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Determination of the trend of highway emissions by means of emission balance measurements
The Science of the Total Environment, 93 (1990) 339-348
339
Elsevier
DETERMINATION OF THE TREND OF HIGHWAY EMISSIONS BY MEANS OF EMISSION BALANCE MEASUREMENTS P. Leisen Technischer 0berwachungsverein Rheinland, Postfach 10 17 50, D-5000 Cologne, Federal Republic of Germany SUMMARY The nitrogen oxide emissions of a motorway are determined by means of an immission measurement. The total emission mass flow of the motorway is determined from the latter in the form of a balance measurement. Vehicle-specific emission factors for nitrogen oxides are then determined by statistical methods from this total emission and the simultaneously measured traffic parameters and the trend in the emissions is plotted on a time basis. TERMS OF REFERENCE Certain substances emitted by motor vehicles are governed by statute. In the case of private cars, the components carbon monoxide, nitrogen oxides and total hydrocarbons have already been subject to maximum emission values for quite some time. In the European exhaust gas legislation a compromise has been reached in that different maximum value requirements will be introduced in stages for three different cubic capacity classes. Exhaust gas reduction systems will be offered which just satisfy the generous maximum values applying in some cases, or else have far lower emissions (e.g. controlled three-way-catalyst for vehicles under 1.4 I cubic capacity). The vehicle population on our roads is broken down into a fairly small number of these "low-pollutant" vehicles together with the vehicles without special pollutant-reducing equipment. Some of the more recent pollutant-reducing systems have different effects for different speed ranges. Moreover, there is still little experience of the ageing behaviour. The real, representative emission behaviour cannot therefore be extrapolated by means of the maximum values or the type test values. In order to make reliable emission or immission calculations or to monitor the effectiveness of the statutory measures, however, the actual emissions have to be known. Comprehensive field tests on roller-type test stands are very cost- and time-intensive (refs 1-2). They are usually conducted on only a small vehicle population and are based on driving patterns which in some cases do not adequately reflect th traffic conditions on the roads. There are currently for commercial vehicle traffic no emission factors which have been determined in an analogous manner on roller type test stands. They have been calculated to date from stationary engine performance graphs (ref. 3). These tests show that with nitrogen oxides in particular, commercial vehicles account for a considerable share of the total emissions of road traffic. Their relative share will rise still further with the progressive reduction in exhaust gases from private cars. Commercial vehicle traffic already accounts for more than half the nitrogen oxide emissions on some German motorways. In order to monitor the effect of statutory measures and to improve the emission behaviour in actual driving conditions, the emissions should as far as possible also be determined under actual driving conditions. This is not possible given the large number of individual vehicles and also not
0048-9697/90/$03.50
© 1990 Elsevier Science Publishers B.V.
340
practicable in the case of a few vehicles. An emission balance measurement enables the total emission mass flow of a road to be recorded. It can then be used to monitor the variations in time of the emissions on an actual road. The mass flow then depends on the traffic intensities, the traffic breakdown and the vehicle speeds. If a fundamental judgement is to be made, the crucial element is not the variation in the emissions of a specific road, but the specific emission variation per vehicle. In order to calculate these values from the total emission mass flow, as accurate data as possible on the afore-mentioned traffic parameters is required. Specific emission factors can then be determined for private and commercial vehicles by means of statistical methods. The measurement cross-section on Federal Autobahn 3 (BAB 3) has a mean daily traffic intensity of approx. 45 000 vehicles/day. This high traffic intensity should help to ensure an overall representative vehicle population, particularly as a high volume of long-distance traffic is encountered on this motorway, which would balance out any local, unrepresentative factors. The measurement produces a mean emission factor, which is representative of the vehicle population typical of the motorway and the speed range encountered there. The emmission factor thus determined for private cars is therefore an integral value for the vehicle population typical of the motorway. A breakdown within the private car category, e.g. diesel- or petrol-driven vehicles, catalyst vehicles, c.c.classes, is not possible, since the traffic measurement cannot distinguish between these criteria. METHODOLOGY The basic concept of emission balancing by means of immission measurements consist in combining the