for one single ascent, started on 14 May 08:07:00 CEST for NO, NO2, O3, CO2 and diffusion charger (DC—measuring the particle surface in mm2 cm3) are depicted. The grey lines are profiles measured on the upwind side of the motorway and the black lines on the downwind side. The concentrations on the upwind side are lower than on the downwind side and almost constant over the measuring height. The increased concentrations on the downwind side were doubtless caused by the exhaust of the traffic on the motorway (Section 2.1), as no other emission source in the near vicinity of the motorway was present. The downwind concentrations decreased with height and reached at approx. 30 m the upwind concentrations. The plume of the motorway has a vertical extension of 30 m with a first maximum at 7 m and a second maximum at
5671
25 m agl. The response times of the analysers were not identical, which can be observed from the vertically shifted profile structure for NO and O3. From correlation calculations with a fast response analyser, it can be determined that the NO analyser e.g. has a time delay of approx. 27 s. The NO data set was shifted 27 s against the rest of the data set. In Fig. 5b five consecutive vertical profiles for NO2 up- and downwind of the motorway are presented (upwind: grey; downwind: black). The upwind profiles are quite constant with height on a level of approx. 20 ppb. The downwind profiles give an idea of the permanently changing concentrations with height as well as with time. The vertical structure and the peak concentrations changed from profile to profile. Even during daytime with relatively constant wind speed and wind direction the downwind concentrations changed a lot. The reason for this behaviour of the air pollutants is that emissions from the motorway are not emitted with a constant flow rate, due to the fact that the vehicle number has temporal fluctuations. Simultaneously, the downwind measurement site is located relatively close to the emission source. As the distance from the emission source to the measurement site becomes longer, the more the high concentrated, inhomogeneously distributed emissions are transformed increasingly into a homogeneous plume. However, due to mechanical and thermal turbulence the vertical extension of the plume increases and so the air pollutants are diluted with cleaner air and thus the difference between up- and downwind concentrations decreases. Neither extensive vertical plumes nor little differences between up- and downwind concentrations are ideal for such a budget study because of the increasing influence of the measurement uncertainty with decreasing difference between up- and downwind concentrations. For these reasons the location of the towers had to be optimized. The inhomogeneous distribution of air pollutants downwind meant that single vertical profiles had to be averaged, similar to the long-time measurements, to mean hourly profiles. In Fig. 5c 12 vertical profiles, collected on 14 May from 08:02:03 to 08:56:59 CEST were combined to mean hourly profiles (solid lines). The original data, sampled every 3 s is marked with grey (upwind) and black (downwind) dots. These mean hourly profiles were used for further calculations. Upwind NO concentrations varied between 1 and 5 ppb, whereas the downwind NO concentrations varied between 1 and 60 ppb. In general, the highest concentrations were measured near ground but there were also profiles with increased concentrations at higher altitudes. Those elevated concentration maxima were observed frequently throughout the entire measurement period. Fig. 6 shows isopleths and wind vectors for 14 May from 07:00 to 18:00 CEST for NOx; white areas indicate
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5672
height in m agl
10
12
14
16
18
50
50
18
45
45
16
40
40
14 12
35
35
30
30
25
25
6
20
20
4
15
15
2
10
10
May 11, 2001 NO X -difference in ppb
10 8
0 -2
10
12
14
16
18
16
18
time in CEST
height in m agl
10
12
14
50
50
45
45
80
40
40
70
35
35
60
30
30
50
25
25
20
20
15
15
10
10
10
-10
10
12
14
16
18
16
18
90 May 11, 2001 CO -difference in ppb
40 30 20
time in CEST 10
12
14
height in m agl
5 45
45
40
40
4
35
35
3.5
30
30
3
25
25
20
20
15
15
1
10
10
0.5
4.5
May 11, 2001 wind speed in ms -1
2.5 2 1.5
0 10
12
14
16
18
time in CEST Fig. 4. Wind direction perpendicular (northerly winds) to the motorway on 11 May 2001. The figure shows the concentration difference for NOx (upper part) and CO (middle) between down- and upwind side of the motorway and additionally the wind speed. The solid line (bold) indicates the height of the plume.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5673
(a)
(b)
(c) Fig. 5. (a) Vertical profiles of two 5 min soundings on 14 May, starting time 08:07:00 CEST. Upwind (grey lines) and downwind (black lines) for NO, NO2, O3, CO2 and diffusion charger (DC). (b) Five consecutive vertical profiles for NO2 from 08:02:03 to 08:21:45 CEST. Upwind (grey lines) and downwind (black lines) of the motorway. (c) Summary of 12 consecutive vertical profiles within one hour of measurements, NO, NO2 and DC upwind (grey dots) and downwind (black dots).
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5674
NOx - south tower (upwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
(a)
12
13
14
15
16
17
18
15
16
17
18
15
16
17
18
time in : CEST NOx - north tower (downwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
(b)
12
13
14
time in : CEST NOx - north tower (downwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
12
13
14
time in : CEST NOx in ppb 0
(c)
10
20
wind speed / wind direction 30
40
50
60
70
< 10 m /s < 5 m /s < 2 m /s
N W
E S
Fig. 6. (a) Isopleths of NOx (3 s values) combined with a wind vector diagram on upwind side on 14 May 07:00 to 18:00 CEST. (b) Isopleths of NOx (3 s values) on downwind side on 14 May 07:00 to 18:00 CEST. (c) Isopleths of NOx (1-h means) on downwind side on 14 May 07:00 to 18:00 CEST.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
instrument calibration periods. Isopleths are a good method of combining a large number of vertical profiles, in order to get an overview over the temporal variation of the vertical distribution of the measured component, in this case the NOx concentration. In Fig. 6b the number of single profiles (in total 106 at each side of the motorway) is visible. The small black dots indicate when and where measurements were available. The interpolation calculation was done using the Krigin method (Zongmin, 1986). The Kriging method was developed for the interpolation of topographical data. This method delivers the most realistic results, for 2D inter- and extrapolation when the results are unequally distributed in the 2D area, by weighting the data points according to the distance from each other. The isolines were smoothed by using the spline method. The database of Fig. 6a and b are the original 3 s values from the single profiles. Fig. 6a is combined with a wind vector diagram. The length of the wind vector depends on the wind speed, and the orientation of the arrow shows the horizontal wind direction. On 14 May, almost throughout the entire observing period, wind from southerly directions prevailed. Thus, the south tower was upwind and the north tower downwind of the motorway. The wind speed increased between 13:00 and 16:00 CEST and caused decreasing concentrations upwind as well as downwind. Within the second calibration period around 16:00 CEST a sudden change in wind direction with high wind speed occurred, caused by a thunderstorm front touching the investigation area. Up- and downwind was changed for about 45 min. The upwind concentrations (Fig. 6a) decreased by the time, but the vertical distribution was always relatively constant. On the downwind side (Fig. 6b), the highest concentrations occurred near ground level but occasionally high concentrations in higher altitudes could be observed. Using the 1-h-averaged values (Fig. 6c), the isopleths are much smoother. The decreasing concentrations all day long can clearly be followed, especially when the wind speed increased, starting on 14:00 CEST. In Fig. 7 the horizontal distribution of the air pollutants (NO, NOx and NO2) on 14 May from 7:00 to 18:00 CEST is depicted. Ground monitoring stations were fixed directly nearby the motorway on the downwind side (no. 2), perpendicular to the motorway at a distance of 60 m (no. 3) and 80 m (no. 5). The values at site no. 1 (upwind) and 4 (downwind) were gained by calculating the 1-h-mean values from the profile measurements whenever the elevators were near ground level between 5 and 9 m above ground. The temporal courses of the NOx concentration at the different sites decreased from 7:00 to 17:00 CEST and increased again from 17:00 to 18:00 CEST. The air masses first passed the upwind side where the lowest concentrations were measured throughout the day. Then they passed the motorway, while loaded with vehicle
5675
emissions, and on the downwind side the highest NOx concentrations were measured directly beside the lanes (no. 2). On their way to the other downwind measurement sites, the NOx concentrations were reduced by dilution with clean air. At site no. 3 and 4 similar NOx concentrations were measured. Just between 12:00 and 15:00 CEST the concentrations at site no. 3 (height of the air inlet 3.5 m above ground) were slightly higher than at site no. 4 (height of air inlet 5 to 9 m). At site no. 5 the concentrations decreased again. The decreasing values of NOx according to the distance from the motorway is only caused by dilution. The temporal course of NO is similar, the highest concentrations are also measured downwind directly beside the lanes (no. 2) and the lowest concentrations on the upwind side (no. 1). Compared to NOx, the NO concentrations are strongly decreasing. NO is mainly emitted from exhaust pipes, for spark ignition engines approx. 95% of NOx is emitted as NO. In the atmosphere NO is transformed in a very quick reaction, following the photo-stationary equation, with O3 to NO2. Thus, NO is decreasing more strongly than NOx, because NO is affected by chemical transformation and by physical dilution, simultaneously. The temporal course for NO2 is different from those for NO and NOx. The lowest concentrations were measured at the upwind side again. On the downwind side, a decrease with distance from the motorway could not be observed. The reasons for this are opposing effects of physical dilution, on the one hand, and the formation of NO2 from NO, on the other hand. These two effects are not temporally constant, but they depend on varying parameters like wind speed, global radiation, temperature, availability of O3, etc. Thus, sometimes the physical dilution predominates and sometimes the NO2 formation and so the site with the highest NO2 concentration changes from time to time. In Fig. 8, mean hourly profiles from the elevator measurements (grey lines: upwind; black lines: downwind), calculated from 12 single profiles (Fig. 5a), are compared to the vertical profiles from the permanent tower measurements (blue lines: upwind; red lines: downwind) and to the values of the ground monitoring stations (red dots: downwind, beside motorway lanes; pink dots: downwind, 60 m from motorway; green dots: downwind, 80 m from motorway). The plume with relatively low and constant concentrations on the upwind side and with increased concentrations near ground level on the downwind side is clearly visible in all the vertical profiles of the measured species. NOx, NO and NO2 measured at the elevator are in good compliance with the tower measurements; the deviation is quite small (upwind: max. 1 ppb; downwind: max. 6 ppb, resp. 12%). Maximum NOx and NO concentrations, measured directly beside the lanes of the motorway (red dots) decreased with distance from
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5676 South Towerr -> upwind
North Tower -> downwind
measurement sites:
1
Motorway A 656
2
4
3
5
1
upwind - elevator 84 m
2
downwind - motorway
3
downwind - 60 m
4
downwind - elevator 60 m
5
downwind - 80 m
10 m 60 m
NO x concentration in ppb
84 m
80 m
150 NO x
2 100
4 50
5
3
1
0 0700
1100
0900
1300
1500
1700
NO concentration in ppb
100
2
NO
75
50
5
25
3
4
1 0 0700
0900
1100
1300
1500
1700
40 NO2 conzentration in ppb
2 30
NO2
3
5
20
10
0 0700
4
1
0900
1100 1300 time in CEST
1500
1700
Fig. 7. Horizontal distribution of NOx, NO and NO2 upwind (84 m south) and downwind (0, 60, and 80 m north) the motorway on 14 May 07:00 to 18:00 CEST.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5677
PM10: 0.3 - 10µm
height in m agl
45
35
25
15
5 0
50
100
150 0
25 50 75 100 0
10 20 30 40 50
NO conc. in ppb
NOx conc. in ppb
0
40,000
80,000 0
200
400
DC - particle surface PM10 2 -3 particle number/liter in m cm
NO2 conc. in ppb
elevator measurements:
WD = 183 degree
downwind - north (60m)
45
upwind - south (80m)
height in m agl
35 tower measurements: downwind - north (60m) upwind - south (80m)
25
ground stations: BUGH (10m north)
15
FFA (60m north) BASF (80m north)
5 400
410
CO2 conc. in ppm
420 0
20
O3 conc. in ppb
40 17,2
17,4
17,6 0
temperature in ˚C
2
4
wind speed in m/s
Fig. 8. Comparison of different measurements. Database: hourly mean values on 14 May 07:00 to 08:00 CEST. Elevator measurements (grey lines: upwind; black lines: downwind), tower measurements (blue lines: upwind; red lines: downwind) and ground monitoring stations (red dots: downwind beside motorway; pink dots: downwind, 60 m from motorway; green dots: downwind, 80 m from motorway).
the motorway (pink dots: 60 m; and green dots: 80 m). The concentrations measured at the ground at the monitoring station (pink dots) and the concentrations at the elevator (black line) are also in good agreement (max. 4 ppb, resp. 14%).
2.3. Emission calculation The considerations stated above, linked to the assumptions of Section 1.2 and measurement data for the variables given in Eq. (1) allow calculating the source strength of traffic emissions on the motorway.
2.3.1. Long-time measurements The plume is divided into layers of constant thickness, each with constant concentration and constant wind speed. Each layer was centred on the measuring height, in which gas concentrations had been measured. The thickness of each layer is 5 m. To close the gap between the lowest measuring level (8 m) and the ground, it has been assumed that the concentration profile between the lowest level and the ground is constant. Measurements made near the base of the north tower (position no. 3 in Fig. 7) indicate the validity of this assumption, whereas linear extrapolation of the concentration profile would have led in some cases to unrealistically high
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
source strength in kg km-1 h-1
concentrations at the ground level. To derive the wind speed in the calculation heights, a logarithmic fit for each measured wind profile was determined. Taking into account that the wind direction is not always perpendicular to the motorway, only the component of the wind speed perpendicular to the motorway has been used for further calculations. The particular height of the plume has been derived from the vertical profile of the gaseous compounds. Using these assumptions, the source intensities along the motorway were determined. To assure that rapidly changing stratification of the atmosphere has no influence on the results, only profiles between 09:00 CEST and 18:00 CEST, when a well-mixed layer within the surface layer has developed, have been analysed. Depending on the traffic density and the instantaneous composition of the traffic fleet, the emissions are not homogeneous. The inhomogeneous structure of the emission source in combination with turbulence leads to an inhomogeneous structure of the plume as well (Fig. 6b). This effect does not allow the calculation of source intensities with high temporal resolution, but it is possible to determine reliable long-time averages. Fig. 9 shows the mean diurnal course of calculated CO and NOx emissions during working days. The number of values averaged for each 1 h interval differs from 6 to 13 and therefore the statistical significance of the results differs, but nevertheless the daily course of the emissions shows a good compliance with traffic census data, with high values in the morning and in the evening during rush hours. On midday, between 13:00 and 14:00 CEST, a secondary maximum is found for CO, which is influenced by the small number of valid profiles and the high variability of emissions indicated by the standard deviation in Fig. 9. For NOx, which is
mainly emitted by diesel motors, a different diurnal course is found. Diesel motors are mainly used by the heavy-duty fraction of the traffic, and in comparison by only a small number of passenger cars. Therefore, the mean daily course of NOx is strongly influenced by emissions of HDV. Passenger cars influence the NOx emissions during the rush hours, purely because of the large number of vehicles during that time. Due to this fact, the ratio of CO/NOx is an indicator for the composition of the traffic fleet. Between 11:00 and 15:00 CEST, when the fraction of heavy-duty traffic is higher than 7%, the ratio of CO/NOx (Fig. 10) is reduced to numbers o2, the ratio rises up to values 42 when the fraction of heavy-duty traffic is lower than 7%. Analysis of the measured data leads to a mean weekly course of the source strength of CO and NOx as shown in Fig. 11. Between Tuesday and Thursday, a mean source strength of 9.1 kg km1 h1 CO was observed. On Monday (9.8 kg km1 h1) and Friday (9.7 kg km1 h1), slightly increased emissions are determined, due to the more intensive traffic on the motorway. The source strength for NOx shows a similar performance throughout the week with the lowest amount between Tuesday and Thursday (4.0 kg km1 h1) and higher values on Monday (5.0 kg km1 h1) and Friday (4.7 kg km1 h1). The CO/NOx ratio is found during working days to be 2.1. A different performance is observed during the weekend (Saturday and Sunday). A high CO source strength is calculated, even higher than during the week (10.3 and 9.8 kg km1 h1), while a reduced NOx value is observed (2.5 kg km1 h1 and 2.0 kg km1 h1). The data for the weekend should be considered with caution because only a few hours were available for analysis, so they are not as representative for a daily mean value as the calculations made for
30
4 800
25
4 000
20
3 200
15
2 400
10
1 600
5
number of vehicles h-1
5678
800
0
0 7
8
9
10
11 CO
12
13
14 15 16 time in CEST
NOX
LDV
17
18
19
20
21
22
HDV
Fig. 9. Mean daily course of the source strength for CO and NOx combined with the mean daily course for passenger cars and heavyduty traffic during working days.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5679
4.0
16 HDV
3.5
14
3.0
12
2.5
10
2.0
8
1.5
6
1.0
4
0.5
2
0.0
HDV in %
CO/NOX
CO/NOX
0 7
8
9
10
11
12
13
14 15 16 time in CEST
17
18
19
20
21
22
Fig. 10. Mean daily course of the ratio CO/NOx combined with the mean daily course of the heavy-duty fraction on the motorway during working days.
12
10.3
11 source strength in kg km-1 h-1
10
9.8
9.7
9.8 9.1
9 8 7 6
5.0
4.7
5 4
2.5
3 2
4.9 4.1
4.0 2.3
2.0
2.1
2.0
1 0 Mo
Tu-Th CO
Fr NOX
Sa
Su
CO/NOX
Fig. 11. Mean source intensities of CO, NOx as well as CO/NOx ratios averaged over the time period of the BAB II campaign.
working days. Yet even then it can be stated that due to the predominant fraction of passenger cars and the reduced fraction of the heavy-duty traffic the NOx emissions are reduced and therefore the CO/NOx ratio rises up to 4. Compared to other studies, mainly conducted at tunnel sites, similar ratios of CO/NOx have been found. All studies differ in traffic fleet composition, in vehicle driving pattern, in road grades and in observed time periods (daytime and week day), but even then similar ratios occur. At the Taipei Tunnel (Hwa et al., 2002) the experiment was conducted during a Saturday. CO/NOx ratios of 4 were found compared to 4.1 at the BAB 656 site. At
the Gubrist Tunnel, Staehelin et al. (1997) found a ratio of 1.6, which is a consequence of the higher heavy-duty fraction (9.5%) of the traffic in the tunnel compared to the BAB II site (5.5%). The Tauerntunnel experiment (Schmid et al., 2001) shows much lower values for CO/ NOx ratios (0.81), which is also due to a much higher percentage of HDV (18% on work days, 3% on weekends) using the transit route. BAB I (Vogel et al., 2000) gives a snap-shot of emission strength of a few hours, but the high values found for CO (17–32 kg km1 h1) are supposed to be due to the instantaneous composition of the traffic fleet on the motorway. Those values found in 1997 are still within the spread of the measurements of 2001.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5680
Using data from traffic census, it is simple to calculate emission factors for both gaseous compounds CO and NOx. To derive emission factors, it is necessary to normalize the source strength by the number of cars on the motorway. For CO a mean emission factor of 2.62 g km1 veh1 and for NOx 1.08 g km1 veh1 was found. By means of multiple regression, it is possible to split the emission factor in fractions of light-duty vehicles (LDV) and HDV. For NOx an emission factor of 0.68 g km1 veh1 for LDV and 6.86 g km1 veh1 for HDV was determined (s. Fig. 12). For CO it was not possible to split the emission factors between LDV and HDV, which is due to the fact, that those emission factors are quite similar and cannot be clearly separated by the means of multiple regression if the fraction of HDV is very low. The results are in good agreement to those from other studies (Table 2), especially to those made in the Gubrist Tunnel 2000 (Steinemann and Zumsteg, 2003). Differences to older studies (Kristensson et al., 2004) can be explained by use of different exhaust gas reduction techniques in the respective traffic fleet. On the motorway at the measuring site a speed limit of 120 km h1 is given. Speed measurements show that the averaged speed of LDV amounts approx. 113 km h1 for HDV a mean speed of 86 km h1 is found (Ku¨hlwein and Friedrich, 2005). A quite different situation is found during traffic jams. In such a case, the vehicle speed is lower than 30 km h1, and this driving pattern leads to increasing emissions (Ku¨hlwein and Friedrich, 2005).
The effect on the CO source strength is shown in Fig. 13. During the BAB II campaign, two traffic jams occurred with wind directions perpendicular to the motorway. According to traffic census, traffic jams were observed on 14 May between 9:00 CEST and 10:22 CEST, and on 23 May between 7:15 CEST and 10:20 CEST. On 14 May, southerly winds prevailed, while on 23 May, north to northeasterly winds were found (Fig. 2). The source intensity for CO increases remarkably during traffic jams, compared to the same time period of the same day of the week but with no traffic jam. The increase of CO concentration is also remarkable, considering that traffic density during the jam remains constant compared to the hours before. This fact underlines the strong impact of the driving pattern on the traffic emissions and illustrates the need of detailed and sophisticated traffic census as conducted during BAB II.