vehicular traffic on the motorway as a source and determining the total pollutant mass flow by measurement techniques. To this end a suitable boundary for the mass balance is placed around the motorway. All pollutant mass flows beyond the balance limits are determined. Balancings of this kind were described in 1980 by BULLIN et.al. (ref. 4). The shortcomings of this investigation can however be avoided by an appropriate measurement act-up and careful evaluation of the result. Pollutant balancings on roadways have also been carried out by the Agricultural college (Landbauhochschule) in Wageningen (Netherlands) (ref. 5). Fig. 1 illustrates the principle of the emission balance measurement. The difference between the pollutant mass flows leaving the balance envelope (O) and those entering with the basic load (I) represents the emission mass flow of the traffic. The mass flow is therefore the product of mass concentration and air volume flow. See more detailed papers (refs 6,7) for a more precise description of the method and the measurement set-up. In order to determine the mass flow, the wind vector must be measured at the respective measuring point in addition to the pollutant concentration. The mass flow is then the integral of the partial mass flows formed over the full height of the exhaust gas cloud leaving the motorway: E = C(z) Ux(z) dz
(1)
In this equation the source intensity E represents the emission mass flow per unit of time and longitudinal section of the source with the dimension kg/(km*h). The integration must extend vertically over the whole exhaust gas cloud (z=H). The concentrations C and Ux (wind velocity at right angles to the motorway) are functions of the height z. To ensure correct balancing, no mass flow may leave the balancing limit at the top. This balance limit must therefore be sufficiently high to ensure
341
H
E = ~t N O x , o -
~t = J" U . C . d F
~ NOx,I
o Balance - Assumption
NO T r a n s p o r t
t
Z=H
n~ N O x , u p p a r
trough upper boundary
t c i (z:
+-S-
I I
Fig. 1. Diagram
I I I
-1 I I
I I
ENOx
I
|
, _~
I.e, C o ( H ) - CI(H)
I ~t N O x , O . Out '
|.~ N O x , l ~ ' In w
J
- 0
I
i~l
~i ~_
,
of the emission balancing
that the exhaust gas cloud remains below the highest measuring point of the lee-side measuring point network. In certain meteorological conditions the balancing requirements are not fulfilled, so that the measured values may not be used for the balancing. The follwing cases are are mainly involved here: Parallel winds prohibit balancing, since there are no pronounced winds at right angles to the balance limit and pollutants are clearly carried upwards through the balance envelope. In calm or in slightly windy weather, pollutants can likewise be carried upwards through the balance envelope. If there are reversals of wind direction during the averaging period, pollutants can be carried over both lateral balance limits or in the direction of the motorway, despite the fact that the mean wind direction is at right angles. MEASUREMENT SET-UP The measurements were carried out at the measurement station on BAB 3 which had been erected for the large-scale exhaust gas experiment (ref. 2). It had been shown that reliable results, compared with those of the test stand measurements, could be achieved at this station with this measurement system. The measurement cross-section lies on BAB 3 at the 30.25 km mark between the junctions Siegburg/Hennef and Siebengebirge. The motorway here runs through a slightly hilly region, 180 m amsl, almost directly north-south. The measurement cross-section lies roughly in the centre of an approx. 1 km long, straight section of motorway, which climbs slightly (approx. 1%) in the direction of Frankfurt. The measurement of pollutants and wind vector took place at the measuring heights: 1.0, 3.0, 5.5, 10.0 and 17.0 m; a further pollutant measurement was made at a height of 26,5 m. The measurement programme was subsidised by funds from the German Federal Environment Agency (Umweltbundssamt), Berlin.
342
Fig. 2. Picture of the immission measurement station on BAB 3 MEASUREMENT RESULTS The measurements started in February 1987 and have not yet been completed. Only provisional results can therefore be reported here. The following analyses refer to the period from 1st April 1987 to 31st March 1989. Fig. 3 shows the mean daily rates for the total nitrogen oxide emission (dashed line with triangles) and the total traffic intensity. The first third of the figure shows daily curves for
343
Saturdays, the second third of the figure those for Sundays and the last third those for weekdays. It is seen that the emissions for the individual kind of day are very similar. The daily maximum is slightly higher on weekdays than on the other days. On sunday the emission suddenly rises steeply from approx. 22.00 hrs onwards. The traffic intensity, on the other hand, shows by far the highest peak rates on Sundays. It can be concluded from the relationship between the two curves that far more vehicles are required for the same emission on Sundays than on a weekday. With the selected Soturday Sundoy Weekdoy
,oo_ 500
5O00
/
4000'
300
!IZ
' 200 ..~
'I00 ~E 8 0
0.00
12 I.m.
0.00
12 a.m.