2.3.2. Special observation periods (SOPs) For SOPs two different data sets for vertical profiles of NOx, one set from tower measurements and one set from elevator measurements, were available. Despite the different concepts for elevator measurements and tower measurements, Eq. (1) is applied for both emission calculations. The NOx emissions calculated with both data sets during the SOPs are compared to each other in Fig. 14. During all SOPs, 24 1-h-mean profiles could be calculated from the elevator data overall. For 15 of these 1-h means, concentration measurements from the tower
9
8
6.86
6
-1
emission factor in g km veh
-1
7
5
4
2.62 3
2
1.08 0.68 1
0 CO CO
NOX NO X
NOXDH ; LD
NOX;DHD
Fig. 12. Mean emission factors for CO, NOx as well as LDV and HDV NOx emission factors averaged over the time period of the BAB II campaign. The error bars indicate the standard error.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5681
Table 2 Emission factors derived from different tunnel studies Study/site Taipai, 2000 Gubrist, 1993
Gubrist, 1997
Gubrist, 2000
Gubrist, 2002
Fort McHenry, 1992
So¨derledstunnel, So¨derledstunnel, So¨derledstunnel, So¨derledstunnel,
Winter 1993 Autumn 1994 Winter 1995/1996 Winter 1998/1999
BAB656, 2001
All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles All vehicles All vehicles All vehicles All vehicles LDV HDV All vehicles LDV HDV
CO (g km1 veh1)
NOx (g km1 veh1)
3.6470.26 4.1870.38 1.1872.33 3.8970.64 1.9–4.4 1.9–7.5 3.41 1.6–4.0 o3.7 2.72 0.8–2.0 0.5–1.5 1.35 3.9570.34 6.1171.75 4.63 6.270.2 6.370.1 5.370.1 5.2770.1
0.970.18 1.0570.09 15.5670.79 2.4571.59 0.61.8 1018 1.99 0.31.0 1014 0.94 0.20.45 815 1.21 0.570.06 8.9770.28 2.0670.48 1.370.1 1.170.1 1.570.1 1.3670.03 1.0770.03 8.070.8 1.0870.05 0.6870.12 6.8671.57
2.6270.13
Hwa et al. (2002) Staehelin et al. (1997)
Steinemann and Zumsteg (2003)
Pierson et al. (1996)
Kristensson et al. (2004)
This paper
20 no jam jam
traffic jam
CO source strength in kg km
-1
h
-1
18 16
traffic jam
14 12 10 8 6 4 2 0 9
10 May 14, 2001
11
12
7 time in CEST
8
9
10
11
May 23, 2001
Fig. 13. Comparison between traffic jams and undisturbed traffic situations. Light grey bars indicate source intensities during the two traffic jam situations on 14 and 23 May. Dark grey bars indicate typical CO source intensities during corresponding working days without traffic jam.
were available simultaneously. Take note that the 24 1h-mean profiles do not represent a daily cycle. For some hours almost similar results were found but for other hours quite large differences between the emissions
calculated by different measurement strategies exist. Some of these discrepancies are explainable. At value no. 2 (between 06:00 and 07:00 CEST) the elevator method’s emissions are approx. three times as high as
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5682 8.000
-1
NOx emissions in g*km *h
-1
NOx emissions elevator method NOx emissions tower method
6.000
4.000
2.000
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
hours
Fig. 14. Comparison of emission data from elevator measurements (red bars) and tower measurements (blue bars). Database: hourly mean values. In total 24 not interrelated 1-h-mean values throughout the total measurement period are given, which do not characterize a daily cycle.
the tower method’s emissions. This result was due to very low downwind concentrations (NOx: approx. 25 ppb) from 06:00 to 06:30 CEST, followed by a strong increase in the concentrations (NOx: approx. 90 ppb), which occurred due to the start of the morning rush hour. The tower measurements started at a height of 8 m when the downwind–upwind difference was little. After 06:30, when the upwind–downwind difference in the plume of the motorway strongly increased, the tower profiling measurements were done at higher altitudes and thus the plume could not correctly be covered by the tower measurements. This illustrates why the calculation of emissions using the tower measurements were temporally restricted to times where no rapid changes either in stratification or in emission sources occurred. However, not all differences in the results of the emission calculations with the two methods could be explained like that. In general, the NOx emissions of the elevator measurement method were 25% higher as calculated from the long-time measurements.
3. Conclusions Based on the experiences during the precursor campaign BAB I in 1997, BAB II was conducted in May 2001 with an enhanced experimental set-up. Using the gaseous compounds CO and NOx as an example, it could be shown that simultaneous measurements upand downwind of a motorway are practical to calculate the source intensity of any compound (Fig. 3). In cases of wind directions perpendicular to the motorway, the
plume caused by the road traffic can be clearly identified downwind of the motorway. Despite the fact that the small number of valid measurements does not allow a detailed interpretation, a plausible explanation for the diurnal course of both gaseous compounds can be found. The mean diurnal course of CO source intensities follows the course of the number of vehicles on the motorway, showing peaks during the rush hours and minima at midday. NOx source intensities are coupled with the heavy-duty fraction of the traffic; therefore, the mean daily course is influenced by the large number of vehicles during the rush hours and by the maximum of heavy-duty traffic at midday (Figs. 9 and 10). The impact of driving patterns on the emission can be shown by a comparison between traffic jam situations and flowing traffic. During traffic jams, when the averaged speed of the vehicles is lower than 30 km h1, a strong increase of the CO source strength is observed (50–100%), even if the number of vehicles remains constant or decreases (Fig. 13). This gives reasons to keep the traffic flowing by structural measures on the road network as well as by speed limit restrictions. It is possible to determine mean weekly source intensities for CO and NOx. Mean source intensities for CO of 9.5 kg km1 h1 on working days and 10.2 kg km1 h1 on weekends were found. In contradiction, NOx emissions, which are strongly influenced by heavy-duty traffic, show a stronger variability with approx. 4.4 kg km1 h1 during working days and 2.4 kg km1 h1 NOx on the weekends, when there is no heavy-duty traffic allowed on German motorways. The use of elevator measurements during the SOPs gives a new insight in the structure of the plume deriving
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
from the vehicles on the motorway. The plume is even inhomogeneous at a distance of 60–80 m, because of the inhomogeneous emissions on the motorway. To handle this problem and to determine an emission data set in a high temporal resolution, it would have been necessary to extend the elevator measurements over the whole investigation period. This was not practical because of the need of intensive manpower. Another finding was that frequently concentration maxima in higher altitudes on the downwind side were observed. This fact underlines the imperative to cover the whole extent of the plume in a high spatial resolution, as it was done by both measurement strategies used. Each strategy has its own advantages and drawbacks, which make it difficult to compare the results of both. Unfortunately, there are only a small number of measurements that are covered by both methods (Fig. 14). Elevator measurements have a high temporal and spatial resolution, but the measurements are instantaneous and only a small time period of measurements is available. The tower measurements are not so instantaneous because the concentrations of the gaseous compounds are averaged over 3 min, and these measurements cover a longer time period than the elevator measurements. The disadvantage of tower measurements is the time span between the first and the last level of a profile. Therefore, times of fast changing stratification have to be excluded from analyses to handle this fact, which reduces the number of available measurements for analysis. Nevertheless, the long-time tower measurements allow the calculation of mean emissions on the motorway. The results of BAB II compared to similar studies, which have been mainly conducted in tunnels, show a good agreement for the CO/NOx ratios and emission factors. Emission factors of 2.62 for CO and 1.08 g km1 veh1 for NOx were determined. Further analysis of the database gained under these considerations allows the comparison of measured source intensities and emission factors with those calculated by model means (Corsmeier et al., 2005b).
Acknowledgements The BAB II project was funded by the Forschungszentrum Karlsruhe, Germany and the Umweltbundesamt (UBA), Berlin, Germany, under contract number 299-42-250. The authors would especially like to thank the participating scientists and technicians for their active participation and intense collaboration.
References Corsmeier, U., Kohler, M., Vogel, B., Vogel, H., Fiedler, F., 2005a. BAB II—a project to evaluate the accuracy of real
5683
world traffic emission for a motorway. Atmospheric Environment this issue. Corsmeier, U., Ku¨hlwein, J., Vogel, B., Kohler, M., Vogel, H., 2005b. BAB II: comparison of measured and model calculated real world traffic emissions of gaseous and particulate compounds. Atmospheric Environment this issue. v. Friedeburg, Ch., Pundt, I., Mettendorf, K.-U., Wagner, T., Platt, U., 2005. Multi-AXis-(MAX) DOAS measurements of NO2 during the BAB II motorway emission campaign. Atmospheric Environment 39, 977–985. Hucho, W.-H., 1981. Aerodynamik des Automobils. Vogel, Wu¨rzburg. Hwa, M.-Y., Hsieh, Ch.-Ch., Wu, T.-Ch., Chang, L.-F.W., 2002. Real-world vehicle emissions and VOCs profile in the Taipei Tunnel located at Taiwan Taipei area. Atmospheric Environment 36, 1993–2002. Imhof, D., Weingartner, E., Vogt, U., Dreiseidler, A., Rosenbohm, E., Scheer, V., Vogt, R., Nielsen, O.J., Kurtenbach, R., Corsmeier, U., Kohler, M., Baltensperger, U., 2005. Vertical distribution of aerosol particles and NOx close to a motorway. Atmospheric Environment this issue. John, Ch., Friedrich, R., Staehelin, J., Schla¨pfer, K., Stahel, W.A., 1999. Comparison of emission factors for road traffic from a tunnel study (Gubrist tunnel, Switzerland) and from emission modeling. Atmospheric Environment 33, 3367–3376. Kristensson, A., Johansson, Ch., Westerholm, R., Swietlicki, E., Gidhagen, L., Wideqvist, U., Vesely, V., 2004. Realworld traffic emission factors of gases and particles measured in a road tunnel in Stockholm, Sweden. Atmospheric Environment 38, 657–673. Ku¨hlwein, J., Friedrich, R., 2005. Traffic measurements and high-performance modelling of motorway emission rates. Atmospheric Environment this issue. Kurtenbach, R., Becker, K.H., Gomes, J.A.G., Kleffmann, J., Lo¨rzer, J.C., Spittler, M., Wiesen, P., Ackermann, R., Geyer, A., Platt, U., 2001. Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel. Atmospheric Environment 35, 3385–3394. Leisen, P., Mu¨ller, W.-R., Heich, H.-J., Hasselbach, W., Mu¨ller, J., Waldeyer, H., 1992. Entwicklung der Abgasbelastung an Autobahnen. Umweltbundesamt, Berlin (UFOPLAN-Nr. 10402585). Peters, J.-L., 1990. Bestimmung des aerodynamischen Widerstands des ICE/V im Tunnel und auf freier Strecke durch Auslaufversuche. Eisenbahntechnische Rundschau 39, 559–564. Petrea, M., Kurtenbach, R., Wiesen, P., Vogt, U., Baumbach, G., Fuchs, J., Jaeschke, W., 2005. NMVOC measurements of motorway emissions during the BAB II campaign. Atmospheric Environment this issue. Pierson, W.R., Gertler, A.W., Robinson, N.F., Sagebiel, J.C., Zielinska, B., Bishop, G.A., Stedman, D.H., Zweidinger, R.B., Ray, W.D., 1996. Real-world automotive emissions— summary of studies in the Fort McHenry and Tuscarora mountain tunnel. Atmospheric Environment 30, 2233–2256. Pundt, I., Mettendorf, K.-U., Laepple, T., Knab, V., Xie, P., Lo¨sch, J., v. Friedeburg, Ch., Platt, U., Wagner, T., 2005. Measurements of trace gas distributions by Long-path DOAS-tomography during the motorway campaign
ARTICLE IN PRESS 5684
M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
BAB II: experimental setup and results for NO2. Atmospheric Environment 39, 967–975. Schmid, H., Pucher, E., Ellinger, R., Biebl, P., Puxbaum, H., 2001. Decadal reductions of traffic emissions on a transit route in Austria—results of the Tauerntunnel experiment 1997. Atmospheric Environment 35, 3585–3593. Staehelin, J., Schla¨pfer, K., Bu¨rgin, T., Steinemann, U., Schneider, S., Brunner, D., Ba¨umle, M., Meier, M., Zahner, C., Keiser, S., Stahel, W., Keller, C., 1995. Emission factors from road traffic from a tunnel study (Gubrist tunnel, Switzerland). Part I: concept and first results. Science and Total Environment 169, 141–147. Staehelin, J., Keller, C., Stahel, W.A., Schla¨pfer, K., Steinemann, U., Burgin, T., Schneider, S., 1997. Modelling emission factors of road traffic from a tunnel study. Environmetrics 8, 219–239. Staehelin, J., Keller, C., Stahel, W., Schla¨pfer, K., Wunderli, S., 1998. Emission factors from road traffic from a tunnel study (Gubrist Tunnel, Switzerland) Part III results of organic
compounds, SO2 and speciation of organic exhaust emission. Atmospheric Environment 32, 999–1009. Steinemann, U., Zumsteg, F. (Eds.), 2003. Verkehrs- und Schadstoffmessungen 2002 im Gubristtunnel. Ostluft, Amt fu¨r Abfall, Wasser, Energie und Luft des Kantons Zu¨rich (AWEL), Auftragsnummer US 89–16, www. ostluft.ch. Vogel, B., Corsmeier, U., Vogel, H., Fiedler, F., Ku¨hlwein, J., Friedrich, R., Obermeier, A., Weppner, J., Kalthoff, N., Ba¨umer, D., Bitzer, A., Jay, K., 2000. Comparison of measured and calculated motorway emission data. Atmospheric Environment 34, 2437–2450. Vogt, U., Baumbach, G., Dreiseidler, A., 2005. The BAB II quality assurance program for measurements of gaseous emissions and particulate matter. Atmospheric Environment this issue. Zongmin, W., 1986. Die Kriging-Methode zur Lo¨sung mehrdimensionaler Interpolationsprobleme. University Dissertation, Go¨ttingen, (1986/4452).
Atmospheric Environment 39 (2005) 5665–5684 www.elsevier.com/locate/atmosenv
Estimation of gaseous real-world traffic emissions downstream a motorway M. Kohlera,, U. Corsmeiera, U. Vogtb, B. Vogela a
Institut fu¨r Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universita¨t Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany b Institut fu¨r Verfahrenstechnik und Dampfkesselwesen, Universita¨t Stuttgart, Stuttgart, Germany Received 22 March 2004; received in revised form 23 August 2004; accepted 2 September 2004
Abstract The consequences of air pollution scenarios caused by road traffic or the impact of exhaust gas reduction techniques are estimated by emission models. To ensure the quality of model results, it is necessary to evaluate the used emission factors under real-world conditions. Therefore, the Institut fu¨r Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe initiated the field campaign BAB II (BundesAutoBahn, Federal motorway). The campaign was conducted in May 2001 with the objective of measuring the traffic emissions at a motorway section and to compare them to modelled emissions. Based on experiences during a precursor campaign (BAB I, 1997), a symmetric experimental set-up was installed which allowed measurements up- and downwind of a motorway nearby Heidelberg, Germany. This paper focuses on the determination of source intensities and emission factors for CO and NOx, whereas other papers in this issue handle VOC and particulate matter. First the basic approach of BAB II measurements up- and downwind of a motorway was approved, showing that it is possible to detect the plume originating from traffic emissions. A case study during a traffic jam illustrates that driving patterns have a strong impact on the emissions and therefore a detailed traffic census is required to obtain reliable emission calculations. Two different strategies were used: (i) long-time measurements during the whole campaign to obtain vertical profiles each 30 min and (ii) measurements during eight special operation periods (SOP) in a higher temporal resolution of 5 min, using instrumentation in elevators. It could be shown that even at a distance of 60–80 m from the motorway the structure of the plume is still inhomogeneous, and concentration changes within short times. The inhomogeneity of the plume not only affects the temporal scale, the spatial scale is also influenced and frequently concentration maxima in higher altitudes are observed. Mean source intensities of 9.5 kg km1 h1 CO and 4.4 kg km1 h1 NOx on working days have been calculated. The ratio of CO/NOx was found to be 2.1 on working days and 4.5 on weekends and holidays. Emission factors of 2.62 g km1 veh1 CO and 1.08 g km1 veh1 NOx have been determined, which agrees well with results derived from similar studies. r 2005 Elsevier Ltd. All rights reserved. Keywords: Source intensity; CO; NOx; Traffic emissions; Emission calculation; Emission model; Plume structure
Corresponding author. Fax: +49 7247 82 4377.
E-mail address: [email protected] (M. Kohler). 1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.088
ARTICLE IN PRESS 5666
M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
1. Introduction 1.1. State of the art Traffic emissions are one of the major sources for anthropogenic air pollution. Carbon monoxide CO and nitrogen oxides NOx are, beside others, important components of these emissions, and therefore it is necessary to improve the knowledge about their quantity and distribution. The estimation of traffic emissions depends strongly on the quality of emission factors used in model calculations. Generally they are acquired by dynamometric test measurements and there is still the demand to compare these factors with those determined under ‘realworld conditions’. Therefore, a couple of field studies had been conducted to evaluate the emission factors for the most important compounds of road-traffic emissions. Most of them are tunnel studies, like those in the Gubrist Tunnel, Switzerland (Staehelin et al., 1995, 1997, 1998; John et al., 1999), the Fort Mc Henry Tunnel of Baltimore Harbor, Maryland, USA (Pierson et al., 1996), in the Taipei Tunnel, Taiwan (Hwa et al., 2002), the Tauern Tunnel in Austria (Schmid et al., 2001), the So¨derleds Tunnel at Stockholm, Sweden (Kristensson et al., 2004) or in the Kiesberg Tunnel at Wiesbaden, Germany (Kurtenbach et al., 2001). Tunnels are an almost closed system with a welldefined mass flow through the tunnel tube but have the disadvantage that the design of the tunnel influences the driving pattern and the airflow through the tunnel. Hucho (1981) and Peters (1990) found that depending on the geometry of vehicles and tunnels the air resistance number, cw, of a vehicle increases by entering a tunnel. The airflow through tunnels, especially through those with one-way tubes, is influenced by the piston effect of moving cars, which was found i.e. at the Gubrist Tunnel (John et al., 1999). This effect causes an artificial tail wind, which leads to a reduced engine power and as a consequence influences the traffic emissions. Another approach to evaluate emission factors is to carry out measurements perpendicular to an open road. The TU¨V Rheinland performed a field campaign at the A3 federal motorway near Siegburg, Germany, with two towers close to the road in up- and downwind directions (Leisen et al., 1992). The position close to the road (8 and 11 m) had the advantage that the vertical extension of the plume did not exceed 20 m, but the contribution of the turbulent flux to the mass flow is not negligible in this case and leads to an uncertainty in the results. This disadvantage could have been only handled by using analysers with high temporal resolution, which were not available. The Institut fu¨r Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe carried out another field campaign in 1997. It took place along the motorway A656 near Heidelberg, Germany
(Vogel et al., 2000). From 10 July to 30 July 1997, continuous ground-based measurements of the major compounds of traffic emissions were conducted. Additional vertical profiles of wind direction, wind speed, temperature and ozone were measured, using a captive balloon system during special operation periods. Parallel, a detailed traffic census was carried out by the Institut fu¨r Energiewirtschaft und Rationelle Energieanwendung (IER), Universita¨t Stuttgart. As a result, a good compliance was found between emissions given by the Umweltbundesamt (UBA) and measured in the field for NOx. In contrast, the CO emissions were found to be up to two times higher than the modelled CO emissions. Emanating from these results and the experiences made in the field campaign in 1997, a second field campaign with enhanced equipment and objectives was carried out in May 2001. 1.2. Method and assumptions The main feature of the study is to perform measurements up- and downwind of a motorway. In the upwind direction of the motorway the background concentration of the trace gases will be measured, while downwind the background concentration in addition to the emissions of the road traffic will be observed. This principle can be expressed by the following equation: Zh Qi ¼
v? ðzÞ ðci ðx2 ; zÞ ci ðx1 ; zÞÞ dz,
(1)
0
where Qi is the source strength, ci is the concentration of the substance i, v? is the wind velocity perpendicular to the road and h is the height of the traffic exhaust gas plume at the position x2. The position x1 is located on the upwind side, the position x2 on the downwind side of the road. To use Eq. (1), a few assumptions have to be made. First, no deposition and no chemical transformations may take place between x1 and x2. Second, v? only depends on the vertical coordinate, and third, the turbulent fluxes are negligible compared to advection. In addition stationary conditions are assumed. Under conditions meeting these assumptions, the source strength of the traffic emissions can be determined by measuring the variables v?(z), ci (x,z) and h. 1.3. Realization and experimental set-up As in 1997, the motorway A656 between Heidelberg and Mannheim, near Heidelberg-Wieblingen (at 10.5 km), was found to be ideal for the field campaign. The motorway runs from northwest (3151) to southeast (1351), which is perpendicular to the main wind direction
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
(Corsmeier et al., 2005a). The four lanes are on an approx. 1 m high, grass-covered embankment and are surrounded by agriculturally used flat terrain. At a distance of 500–1000 m northeast of the motorway there are six small farm houses; to the east a bridge crosses the motorway in a distance of 400 m from the measurement site. The field campaign started on 1 May 2001 and ended on 25 May 2001. The campaign was preceded and followed by a quality assurance programme (Vogt et al., 2005). Within the whole period, eight special operation periods (SOPs), with extended measurement activities were carried out (Corsmeier et al., 2005a). Essential parts of the measurement site were two towers, one on each side of the motorway, both of them 52 m high. They were established 60 m north and 80 m south of the motorway (Corsmeier et al., 2005a). For budget calculations it is necessary to cover the total vertical extension of the plume. On the other hand, the towers had to be built up at a distance far enough from the motorway that turbulence is reduced but not so far that the emissions are overly weakened by dilution. Therefore, the distance of the towers from the motorway had to be optimized to fulfil the guidelines. The symmetric set-up of the measurement site allowed analysing of both main wind directions with northeasterly and southwesterly winds. Meteorological measurements of wind speed, temperature and humidity were conducted at six heights (Fig. 1). The wind
5667
direction was measured at the top of the towers and additionally in four different heights on the towers. For the measurement of CO an Aero Laser AL 5001 and for NOx a Monitor Labs 9841 analyser was used on each side of the motorway. Sophisticated measuring systems were installed on each tower, which allowed measurements at different heights, using only one analyser for each gas. It consists of 10 inlets on each tower at heights of 8, 13, 18, 23, 28, 33, 38, 43, 48 and 51 m above ground level (agl). The inlets were connected through Teflon tubes with the analysers, which were based in a housing at the ground. Bypass pumps provided steady airflow through each tube. A system of valves redirected the airflow from the inlets to the analysers at a specific time schedule. At each height, three 1-min averages were taken, so in total one vertical profile was measured within 30 min. This set-up allowed continuous measurements over the whole time period of the campaign, interrupted only by regular calibration periods. In addition to the long-time measurements additional equipment was used during the eight SOPs. At the towers elevators were installed (Fig. 1), which were running continuously during SOPs, interrupted only for calibration purposes every few hours or in cases of gusty winds. The elevators were equipped with analysers for NO, NO2, O3, CO2, VOCs and particulate matter (Table 1). The results of NO, NO2, O3 and CO2 measurements are the subject of this paper, VOC and
height 51.5 m
elevator
Fig. 1. Sketch of the experimental set-up for gaseous compounds and meteorological measurements at the north tower: temperature T, wet bulb temperature Tf, wind velocity v and wind direction wd. Parameters followed by a quotation mark indicate turbulence measurements.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5668
Table 1 Components, measurement methods, manufacturer and type of analysers used in the elevators Component
Method
Manufacturer
Bendix NO Chemiluminescence with O3 NO2 Liquid chemiluminescence with Luminol—Luminox Scintrex UV absorption Thermo Electron, Horiba O3 O3 Solid chemiluminescence GEFAS CO2 IR absorption Fischer-Rosemount VOC Adsorption on charcoal, CS2 extraction and GC-FID analysis Dra¨ger PM particle number Optical particle counter Grimm PM particle number Electrical low pressure impactor—ELPI Dekati/TSI GmbH PM particle surface Diffusion charger Matter Engineering AG
particle measurements are analysed in other papers of this issue (Petrea et al., 2005; Imhof et al., 2005). The ascent speed of the elevator was about 9 m min1, which meant that an ascent from ground (5 m agl) to the top of the tower (50 m agl) took about 5 min. Thus in one hour 12 profiles (ascents and descents) were taken. The data were collected at a temporal resolution of 3 s, resulting in continuous vertical profiles. Traffic observations were carried out by the IER (Ku¨hlwein and Friedrich, 2005) with detailed speed measurements, license plate observations and traffic census. Analysis of license plate observations supported by the German Kraftfahrzeugbundesamt resulted in detailed information about the composition of the traffic fleet. The number of vehicles on the motorway could be obtained by an automatic traffic monitor in the neighbourhood of the measurement site.