0.00
12 a.m.
12 ,.m.
day/time of day Fig. 3. Mean daily rates of the total NO=emission (dashed line with triangles) and the traffic intensitiy for 3 different kinds of day. Left-hand third of figure: Saturday centre: Sunday right: weekday Sunday
Saturday
Weekday
20
so .~
15'
10
l
20 E
o 12 o.m. 12 ~.m. day/tlme of day Fig. 4. Mean daily rates of the total NOxemission per vehicle (dashed line with triangles) and the lorry share for 3 different kinds of day. Left-hand third of figure: Saturday centre: Sunday right: weekday 0
~.00
12 a.m.
0.00
12 .m.
0.00
344
scale the traffic intensity curve generally runs above the emission curve. Only during the night hours on weekdays and on Sundays from 22.00 hrs onwards does the emission predominate. The reason for these peculiarities can be seen from the following Fig. 4. Here are plotted in an analogue graph the mean daily rates for the specific NO Xemission per vehicle and the share of commercial vehicles in the traffic load (share of lorries). A mainly parallel pattern for both curves is obtained. At periods with high lorry share the specific emissions are also high. This correlation becomes particularly clear from the mean daily rates for Sundays (centre) and weekdays. On Sunday the share of lorries is particularly low in the clay-time hours. At these times the emission per vehicle is also particularly low. At 22.00 hrs the motorway is again open to heavy traffic and the share of lorries increases sharply. At the same time the specific emission also rises. On weekdays the share of lorries and the specific emission are correspondingly higher in the day-time hours than on Saturdays and Sundays. In the night hours on weekdays the values are comparatively high for both variables. The parallel shape of both curves shows that the commercial vehicle traffic has a considerable influence on the NOx emission rate. The total emission of the motorway is composed of the shares of private car traffic (PC) and commercial vehicle traffic (lorries) added together: E(NOx) = E (PC) + E (LORR) = e (PC) N (PC) + e (LORR) N (LORR)
(2)
The emission of each of these two types of vehicle is calculated here as the product of the number of vehicles and a mean emission factor (emission per vehicle). In the case of private cars the emission is a function of the driving speed. This is known from test-bed tests and can be confirmed from the data determined here. With lorries there is only a slight variation in speed, so that a constant emission factor can be employed here.
Sunday
Saturday
Weekday lOO
140
30'
120
m 7o
~1110'
8O
1CO
0.00
i
12 a.rn.
0.00
12 a.rn.
0.00
12 a.rn.
12 p.m.
day/time of day Fig. 5. Mean daily rates of the speed of private cars (dashed line with triangles) and commercial vehicles for 3 different kind of day.
Left-hand third of figure: Saturday
centre: Sunday
right: weekday
345
Fig. 5 shows the mean speed curves for private cars and lorries for the different kinds of day mentioned above. A very high speed level for the private car is noted. The daily rate characteristic curve is not very pronounced. The speed level of the commercial vehicles is far lower. It is noticeable that on Saturdays and Sundays the speed level is far higher than on weekdays. This is a reflection of a problem in the measurement of lorries. On Sundays the composition of the vehicles counted as lorries is obviously different to that on weekdays. Coaches and small lorries predominate on Sundays, while on weekdays the heavier lorries dominate. Moreover, the induction coils use as criterion for distinguishing the type of vehicle the vehicle length. Private cars with trailers are therefore also counted as lorries. At weekends during the holiday season, in particular, a relatively large number of these vehicles are on the roads. In general, however, the latter travel faster than real lorries. Consequently the speed level at weekends is also higher. Fig. 6 shows a time plot of the total nitrogen oxide emission per hour and km travelling distance of the motorway, together with the total traffic intensity for the balancing periods. The number of usable half-hours per month varies here from 1002 to 28. The traffic intensity shows a clear annual rate with a band width of the mean monthly values from approx. 1600 to approx. 3000 vehicles/h. The emission shows a parallel pattern but with somewhat flattened maxima in the summer months. N O x emission (kg/(kmoh))
traffic Intensity (vehicle/h)
30
3000
24
2400
18
1800
12
1200 600
0 87104
87107
87/10
88101
88•04
88•07
88110
89101
year/month ENOx
I
N total
Fig. 6. Mean monthly values of the total nitrogen oxide emission (ENO,) and traffic intensity ( N total) When evaluating the results, it is important to know the speed behaviour in the measurement period. In Fig. 7 are plotted the mean monthly values for the private car and commercial vehicle speeds. The drop in private car speeds in the months June to November 1987 is noteworthy. In this period there were roadworks directly behind the measurement cross section in the direction of Frankfurt. The speed limit of 100 km/h came into force just before the measurement cross-section, the reduction to 80 km/h directly behind it. This speed limit had no influence in the case of heavy lorries. The upward projecting values in October and November 1987 are due to a faulty measurement in the direction of Cologne. A coil end defect on the fight-hand traffic line caused some private cars to be counted as commercial vehicles. The main result of this was measurement of an excessively high lorry speed.