1.4. Period of investigation During the entire investigation period, in total more than 1230 vertical profiles of each, NOx and CO, were measured. Additionally, the use of the elevators during the SOPs produced another 500 profiles. According to the assumptions stated in Section 1.2, limits were defined to filter the data. The profiles were aggregated to 1-hourly means. For further investigation, profiles were selected, where the wind direction was within a sector of 7451, perpendicular to the motorway. This means wind directions within the sectors 01 to 901 and 1801 to 2701 were valid. To select cases with a more homogeneous structure of the plume, it was postulated that the number of vehicles should exceed more than 1000 h1. This was fulfilled between 06:00 CEST and 24:00 CEST on working days and between 08:00 CEST and 24:00 CEST on weekends. Further analysis was restricted to clearly identifiable plumes. Therefore, a concentration difference, averaged over the heights less than 18 m, between up- and downwind side was required, which exceeded 6 ppb for
Type
LMA3, LMA4 TE 49C, APOA 360 OS-2-B BINOS NIOSH 1.108 ELPI DC
NOx and 20 ppb for CO. The number of valid profiles was increased by allowing data even when the wind direction was just beyond the defined wind direction sector or if single profiles just completed a contiguous time period. Applying these restrictions finally resulted in a total number of 280 profiles, out of 1230 from the long-time measurements, for further evaluation. Approximately 60% of the selected profiles were measured with prevailing northerly winds, 40% with southerly winds (Fig. 2). During working days the traffic density is nearly two thirds higher than on weekends. The highest numbers of vehicles were observed during the rush hours in the morning (07:00–08:00 CEST) and in the evening (17:00–18:00 CEST). On weekends and during holidays (1 and 25 May), only one peak of traffic density in the afternoon is observed. The fraction of heavy-duty vehicles (HDV) ranges from 5.5% on working days to 1.1% on Sundays and holidays (Ku¨hlwein and Friedrich, 2004). A particular traffic situation occurred between Saturday, 19 May at 17:00 CEST and Sunday, 20 May 17:00 CEST when the motorway was closed due to the demolition of a bridge nearby the site.
2. Results 2.1. Long-time measurements of NOx and CO As described in the section above it is expected, that in case of a wind direction perpendicular to the motorway, a plume, originating from the traffic on the motorway, will be found at the downwind side. To illustrate the validity of this assumption, a special situation during the campaign can be used. Between 19 May 17:00 CEST and 20 May 17:00 CEST a bridge nearby the measurement site was torn down and the motorway was closed for 24 h. With the closure the
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5669 360
6000
225 180
3000
135
2000
90 1000
45 0
0 1
2
3
4
5
6
7
8
9 360
6000
vehicles h-1
270 4000
225 180
3000
135
2000
90 1000
45 0
0 9
10
11
12
13
14
15
16
17 360
6000
315
5000
270
vehicles h-1
wind direction in degrees
315
5000
4000
225
3000
180 135
2000
90 1000
wind direction in degrees
vehicles h-1
270 4000
wind direction in degrees
315
5000
45
0
0 17
18
19
20
21
22
23
24
25
May 2001
Fig. 2. Wind direction and traffic density during the BAB II campaign. The solid line indicates the number of vehicles per hour on the motorway; dots indicate the wind direction at 51 m agl. Filled dots indicate profiles used for emission calculation. The light shaded areas mark sectors with a wind direction perpendicular to the motorway, dark shaded areas mark time periods with a well-mixed layer between 09:00 and 18:00 CEST.
number of vehicles decreases to zero (Fig. 3). The wind direction was northeast to north and therefore perpendicular to the motorway. As long as there was traffic on the motorway a plume was observed with concentration differences of 100 ppb CO and 10 ppb NOx (Fig. 3). After the road was closed, the same concentration level was monitored on both sides of the motorway. At the end of the closing period a similar process was observed.
One hour before the motorway was opened the concentrations of CO and NOx up- and downwind of the motorway were at the same level; 1 h after the opening a plume was found, which reached a height of approx. 23 m agl. A difference of 40 ppb CO and 10 ppb NOx, at 8 m agl, was observed. It is clearly evident that plumes downwind the motorway originate from traffic emission and can be used to calculate the source
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
3500
May 19/20, 2001
80 vehicles
wd
70
vehicles h-1
3000
wind direction in degrees
5670
60
2500
50
2000
40 1500
30
1000
20
500
10
0
0 18
00
06
12
18
time in CEST 60
60
height in m agl
May, 19 15:00
60 May, 20 17:00
May, 20 19:00
50
50
50
50
40
40
40
40
30
30
30
30
20
20
20
20
10
10
10
10
0
0 100
200
300
0 100
CO in ppb
200
300
0 100
60 May, 19 15:00
200
300
100
CO in ppb
CO in ppb
60
height in m agl
60
May, 19 18:00
60 May, 20 17:00
May, 20 19:00
50
50
50
40
40
40
40
30
30
30
30
20
20
20
20
10
10
10
10
0 0
20 NOX in ppb
40
0 0
20
40
NOX in ppb upwind
300
60
May, 19 18:00
50
0
200
CO in ppb
0 0
20
40
NOX in ppb
0
20
40
NOX in ppb
downwind
Fig. 3. Development of a plume along the motorway on 19 and 20 May, before and after the motorway was closed. Upper part: number of vehicles per hour and wind direction. Middle: CO concentration profiles before (left), during (middle left and middle right) and after (right) the closure of the motorway. Lower part: the same for NOx. Solid lines indicate downwind, dashed lines upwind measurements.
intensities from the measured concentration differences. The formation of the plume is also validated by the results of differential optical absorption spectroscopy
(DOAS) measurements, which were additionally carried out in the first few days of the BAB II campaign (v. Friedeburg et al., 2005; Pundt et al., 2005).
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
Fig. 4 shows the typical development of the plume during a day with a nearly constant wind direction perpendicular to the motorway. On Friday, 11 May 2001 from 10:00 CEST to 18:00 CEST northeasterly winds prevailed (Fig. 2). The number of vehicles on the motorway decreased from 3600 veh h1 at 10:00 CEST to 3200 veh h1 at 11:00 CEST. Afterwards the number of cars increased to 4800 veh h1 during the rush hour at 17:00 CEST (Fig. 2). The wind speed increased from less than 1 m s1 at 10:00 CEST in the morning to 3 m s1 at 8 m agl and 5 m s1 at 51 m agl. At 10:00 CEST large concentration differences were found (16 and 80 ppb, respectively), for both, NOx and CO, coupled with low wind speed. The plume reached a height of about 43 m. During the day the concentration differences varied within the plume from 4 to 16 ppb NOx and from 10 to 80 ppb CO. For NOx two maxima occurred at 12:00 CEST (16 ppb) and at 16:00 CEST (18 ppb), whereas at 13:00 CEST and 16:00 CEST maxima were found for CO (80 ppb). During the day the height of the plume was approx. 18–23 m.