346
Mean emission factors for private cars and lorries were calculated by means of a multiple regression according to equation (2) and a private car emission factor statement: e ( P C ) = a o+a 1 x + a 2 x 2
(3)
In order to illustrate the variation in time, this was based on monthly values, although only small populations occurred in some months. Because of the low population levels, the breakdown of some of the values was very unrepresentative as regards the traffic parameters, so that two months had to be eliminated. vehicle speed Vm (km/h) 140
120
100
80
60 87/04
87/07
87/10
88/01
88/04
88/07
88/10
89/01
year/month lorry
~
private oar
Fig. 7. Mean monthly values of the speed of private cars and the commercial traffic (Iorriss) private car NOx emission factor (g/kin)
6-t
7
5
/
A
f A
/
3 7P-A / ,
/..1
i
/A
1 -~'/~/~-~ 0
V ~•
87/04
'
J
r
87/07
i
i
~
87/10
r
r
.
I
88/01
'
88/04
88/07
88/10
89/01
year/month actual V
~
V-128 km/h
Fig. 8. Mean monthly values of the private car NO xemission factor for the respective actual speed and conversion to speed basis V = 125 km/h
347
The private car emission factors calculated in this way are shown in Fig. 8 (dashed bars). The mean private car emission is far lower in the roadworks period than in the remaining period. A slight reduction in the specific private car emission seems to occur in the period from December 1988 onwards. In order to eliminate the slight speed differences, extrapolation to a mean car speed of 125 km/h (dark bars) was undertaken by means of the speed function determined according to equ.(3). Clear differences in comparison with the emission factors at actual speed occur only in the work period. The reduction of the specific emission in the roadworks period is only partly attributable to the reduced speed. In addition, because of the progressive reduction in speed, the vehicles passed through the measurement cress-section mainly in coasting mode. A smaller reduction in emission would have been obtained according to the mean speed function. NOx emission factor (g/kin) 30 25 20 15 10
0 87•04
87107
87110
88101
88/04
88/07
88/10
89/01
year/month priv. car V-125 km/h
~
lorry
Fig. 9. Mean monthly values of the NO x emission factors of private car traffic (calculated for V=125 km/h) and commercial vehicle traffic (lorries) Fig. 9 shows the emission factors of commercial traffic on a monthly basis (dark bars). A far stronger variation occurs here. This is partly explainable by the above-mentioned classification difficulties in identifying types of vehicle. In addition, the composition of the commercial vehicle group is far more heterogeneous than that of private car traffic, with a band width in the emissions which can amount to a factor as high as 10 from the empty small lorry up to the fully-loaded juggernaut. The emissions in winter are far higher than in summer, which confirms the suspicion that in summer the number of "false" lorries is much higher. In the raodworks period the lorry emissions are also lower, although the mean speed remained practically unaffected (Fig. 7). Here again the roadworks with a diversion in the area of the measurement cross-section seems to have caused the lorry drivers to go steady on the accelerator, thus causing less emissions due to the thrust mode. In addition, however, there seems to be a slight tendency for the specific lorry emission rate to rise. This cannot be explained at present. A rise in average loads plus a move towards heavier commercial vehicles - a reflection of the improved economic climate - are two possible reasons. In order to illustrate absolute differences between private car and lorry emissions, the private car emission factors have once again been included in this diagram (hatched bars).
348 NOx emission factor (g/km)
30 25 20 15
10 ¸ 5
87/2
87/3
87/4
88/1
88/2
88/3
88/4
89/1
year/quarter priv. car V-125 km/h ~ lorry Fig. 10. Mean quarterly values of the NO~ emission factors of private car traffic (calculated for V=125 km/h) and commercial vehicle traffm (lorries) In Fig. 10 the calculated emission factors for both vehicle groups are shown by quarters. Here again we see both a winter increase from the last quarter of 1988 onwards an a rise in the lorry emission rate. The private car emission factors show in the quarterly representation only a small scatter, with a slightly downward trend. CONCLUSION AND PROSPECTS Analysis of the data has not yet been completed. Some additional corrections will undoubtedly still have to be made to the lorry counts in the light of data on the different traffic breakdowns at holiday periods and on the various weekdays. This could lead to reduced scatter. No major changes in the findings are expected however. Recent developments in the traffic measurement techniques could undoubtedly improve the results of such measurements through a better and more finely grades distinction of vehicle types. The increase in the number of low-pollutant vehicles seems to be having the first effect on the emissions. Emissions from commercial vehicles, on the other hand, seem to exhibit a slightly upward trend. REFERENCES: 1. The exhaust gas mission behaviour of private cars in the Federal Republic of Germany in the reference year 1985 (in german), UBA-Berichte 7/87, Erich Schmidt Verlag, Berlin, 1987. 2. Vereinigung der Technischen Uberwachungsvereine (Editor): Large-scale experiment for investigating the effects of a speed limit on the exhaust gas emission behaviour of private cars on motorways (in german), Verlag TUV Rheinland, Cologne, 1986. 3. The exhaust gas emission behaviour of commercial vehicles in the Federal Republic of Germany in the reference year 1980 (in german), UBA-Berichte 11/83, Erich Schmidt Verlag, Berlin, 1983. 4. J.A. Bullin et.al., Determination of vehicle emission rates from roadways by mass balance techniques, Environ. Sol. & Techn. 15 (1980), pp. 700-705. 5. J.S. Hooghiemstra, Emissieschattigen weegverkeer d.m.v, een massabalas, Einsverlag project LB 135, May 1985, Landbauhochschule Wageningen (NL). 6. P. Leisen et.al., Immission measurements for emission balancing, in (2), (in german). 7. P.Leisen, Determination of motor vehicle emission factors by means of immission measurements (in german), VDI-Berichte No. 608 (1987), pp. 505-535.
339
Elsevier
DETERMINATION OF THE TREND OF HIGHWAY EMISSIONS BY MEANS OF EMISSION BALANCE MEASUREMENTS P. Leisen Technischer 0berwachungsverein Rheinland, Postfach 10 17 50, D-5000 Cologne, Federal Republic of Germany SUMMARY The nitrogen oxide emissions of a motorway are determined by means of an immission measurement. The total emission mass flow of the motorway is determined from the latter in the form of a balance measurement. Vehicle-specific emission factors for nitrogen oxides are then determined by statistical methods from this total emission and the simultaneously measured traffic parameters and the trend in the emissions is plotted on a time basis. TERMS OF REFERENCE Certain substances emitted by motor vehicles are governed by statute. In the case of private cars, the components carbon monoxide, nitrogen oxides and total hydrocarbons have already been subject to maximum emission values for quite some time. In the European exhaust gas legislation a compromise has been reached in that different maximum value requirements will be introduced in stages for three different cubic capacity classes. Exhaust gas reduction systems will be offered which just satisfy the generous maximum values applying in some cases, or else have far lower emissions (e.g. controlled three-way-catalyst for vehicles under 1.4 I cubic capacity). The vehicle population on our roads is broken down into a fairly small number of these "low-pollutant" vehicles together with the vehicles without special pollutant-reducing equipment. Some of the more recent pollutant-reducing systems have different effects for different speed ranges. Moreover, there is still little experience of the ageing behaviour. The real, representative emission behaviour cannot therefore be extrapolated by means of the maximum values or the type test values. In order to make reliable emission or immission calculations or to monitor the effectiveness of the statutory measures, however, the actual emissions have to be known. Comprehensive field tests on roller-type test stands are very cost- and time-intensive (refs 1-2). They are usually conducted on only a small vehicle population and are based on driving patterns which in some cases do not adequately reflect th traffic conditions on the roads. There are currently for commercial vehicle traffic no emission factors which have been determined in an analogous manner on roller type test stands. They have been calculated to date from stationary engine performance graphs (ref. 3). These tests show that with nitrogen oxides in particular, commercial vehicles account for a considerable share of the total emissions of road traffic. Their relative share will rise still further with the progressive reduction in exhaust gases from private cars. Commercial vehicle traffic already accounts for more than half the nitrogen oxide emissions on some German motorways. In order to monitor the effect of statutory measures and to improve the emission behaviour in actual driving conditions, the emissions should as far as possible also be determined under actual driving conditions. This is not possible given the large number of individual vehicles and also not
0048-9697/90/$03.50
© 1990 Elsevier Science Publishers B.V.