2.2. Special operation periods—SOPs Eight special operation periods (SOPs) were conducted with enhanced instrumentation. For reasons of manpower, the use of additional equipment had to be restricted to time periods when an ideal wind direction perpendicular to the motorway was expected. The main objective of the project was to calculate the air pollutants emitted from the traffic on the motorway. Budget studies require information about vertical distribution of meteorological parameters and at least the vertical extension of the plume or, if available, its vertical profile. Therefore the measurements, which were carried out with the help of elevators up- and downwind of the motorway, were along with long-time measurements a decisive part of the measurement strategy. In Fig. 5a typical vertical profiles for an upwind– downwind situation for one single ascent, started on 14 May 08:07:00 CEST for NO, NO2, O3, CO2 and diffusion charger (DC—measuring the particle surface in mm2 cm3) are depicted. The grey lines are profiles measured on the upwind side of the motorway and the black lines on the downwind side. The concentrations on the upwind side are lower than on the downwind side and almost constant over the measuring height. The increased concentrations on the downwind side were doubtless caused by the exhaust of the traffic on the motorway (Section 2.1), as no other emission source in the near vicinity of the motorway was present. The downwind concentrations decreased with height and reached at approx. 30 m the upwind concentrations. The plume of the motorway has a vertical extension of 30 m with a first maximum at 7 m and a second maximum at
5671
25 m agl. The response times of the analysers were not identical, which can be observed from the vertically shifted profile structure for NO and O3. From correlation calculations with a fast response analyser, it can be determined that the NO analyser e.g. has a time delay of approx. 27 s. The NO data set was shifted 27 s against the rest of the data set. In Fig. 5b five consecutive vertical profiles for NO2 up- and downwind of the motorway are presented (upwind: grey; downwind: black). The upwind profiles are quite constant with height on a level of approx. 20 ppb. The downwind profiles give an idea of the permanently changing concentrations with height as well as with time. The vertical structure and the peak concentrations changed from profile to profile. Even during daytime with relatively constant wind speed and wind direction the downwind concentrations changed a lot. The reason for this behaviour of the air pollutants is that emissions from the motorway are not emitted with a constant flow rate, due to the fact that the vehicle number has temporal fluctuations. Simultaneously, the downwind measurement site is located relatively close to the emission source. As the distance from the emission source to the measurement site becomes longer, the more the high concentrated, inhomogeneously distributed emissions are transformed increasingly into a homogeneous plume. However, due to mechanical and thermal turbulence the vertical extension of the plume increases and so the air pollutants are diluted with cleaner air and thus the difference between up- and downwind concentrations decreases. Neither extensive vertical plumes nor little differences between up- and downwind concentrations are ideal for such a budget study because of the increasing influence of the measurement uncertainty with decreasing difference between up- and downwind concentrations. For these reasons the location of the towers had to be optimized. The inhomogeneous distribution of air pollutants downwind meant that single vertical profiles had to be averaged, similar to the long-time measurements, to mean hourly profiles. In Fig. 5c 12 vertical profiles, collected on 14 May from 08:02:03 to 08:56:59 CEST were combined to mean hourly profiles (solid lines). The original data, sampled every 3 s is marked with grey (upwind) and black (downwind) dots. These mean hourly profiles were used for further calculations. Upwind NO concentrations varied between 1 and 5 ppb, whereas the downwind NO concentrations varied between 1 and 60 ppb. In general, the highest concentrations were measured near ground but there were also profiles with increased concentrations at higher altitudes. Those elevated concentration maxima were observed frequently throughout the entire measurement period. Fig. 6 shows isopleths and wind vectors for 14 May from 07:00 to 18:00 CEST for NOx; white areas indicate
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5672
height in m agl
10
12
14
16
18
50
50
18
45
45
16
40
40
14 12
35
35
30
30
25
25
6
20
20
4
15
15
2
10
10
May 11, 2001 NO X -difference in ppb
10 8
0 -2
10
12
14
16
18
16
18
time in CEST
height in m agl
10
12
14
50
50
45
45
80
40
40
70
35
35
60
30
30
50
25
25
20
20
15
15
10
10
10
-10
10
12
14
16
18
16
18
90 May 11, 2001 CO -difference in ppb
40 30 20
time in CEST 10
12
14
height in m agl
5 45
45
40
40
4
35
35
3.5
30
30
3
25
25
20
20
15
15
1
10
10
0.5
4.5
May 11, 2001 wind speed in ms -1
2.5 2 1.5
0 10
12
14
16
18
time in CEST Fig. 4. Wind direction perpendicular (northerly winds) to the motorway on 11 May 2001. The figure shows the concentration difference for NOx (upper part) and CO (middle) between down- and upwind side of the motorway and additionally the wind speed. The solid line (bold) indicates the height of the plume.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5673
(a)
(b)
(c) Fig. 5. (a) Vertical profiles of two 5 min soundings on 14 May, starting time 08:07:00 CEST. Upwind (grey lines) and downwind (black lines) for NO, NO2, O3, CO2 and diffusion charger (DC). (b) Five consecutive vertical profiles for NO2 from 08:02:03 to 08:21:45 CEST. Upwind (grey lines) and downwind (black lines) of the motorway. (c) Summary of 12 consecutive vertical profiles within one hour of measurements, NO, NO2 and DC upwind (grey dots) and downwind (black dots).
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5674
NOx - south tower (upwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
(a)
12
13
14
15
16
17
18
15
16
17
18
15
16
17
18
time in : CEST NOx - north tower (downwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
(b)
12
13
14
time in : CEST NOx - north tower (downwind) 50
height in m agl
40
30
20
10 7
8
9
10
11
12
13
14
time in : CEST NOx in ppb 0
(c)
10
20
wind speed / wind direction 30
40
50
60
70
< 10 m /s < 5 m /s < 2 m /s
N W
E S
Fig. 6. (a) Isopleths of NOx (3 s values) combined with a wind vector diagram on upwind side on 14 May 07:00 to 18:00 CEST. (b) Isopleths of NOx (3 s values) on downwind side on 14 May 07:00 to 18:00 CEST. (c) Isopleths of NOx (1-h means) on downwind side on 14 May 07:00 to 18:00 CEST.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
instrument calibration periods. Isopleths are a good method of combining a large number of vertical profiles, in order to get an overview over the temporal variation of the vertical distribution of the measured component, in this case the NOx concentration. In Fig. 6b the number of single profiles (in total 106 at each side of the motorway) is visible. The small black dots indicate when and where measurements were available. The interpolation calculation was done using the Krigin method (Zongmin, 1986). The Kriging method was developed for the interpolation of topographical data. This method delivers the most realistic results, for 2D inter- and extrapolation when the results are unequally distributed in the 2D area, by weighting the data points according to the distance from each other. The isolines were smoothed by using the spline method. The database of Fig. 6a and b are the original 3 s values from the single profiles. Fig. 6a is combined with a wind vector diagram. The length of the wind vector depends on the wind speed, and the orientation of the arrow shows the horizontal wind direction. On 14 May, almost throughout the entire observing period, wind from southerly directions prevailed. Thus, the south tower was upwind and the north tower downwind of the motorway. The wind speed increased between 13:00 and 16:00 CEST and caused decreasing concentrations upwind as well as downwind. Within the second calibration period around 16:00 CEST a sudden change in wind direction with high wind speed occurred, caused by a thunderstorm front touching the investigation area. Up- and downwind was changed for about 45 min. The upwind concentrations (Fig. 6a) decreased by the time, but the vertical distribution was always relatively constant. On the downwind side (Fig. 6b), the highest concentrations occurred near ground level but occasionally high concentrations in higher altitudes could be observed. Using the 1-h-averaged values (Fig. 6c), the isopleths are much smoother. The decreasing concentrations all day long can clearly be followed, especially when the wind speed increased, starting on 14:00 CEST. In Fig. 7 the horizontal distribution of the air pollutants (NO, NOx and NO2) on 14 May from 7:00 to 18:00 CEST is depicted. Ground monitoring stations were fixed directly nearby the motorway on the downwind side (no. 2), perpendicular to the motorway at a distance of 60 m (no. 3) and 80 m (no. 5). The values at site no. 1 (upwind) and 4 (downwind) were gained by calculating the 1-h-mean values from the profile measurements whenever the elevators were near ground level between 5 and 9 m above ground. The temporal courses of the NOx concentration at the different sites decreased from 7:00 to 17:00 CEST and increased again from 17:00 to 18:00 CEST. The air masses first passed the upwind side where the lowest concentrations were measured throughout the day. Then they passed the motorway, while loaded with vehicle
5675
emissions, and on the downwind side the highest NOx concentrations were measured directly beside the lanes (no. 2). On their way to the other downwind measurement sites, the NOx concentrations were reduced by dilution with clean air. At site no. 3 and 4 similar NOx concentrations were measured. Just between 12:00 and 15:00 CEST the concentrations at site no. 3 (height of the air inlet 3.5 m above ground) were slightly higher than at site no. 4 (height of air inlet 5 to 9 m). At site no. 5 the concentrations decreased again. The decreasing values of NOx according to the distance from the motorway is only caused by dilution. The temporal course of NO is similar, the highest concentrations are also measured downwind directly beside the lanes (no. 2) and the lowest concentrations on the upwind side (no. 1). Compared to NOx, the NO concentrations are strongly decreasing. NO is mainly emitted from exhaust pipes, for spark ignition engines approx. 95% of NOx is emitted as NO. In the atmosphere NO is transformed in a very quick reaction, following the photo-stationary equation, with O3 to NO2. Thus, NO is decreasing more strongly than NOx, because NO is affected by chemical transformation and by physical dilution, simultaneously. The temporal course for NO2 is different from those for NO and NOx. The lowest concentrations were measured at the upwind side again. On the downwind side, a decrease with distance from the motorway could not be observed. The reasons for this are opposing effects of physical dilution, on the one hand, and the formation of NO2 from NO, on the other hand. These two effects are not temporally constant, but they depend on varying parameters like wind speed, global radiation, temperature, availability of O3, etc. Thus, sometimes the physical dilution predominates and sometimes the NO2 formation and so the site with the highest NO2 concentration changes from time to time. In Fig. 8, mean hourly profiles from the elevator measurements (grey lines: upwind; black lines: downwind), calculated from 12 single profiles (Fig. 5a), are compared to the vertical profiles from the permanent tower measurements (blue lines: upwind; red lines: downwind) and to the values of the ground monitoring stations (red dots: downwind, beside motorway lanes; pink dots: downwind, 60 m from motorway; green dots: downwind, 80 m from motorway). The plume with relatively low and constant concentrations on the upwind side and with increased concentrations near ground level on the downwind side is clearly visible in all the vertical profiles of the measured species. NOx, NO and NO2 measured at the elevator are in good compliance with the tower measurements; the deviation is quite small (upwind: max. 1 ppb; downwind: max. 6 ppb, resp. 12%). Maximum NOx and NO concentrations, measured directly beside the lanes of the motorway (red dots) decreased with distance from
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5676 South Towerr -> upwind
North Tower -> downwind
measurement sites:
1
Motorway A 656
2
4
3
5
1
upwind - elevator 84 m
2
downwind - motorway
3
downwind - 60 m
4
downwind - elevator 60 m
5
downwind - 80 m
10 m 60 m
NO x concentration in ppb
84 m
80 m
150 NO x
2 100
4 50
5
3
1
0 0700
1100
0900
1300
1500
1700
NO concentration in ppb
100
2
NO
75
50
5
25
3
4
1 0 0700
0900
1100
1300
1500
1700
40 NO2 conzentration in ppb
2 30
NO2
3
5
20
10
0 0700
4
1
0900
1100 1300 time in CEST
1500
1700
Fig. 7. Horizontal distribution of NOx, NO and NO2 upwind (84 m south) and downwind (0, 60, and 80 m north) the motorway on 14 May 07:00 to 18:00 CEST.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5677
PM10: 0.3 - 10µm
height in m agl
45
35
25
15
5 0
50
100
150 0
25 50 75 100 0
10 20 30 40 50
NO conc. in ppb
NOx conc. in ppb
0
40,000
80,000 0
200
400
DC - particle surface PM10 2 -3 particle number/liter in m cm
NO2 conc. in ppb
elevator measurements:
WD = 183 degree
downwind - north (60m)
45
upwind - south (80m)
height in m agl
35 tower measurements: downwind - north (60m) upwind - south (80m)
25
ground stations: BUGH (10m north)
15
FFA (60m north) BASF (80m north)
5 400
410
CO2 conc. in ppm
420 0
20
O3 conc. in ppb
40 17,2
17,4
17,6 0
temperature in ˚C
2
4
wind speed in m/s
Fig. 8. Comparison of different measurements. Database: hourly mean values on 14 May 07:00 to 08:00 CEST. Elevator measurements (grey lines: upwind; black lines: downwind), tower measurements (blue lines: upwind; red lines: downwind) and ground monitoring stations (red dots: downwind beside motorway; pink dots: downwind, 60 m from motorway; green dots: downwind, 80 m from motorway).
the motorway (pink dots: 60 m; and green dots: 80 m). The concentrations measured at the ground at the monitoring station (pink dots) and the concentrations at the elevator (black line) are also in good agreement (max. 4 ppb, resp. 14%).
2.3. Emission calculation The considerations stated above, linked to the assumptions of Section 1.2 and measurement data for the variables given in Eq. (1) allow calculating the source strength of traffic emissions on the motorway.