340
practicable in the case of a few vehicles. An emission balance measurement enables the total emission mass flow of a road to be recorded. It can then be used to monitor the variations in time of the emissions on an actual road. The mass flow then depends on the traffic intensities, the traffic breakdown and the vehicle speeds. If a fundamental judgement is to be made, the crucial element is not the variation in the emissions of a specific road, but the specific emission variation per vehicle. In order to calculate these values from the total emission mass flow, as accurate data as possible on the afore-mentioned traffic parameters is required. Specific emission factors can then be determined for private and commercial vehicles by means of statistical methods. The measurement cross-section on Federal Autobahn 3 (BAB 3) has a mean daily traffic intensity of approx. 45 000 vehicles/day. This high traffic intensity should help to ensure an overall representative vehicle population, particularly as a high volume of long-distance traffic is encountered on this motorway, which would balance out any local, unrepresentative factors. The measurement produces a mean emission factor, which is representative of the vehicle population typical of the motorway and the speed range encountered there. The emmission factor thus determined for private cars is therefore an integral value for the vehicle population typical of the motorway. A breakdown within the private car category, e.g. diesel- or petrol-driven vehicles, catalyst vehicles, c.c.classes, is not possible, since the traffic measurement cannot distinguish between these criteria. METHODOLOGY The basic concept of emission balancing by means of immission measurements consist in combining the vehicular traffic on the motorway as a source and determining the total pollutant mass flow by measurement techniques. To this end a suitable boundary for the mass balance is placed around the motorway. All pollutant mass flows beyond the balance limits are determined. Balancings of this kind were described in 1980 by BULLIN et.al. (ref. 4). The shortcomings of this investigation can however be avoided by an appropriate measurement act-up and careful evaluation of the result. Pollutant balancings on roadways have also been carried out by the Agricultural college (Landbauhochschule) in Wageningen (Netherlands) (ref. 5). Fig. 1 illustrates the principle of the emission balance measurement. The difference between the pollutant mass flows leaving the balance envelope (O) and those entering with the basic load (I) represents the emission mass flow of the traffic. The mass flow is therefore the product of mass concentration and air volume flow. See more detailed papers (refs 6,7) for a more precise description of the method and the measurement set-up. In order to determine the mass flow, the wind vector must be measured at the respective measuring point in addition to the pollutant concentration. The mass flow is then the integral of the partial mass flows formed over the full height of the exhaust gas cloud leaving the motorway: E = C(z) Ux(z) dz
(1)
In this equation the source intensity E represents the emission mass flow per unit of time and longitudinal section of the source with the dimension kg/(km*h). The integration must extend vertically over the whole exhaust gas cloud (z=H). The concentrations C and Ux (wind velocity at right angles to the motorway) are functions of the height z. To ensure correct balancing, no mass flow may leave the balancing limit at the top. This balance limit must therefore be sufficiently high to ensure
341
H
E = ~t N O x , o -
~t = J" U . C . d F
~ NOx,I
o Balance - Assumption
NO T r a n s p o r t
t
Z=H
n~ N O x , u p p a r
trough upper boundary
t c i (z:
+-S-
I I
Fig. 1. Diagram
I I I
-1 I I
I I
ENOx
I
|
, _~
I.e, C o ( H ) - CI(H)
I ~t N O x , O . Out '
|.~ N O x , l ~ ' In w
J
- 0
I
i~l
~i ~_
,
of the emission balancing
that the exhaust gas cloud remains below the highest measuring point of the lee-side measuring point network. In certain meteorological conditions the balancing requirements are not fulfilled, so that the measured values may not be used for the balancing. The follwing cases are are mainly involved here: Parallel winds prohibit balancing, since there are no pronounced winds at right angles to the balance limit and pollutants are clearly carried upwards through the balance envelope. In calm or in slightly windy weather, pollutants can likewise be carried upwards through the balance envelope. If there are reversals of wind direction during the averaging period, pollutants can be carried over both lateral balance limits or in the direction of the motorway, despite the fact that the mean wind direction is at right angles. MEASUREMENT SET-UP The measurements were carried out at the measurement station on BAB 3 which had been erected for the large-scale exhaust gas experiment (ref. 2). It had been shown that reliable results, compared with those of the test stand measurements, could be achieved at this station with this measurement system. The measurement cross-section lies on BAB 3 at the 30.25 km mark between the junctions Siegburg/Hennef and Siebengebirge. The motorway here runs through a slightly hilly region, 180 m amsl, almost directly north-south. The measurement cross-section lies roughly in the centre of an approx. 1 km long, straight section of motorway, which climbs slightly (approx. 1%) in the direction of Frankfurt. The measurement of pollutants and wind vector took place at the measuring heights: 1.0, 3.0, 5.5, 10.0 and 17.0 m; a further pollutant measurement was made at a height of 26,5 m. The measurement programme was subsidised by funds from the German Federal Environment Agency (Umweltbundssamt), Berlin.
342
Fig. 2. Picture of the immission measurement station on BAB 3 MEASUREMENT RESULTS The measurements started in February 1987 and have not yet been completed. Only provisional results can therefore be reported here. The following analyses refer to the period from 1st April 1987 to 31st March 1989. Fig. 3 shows the mean daily rates for the total nitrogen oxide emission (dashed line with triangles) and the total traffic intensity. The first third of the figure shows daily curves for
343
Saturdays, the second third of the figure those for Sundays and the last third those for weekdays. It is seen that the emissions for the individual kind of day are very similar. The daily maximum is slightly higher on weekdays than on the other days. On sunday the emission suddenly rises steeply from approx. 22.00 hrs onwards. The traffic intensity, on the other hand, shows by far the highest peak rates on Sundays. It can be concluded from the relationship between the two curves that far more vehicles are required for the same emission on Sundays than on a weekday. With the selected Soturday Sundoy Weekdoy
,oo_ 500
5O00
/
4000'
300
!IZ
' 200 ..~
'I00 ~E 8 0
0.00
12 I.m.
0.00
12 a.m.
0.00
12 a.m.
12 ,.m.
day/time of day Fig. 3. Mean daily rates of the total NO=emission (dashed line with triangles) and the traffic intensitiy for 3 different kinds of day. Left-hand third of figure: Saturday centre: Sunday right: weekday Sunday
Saturday
Weekday
20
so .~
15'
10
l
20 E
o 12 o.m. 12 ~.m. day/tlme of day Fig. 4. Mean daily rates of the total NOxemission per vehicle (dashed line with triangles) and the lorry share for 3 different kinds of day. Left-hand third of figure: Saturday centre: Sunday right: weekday 0
~.00
12 a.m.
0.00
12 .m.
0.00
344
scale the traffic intensity curve generally runs above the emission curve. Only during the night hours on weekdays and on Sundays from 22.00 hrs onwards does the emission predominate. The reason for these peculiarities can be seen from the following Fig. 4. Here are plotted in an analogue graph the mean daily rates for the specific NO Xemission per vehicle and the share of commercial vehicles in the traffic load (share of lorries). A mainly parallel pattern for both curves is obtained. At periods with high lorry share the specific emissions are also high. This correlation becomes particularly clear from the mean daily rates for Sundays (centre) and weekdays. On Sunday the share of lorries is particularly low in the clay-time hours. At these times the emission per vehicle is also particularly low. At 22.00 hrs the motorway is again open to heavy traffic and the share of lorries increases sharply. At the same time the specific emission also rises. On weekdays the share of lorries and the specific emission are correspondingly higher in the day-time hours than on Saturdays and Sundays. In the night hours on weekdays the values are comparatively high for both variables. The parallel shape of both curves shows that the commercial vehicle traffic has a considerable influence on the NOx emission rate. The total emission of the motorway is composed of the shares of private car traffic (PC) and commercial vehicle traffic (lorries) added together: E(NOx) = E (PC) + E (LORR) = e (PC) N (PC) + e (LORR) N (LORR)
(2)
The emission of each of these two types of vehicle is calculated here as the product of the number of vehicles and a mean emission factor (emission per vehicle). In the case of private cars the emission is a function of the driving speed. This is known from test-bed tests and can be confirmed from the data determined here. With lorries there is only a slight variation in speed, so that a constant emission factor can be employed here.