2.3.1. Long-time measurements The plume is divided into layers of constant thickness, each with constant concentration and constant wind speed. Each layer was centred on the measuring height, in which gas concentrations had been measured. The thickness of each layer is 5 m. To close the gap between the lowest measuring level (8 m) and the ground, it has been assumed that the concentration profile between the lowest level and the ground is constant. Measurements made near the base of the north tower (position no. 3 in Fig. 7) indicate the validity of this assumption, whereas linear extrapolation of the concentration profile would have led in some cases to unrealistically high
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
source strength in kg km-1 h-1
concentrations at the ground level. To derive the wind speed in the calculation heights, a logarithmic fit for each measured wind profile was determined. Taking into account that the wind direction is not always perpendicular to the motorway, only the component of the wind speed perpendicular to the motorway has been used for further calculations. The particular height of the plume has been derived from the vertical profile of the gaseous compounds. Using these assumptions, the source intensities along the motorway were determined. To assure that rapidly changing stratification of the atmosphere has no influence on the results, only profiles between 09:00 CEST and 18:00 CEST, when a well-mixed layer within the surface layer has developed, have been analysed. Depending on the traffic density and the instantaneous composition of the traffic fleet, the emissions are not homogeneous. The inhomogeneous structure of the emission source in combination with turbulence leads to an inhomogeneous structure of the plume as well (Fig. 6b). This effect does not allow the calculation of source intensities with high temporal resolution, but it is possible to determine reliable long-time averages. Fig. 9 shows the mean diurnal course of calculated CO and NOx emissions during working days. The number of values averaged for each 1 h interval differs from 6 to 13 and therefore the statistical significance of the results differs, but nevertheless the daily course of the emissions shows a good compliance with traffic census data, with high values in the morning and in the evening during rush hours. On midday, between 13:00 and 14:00 CEST, a secondary maximum is found for CO, which is influenced by the small number of valid profiles and the high variability of emissions indicated by the standard deviation in Fig. 9. For NOx, which is
mainly emitted by diesel motors, a different diurnal course is found. Diesel motors are mainly used by the heavy-duty fraction of the traffic, and in comparison by only a small number of passenger cars. Therefore, the mean daily course of NOx is strongly influenced by emissions of HDV. Passenger cars influence the NOx emissions during the rush hours, purely because of the large number of vehicles during that time. Due to this fact, the ratio of CO/NOx is an indicator for the composition of the traffic fleet. Between 11:00 and 15:00 CEST, when the fraction of heavy-duty traffic is higher than 7%, the ratio of CO/NOx (Fig. 10) is reduced to numbers o2, the ratio rises up to values 42 when the fraction of heavy-duty traffic is lower than 7%. Analysis of the measured data leads to a mean weekly course of the source strength of CO and NOx as shown in Fig. 11. Between Tuesday and Thursday, a mean source strength of 9.1 kg km1 h1 CO was observed. On Monday (9.8 kg km1 h1) and Friday (9.7 kg km1 h1), slightly increased emissions are determined, due to the more intensive traffic on the motorway. The source strength for NOx shows a similar performance throughout the week with the lowest amount between Tuesday and Thursday (4.0 kg km1 h1) and higher values on Monday (5.0 kg km1 h1) and Friday (4.7 kg km1 h1). The CO/NOx ratio is found during working days to be 2.1. A different performance is observed during the weekend (Saturday and Sunday). A high CO source strength is calculated, even higher than during the week (10.3 and 9.8 kg km1 h1), while a reduced NOx value is observed (2.5 kg km1 h1 and 2.0 kg km1 h1). The data for the weekend should be considered with caution because only a few hours were available for analysis, so they are not as representative for a daily mean value as the calculations made for
30
4 800
25
4 000
20
3 200
15
2 400
10
1 600
5
number of vehicles h-1
5678
800
0
0 7
8
9
10
11 CO
12
13
14 15 16 time in CEST
NOX
LDV
17
18
19
20
21
22
HDV
Fig. 9. Mean daily course of the source strength for CO and NOx combined with the mean daily course for passenger cars and heavyduty traffic during working days.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5679
4.0
16 HDV
3.5
14
3.0
12
2.5
10
2.0
8
1.5
6
1.0
4
0.5
2
0.0
HDV in %
CO/NOX
CO/NOX
0 7
8
9
10
11
12
13
14 15 16 time in CEST
17
18
19
20
21
22
Fig. 10. Mean daily course of the ratio CO/NOx combined with the mean daily course of the heavy-duty fraction on the motorway during working days.
12
10.3
11 source strength in kg km-1 h-1
10
9.8
9.7
9.8 9.1
9 8 7 6
5.0
4.7
5 4
2.5
3 2
4.9 4.1
4.0 2.3
2.0
2.1
2.0
1 0 Mo
Tu-Th CO
Fr NOX
Sa
Su
CO/NOX
Fig. 11. Mean source intensities of CO, NOx as well as CO/NOx ratios averaged over the time period of the BAB II campaign.
working days. Yet even then it can be stated that due to the predominant fraction of passenger cars and the reduced fraction of the heavy-duty traffic the NOx emissions are reduced and therefore the CO/NOx ratio rises up to 4. Compared to other studies, mainly conducted at tunnel sites, similar ratios of CO/NOx have been found. All studies differ in traffic fleet composition, in vehicle driving pattern, in road grades and in observed time periods (daytime and week day), but even then similar ratios occur. At the Taipei Tunnel (Hwa et al., 2002) the experiment was conducted during a Saturday. CO/NOx ratios of 4 were found compared to 4.1 at the BAB 656 site. At
the Gubrist Tunnel, Staehelin et al. (1997) found a ratio of 1.6, which is a consequence of the higher heavy-duty fraction (9.5%) of the traffic in the tunnel compared to the BAB II site (5.5%). The Tauerntunnel experiment (Schmid et al., 2001) shows much lower values for CO/ NOx ratios (0.81), which is also due to a much higher percentage of HDV (18% on work days, 3% on weekends) using the transit route. BAB I (Vogel et al., 2000) gives a snap-shot of emission strength of a few hours, but the high values found for CO (17–32 kg km1 h1) are supposed to be due to the instantaneous composition of the traffic fleet on the motorway. Those values found in 1997 are still within the spread of the measurements of 2001.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5680
Using data from traffic census, it is simple to calculate emission factors for both gaseous compounds CO and NOx. To derive emission factors, it is necessary to normalize the source strength by the number of cars on the motorway. For CO a mean emission factor of 2.62 g km1 veh1 and for NOx 1.08 g km1 veh1 was found. By means of multiple regression, it is possible to split the emission factor in fractions of light-duty vehicles (LDV) and HDV. For NOx an emission factor of 0.68 g km1 veh1 for LDV and 6.86 g km1 veh1 for HDV was determined (s. Fig. 12). For CO it was not possible to split the emission factors between LDV and HDV, which is due to the fact, that those emission factors are quite similar and cannot be clearly separated by the means of multiple regression if the fraction of HDV is very low. The results are in good agreement to those from other studies (Table 2), especially to those made in the Gubrist Tunnel 2000 (Steinemann and Zumsteg, 2003). Differences to older studies (Kristensson et al., 2004) can be explained by use of different exhaust gas reduction techniques in the respective traffic fleet. On the motorway at the measuring site a speed limit of 120 km h1 is given. Speed measurements show that the averaged speed of LDV amounts approx. 113 km h1 for HDV a mean speed of 86 km h1 is found (Ku¨hlwein and Friedrich, 2005). A quite different situation is found during traffic jams. In such a case, the vehicle speed is lower than 30 km h1, and this driving pattern leads to increasing emissions (Ku¨hlwein and Friedrich, 2005).
The effect on the CO source strength is shown in Fig. 13. During the BAB II campaign, two traffic jams occurred with wind directions perpendicular to the motorway. According to traffic census, traffic jams were observed on 14 May between 9:00 CEST and 10:22 CEST, and on 23 May between 7:15 CEST and 10:20 CEST. On 14 May, southerly winds prevailed, while on 23 May, north to northeasterly winds were found (Fig. 2). The source intensity for CO increases remarkably during traffic jams, compared to the same time period of the same day of the week but with no traffic jam. The increase of CO concentration is also remarkable, considering that traffic density during the jam remains constant compared to the hours before. This fact underlines the strong impact of the driving pattern on the traffic emissions and illustrates the need of detailed and sophisticated traffic census as conducted during BAB II.
2.3.2. Special observation periods (SOPs) For SOPs two different data sets for vertical profiles of NOx, one set from tower measurements and one set from elevator measurements, were available. Despite the different concepts for elevator measurements and tower measurements, Eq. (1) is applied for both emission calculations. The NOx emissions calculated with both data sets during the SOPs are compared to each other in Fig. 14. During all SOPs, 24 1-h-mean profiles could be calculated from the elevator data overall. For 15 of these 1-h means, concentration measurements from the tower
9
8
6.86
6
-1
emission factor in g km veh
-1
7
5
4
2.62 3
2
1.08 0.68 1
0 CO CO
NOX NO X
NOXDH ; LD
NOX;DHD
Fig. 12. Mean emission factors for CO, NOx as well as LDV and HDV NOx emission factors averaged over the time period of the BAB II campaign. The error bars indicate the standard error.
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5681
Table 2 Emission factors derived from different tunnel studies Study/site Taipai, 2000 Gubrist, 1993
Gubrist, 1997
Gubrist, 2000
Gubrist, 2002
Fort McHenry, 1992
So¨derledstunnel, So¨derledstunnel, So¨derledstunnel, So¨derledstunnel,
Winter 1993 Autumn 1994 Winter 1995/1996 Winter 1998/1999
BAB656, 2001
All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles LDV HDV All vehicles All vehicles All vehicles All vehicles All vehicles LDV HDV All vehicles LDV HDV
CO (g km1 veh1)
NOx (g km1 veh1)
3.6470.26 4.1870.38 1.1872.33 3.8970.64 1.9–4.4 1.9–7.5 3.41 1.6–4.0 o3.7 2.72 0.8–2.0 0.5–1.5 1.35 3.9570.34 6.1171.75 4.63 6.270.2 6.370.1 5.370.1 5.2770.1
0.970.18 1.0570.09 15.5670.79 2.4571.59 0.61.8 1018 1.99 0.31.0 1014 0.94 0.20.45 815 1.21 0.570.06 8.9770.28 2.0670.48 1.370.1 1.170.1 1.570.1 1.3670.03 1.0770.03 8.070.8 1.0870.05 0.6870.12 6.8671.57
2.6270.13
Hwa et al. (2002) Staehelin et al. (1997)
Steinemann and Zumsteg (2003)
Pierson et al. (1996)
Kristensson et al. (2004)
This paper
20 no jam jam
traffic jam
CO source strength in kg km
-1
h
-1
18 16
traffic jam
14 12 10 8 6 4 2 0 9
10 May 14, 2001
11
12
7 time in CEST
8
9
10
11
May 23, 2001
Fig. 13. Comparison between traffic jams and undisturbed traffic situations. Light grey bars indicate source intensities during the two traffic jam situations on 14 and 23 May. Dark grey bars indicate typical CO source intensities during corresponding working days without traffic jam.
were available simultaneously. Take note that the 24 1h-mean profiles do not represent a daily cycle. For some hours almost similar results were found but for other hours quite large differences between the emissions
calculated by different measurement strategies exist. Some of these discrepancies are explainable. At value no. 2 (between 06:00 and 07:00 CEST) the elevator method’s emissions are approx. three times as high as
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
5682 8.000
-1
NOx emissions in g*km *h
-1
NOx emissions elevator method NOx emissions tower method
6.000
4.000
2.000
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
hours
Fig. 14. Comparison of emission data from elevator measurements (red bars) and tower measurements (blue bars). Database: hourly mean values. In total 24 not interrelated 1-h-mean values throughout the total measurement period are given, which do not characterize a daily cycle.
the tower method’s emissions. This result was due to very low downwind concentrations (NOx: approx. 25 ppb) from 06:00 to 06:30 CEST, followed by a strong increase in the concentrations (NOx: approx. 90 ppb), which occurred due to the start of the morning rush hour. The tower measurements started at a height of 8 m when the downwind–upwind difference was little. After 06:30, when the upwind–downwind difference in the plume of the motorway strongly increased, the tower profiling measurements were done at higher altitudes and thus the plume could not correctly be covered by the tower measurements. This illustrates why the calculation of emissions using the tower measurements were temporally restricted to times where no rapid changes either in stratification or in emission sources occurred. However, not all differences in the results of the emission calculations with the two methods could be explained like that. In general, the NOx emissions of the elevator measurement method were 25% higher as calculated from the long-time measurements.
3. Conclusions Based on the experiences during the precursor campaign BAB I in 1997, BAB II was conducted in May 2001 with an enhanced experimental set-up. Using the gaseous compounds CO and NOx as an example, it could be shown that simultaneous measurements upand downwind of a motorway are practical to calculate the source intensity of any compound (Fig. 3). In cases of wind directions perpendicular to the motorway, the
plume caused by the road traffic can be clearly identified downwind of the motorway. Despite the fact that the small number of valid measurements does not allow a detailed interpretation, a plausible explanation for the diurnal course of both gaseous compounds can be found. The mean diurnal course of CO source intensities follows the course of the number of vehicles on the motorway, showing peaks during the rush hours and minima at midday. NOx source intensities are coupled with the heavy-duty fraction of the traffic; therefore, the mean daily course is influenced by the large number of vehicles during the rush hours and by the maximum of heavy-duty traffic at midday (Figs. 9 and 10). The impact of driving patterns on the emission can be shown by a comparison between traffic jam situations and flowing traffic. During traffic jams, when the averaged speed of the vehicles is lower than 30 km h1, a strong increase of the CO source strength is observed (50–100%), even if the number of vehicles remains constant or decreases (Fig. 13). This gives reasons to keep the traffic flowing by structural measures on the road network as well as by speed limit restrictions. It is possible to determine mean weekly source intensities for CO and NOx. Mean source intensities for CO of 9.5 kg km1 h1 on working days and 10.2 kg km1 h1 on weekends were found. In contradiction, NOx emissions, which are strongly influenced by heavy-duty traffic, show a stronger variability with approx. 4.4 kg km1 h1 during working days and 2.4 kg km1 h1 NOx on the weekends, when there is no heavy-duty traffic allowed on German motorways. The use of elevator measurements during the SOPs gives a new insight in the structure of the plume deriving
ARTICLE IN PRESS M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
from the vehicles on the motorway. The plume is even inhomogeneous at a distance of 60–80 m, because of the inhomogeneous emissions on the motorway. To handle this problem and to determine an emission data set in a high temporal resolution, it would have been necessary to extend the elevator measurements over the whole investigation period. This was not practical because of the need of intensive manpower. Another finding was that frequently concentration maxima in higher altitudes on the downwind side were observed. This fact underlines the imperative to cover the whole extent of the plume in a high spatial resolution, as it was done by both measurement strategies used. Each strategy has its own advantages and drawbacks, which make it difficult to compare the results of both. Unfortunately, there are only a small number of measurements that are covered by both methods (Fig. 14). Elevator measurements have a high temporal and spatial resolution, but the measurements are instantaneous and only a small time period of measurements is available. The tower measurements are not so instantaneous because the concentrations of the gaseous compounds are averaged over 3 min, and these measurements cover a longer time period than the elevator measurements. The disadvantage of tower measurements is the time span between the first and the last level of a profile. Therefore, times of fast changing stratification have to be excluded from analyses to handle this fact, which reduces the number of available measurements for analysis. Nevertheless, the long-time tower measurements allow the calculation of mean emissions on the motorway. The results of BAB II compared to similar studies, which have been mainly conducted in tunnels, show a good agreement for the CO/NOx ratios and emission factors. Emission factors of 2.62 for CO and 1.08 g km1 veh1 for NOx were determined. Further analysis of the database gained under these considerations allows the comparison of measured source intensities and emission factors with those calculated by model means (Corsmeier et al., 2005b).
Acknowledgements The BAB II project was funded by the Forschungszentrum Karlsruhe, Germany and the Umweltbundesamt (UBA), Berlin, Germany, under contract number 299-42-250. The authors would especially like to thank the participating scientists and technicians for their active participation and intense collaboration.
References Corsmeier, U., Kohler, M., Vogel, B., Vogel, H., Fiedler, F., 2005a. BAB II—a project to evaluate the accuracy of real
5683
world traffic emission for a motorway. Atmospheric Environment this issue. Corsmeier, U., Ku¨hlwein, J., Vogel, B., Kohler, M., Vogel, H., 2005b. BAB II: comparison of measured and model calculated real world traffic emissions of gaseous and particulate compounds. Atmospheric Environment this issue. v. Friedeburg, Ch., Pundt, I., Mettendorf, K.-U., Wagner, T., Platt, U., 2005. Multi-AXis-(MAX) DOAS measurements of NO2 during the BAB II motorway emission campaign. Atmospheric Environment 39, 977–985. Hucho, W.-H., 1981. Aerodynamik des Automobils. Vogel, Wu¨rzburg. Hwa, M.-Y., Hsieh, Ch.-Ch., Wu, T.-Ch., Chang, L.-F.W., 2002. Real-world vehicle emissions and VOCs profile in the Taipei Tunnel located at Taiwan Taipei area. Atmospheric Environment 36, 1993–2002. Imhof, D., Weingartner, E., Vogt, U., Dreiseidler, A., Rosenbohm, E., Scheer, V., Vogt, R., Nielsen, O.J., Kurtenbach, R., Corsmeier, U., Kohler, M., Baltensperger, U., 2005. Vertical distribution of aerosol particles and NOx close to a motorway. Atmospheric Environment this issue. John, Ch., Friedrich, R., Staehelin, J., Schla¨pfer, K., Stahel, W.A., 1999. Comparison of emission factors for road traffic from a tunnel study (Gubrist tunnel, Switzerland) and from emission modeling. Atmospheric Environment 33, 3367–3376. Kristensson, A., Johansson, Ch., Westerholm, R., Swietlicki, E., Gidhagen, L., Wideqvist, U., Vesely, V., 2004. Realworld traffic emission factors of gases and particles measured in a road tunnel in Stockholm, Sweden. Atmospheric Environment 38, 657–673. Ku¨hlwein, J., Friedrich, R., 2005. Traffic measurements and high-performance modelling of motorway emission rates. Atmospheric Environment this issue. Kurtenbach, R., Becker, K.H., Gomes, J.A.G., Kleffmann, J., Lo¨rzer, J.C., Spittler, M., Wiesen, P., Ackermann, R., Geyer, A., Platt, U., 2001. Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel. Atmospheric Environment 35, 3385–3394. Leisen, P., Mu¨ller, W.-R., Heich, H.-J., Hasselbach, W., Mu¨ller, J., Waldeyer, H., 1992. Entwicklung der Abgasbelastung an Autobahnen. Umweltbundesamt, Berlin (UFOPLAN-Nr. 10402585). Peters, J.-L., 1990. Bestimmung des aerodynamischen Widerstands des ICE/V im Tunnel und auf freier Strecke durch Auslaufversuche. Eisenbahntechnische Rundschau 39, 559–564. Petrea, M., Kurtenbach, R., Wiesen, P., Vogt, U., Baumbach, G., Fuchs, J., Jaeschke, W., 2005. NMVOC measurements of motorway emissions during the BAB II campaign. Atmospheric Environment this issue. Pierson, W.R., Gertler, A.W., Robinson, N.F., Sagebiel, J.C., Zielinska, B., Bishop, G.A., Stedman, D.H., Zweidinger, R.B., Ray, W.D., 1996. Real-world automotive emissions— summary of studies in the Fort McHenry and Tuscarora mountain tunnel. Atmospheric Environment 30, 2233–2256. Pundt, I., Mettendorf, K.-U., Laepple, T., Knab, V., Xie, P., Lo¨sch, J., v. Friedeburg, Ch., Platt, U., Wagner, T., 2005. Measurements of trace gas distributions by Long-path DOAS-tomography during the motorway campaign
ARTICLE IN PRESS 5684
M. Kohler et al. / Atmospheric Environment 39 (2005) 5665–5684
BAB II: experimental setup and results for NO2. Atmospheric Environment 39, 967–975. Schmid, H., Pucher, E., Ellinger, R., Biebl, P., Puxbaum, H., 2001. Decadal reductions of traffic emissions on a transit route in Austria—results of the Tauerntunnel experiment 1997. Atmospheric Environment 35, 3585–3593. Staehelin, J., Schla¨pfer, K., Bu¨rgin, T., Steinemann, U., Schneider, S., Brunner, D., Ba¨umle, M., Meier, M., Zahner, C., Keiser, S., Stahel, W., Keller, C., 1995. Emission factors from road traffic from a tunnel study (Gubrist tunnel, Switzerland). Part I: concept and first results. Science and Total Environment 169, 141–147. Staehelin, J., Keller, C., Stahel, W.A., Schla¨pfer, K., Steinemann, U., Burgin, T., Schneider, S., 1997. Modelling emission factors of road traffic from a tunnel study. Environmetrics 8, 219–239. Staehelin, J., Keller, C., Stahel, W., Schla¨pfer, K., Wunderli, S., 1998. Emission factors from road traffic from a tunnel study (Gubrist Tunnel, Switzerland) Part III results of organic
compounds, SO2 and speciation of organic exhaust emission. Atmospheric Environment 32, 999–1009. Steinemann, U., Zumsteg, F. (Eds.), 2003. Verkehrs- und Schadstoffmessungen 2002 im Gubristtunnel. Ostluft, Amt fu¨r Abfall, Wasser, Energie und Luft des Kantons Zu¨rich (AWEL), Auftragsnummer US 89–16, www. ostluft.ch. Vogel, B., Corsmeier, U., Vogel, H., Fiedler, F., Ku¨hlwein, J., Friedrich, R., Obermeier, A., Weppner, J., Kalthoff, N., Ba¨umer, D., Bitzer, A., Jay, K., 2000. Comparison of measured and calculated motorway emission data. Atmospheric Environment 34, 2437–2450. Vogt, U., Baumbach, G., Dreiseidler, A., 2005. The BAB II quality assurance program for measurements of gaseous emissions and particulate matter. Atmospheric Environment this issue. Zongmin, W., 1986. Die Kriging-Methode zur Lo¨sung mehrdimensionaler Interpolationsprobleme. University Dissertation, Go¨ttingen, (1986/4452).