Sunday
Saturday
Weekday lOO
140
30'
120
m 7o
~1110'
8O
1CO
0.00
i
12 a.rn.
0.00
12 a.rn.
0.00
12 a.rn.
12 p.m.
day/time of day Fig. 5. Mean daily rates of the speed of private cars (dashed line with triangles) and commercial vehicles for 3 different kind of day.
Left-hand third of figure: Saturday
centre: Sunday
right: weekday
345
Fig. 5 shows the mean speed curves for private cars and lorries for the different kinds of day mentioned above. A very high speed level for the private car is noted. The daily rate characteristic curve is not very pronounced. The speed level of the commercial vehicles is far lower. It is noticeable that on Saturdays and Sundays the speed level is far higher than on weekdays. This is a reflection of a problem in the measurement of lorries. On Sundays the composition of the vehicles counted as lorries is obviously different to that on weekdays. Coaches and small lorries predominate on Sundays, while on weekdays the heavier lorries dominate. Moreover, the induction coils use as criterion for distinguishing the type of vehicle the vehicle length. Private cars with trailers are therefore also counted as lorries. At weekends during the holiday season, in particular, a relatively large number of these vehicles are on the roads. In general, however, the latter travel faster than real lorries. Consequently the speed level at weekends is also higher. Fig. 6 shows a time plot of the total nitrogen oxide emission per hour and km travelling distance of the motorway, together with the total traffic intensity for the balancing periods. The number of usable half-hours per month varies here from 1002 to 28. The traffic intensity shows a clear annual rate with a band width of the mean monthly values from approx. 1600 to approx. 3000 vehicles/h. The emission shows a parallel pattern but with somewhat flattened maxima in the summer months. N O x emission (kg/(kmoh))
traffic Intensity (vehicle/h)
30
3000
24
2400
18
1800
12
1200 600
0 87104
87107
87/10
88101
88•04
88•07
88110
89101
year/month ENOx
I
N total
Fig. 6. Mean monthly values of the total nitrogen oxide emission (ENO,) and traffic intensity ( N total) When evaluating the results, it is important to know the speed behaviour in the measurement period. In Fig. 7 are plotted the mean monthly values for the private car and commercial vehicle speeds. The drop in private car speeds in the months June to November 1987 is noteworthy. In this period there were roadworks directly behind the measurement cross section in the direction of Frankfurt. The speed limit of 100 km/h came into force just before the measurement cross-section, the reduction to 80 km/h directly behind it. This speed limit had no influence in the case of heavy lorries. The upward projecting values in October and November 1987 are due to a faulty measurement in the direction of Cologne. A coil end defect on the fight-hand traffic line caused some private cars to be counted as commercial vehicles. The main result of this was measurement of an excessively high lorry speed.
346
Mean emission factors for private cars and lorries were calculated by means of a multiple regression according to equation (2) and a private car emission factor statement: e ( P C ) = a o+a 1 x + a 2 x 2
(3)
In order to illustrate the variation in time, this was based on monthly values, although only small populations occurred in some months. Because of the low population levels, the breakdown of some of the values was very unrepresentative as regards the traffic parameters, so that two months had to be eliminated. vehicle speed Vm (km/h) 140
120
100
80
60 87/04
87/07
87/10
88/01
88/04
88/07
88/10
89/01
year/month lorry
~
private oar
Fig. 7. Mean monthly values of the speed of private cars and the commercial traffic (Iorriss) private car NOx emission factor (g/kin)
6-t
7
5
/
A
f A
/
3 7P-A / ,
/..1
i
/A
1 -~'/~/~-~ 0
V ~•
87/04
'
J
r
87/07
i
i
~
87/10
r
r
.
I
88/01
'
88/04
88/07
88/10
89/01
year/month actual V
~
V-128 km/h
Fig. 8. Mean monthly values of the private car NO xemission factor for the respective actual speed and conversion to speed basis V = 125 km/h
347
The private car emission factors calculated in this way are shown in Fig. 8 (dashed bars). The mean private car emission is far lower in the roadworks period than in the remaining period. A slight reduction in the specific private car emission seems to occur in the period from December 1988 onwards. In order to eliminate the slight speed differences, extrapolation to a mean car speed of 125 km/h (dark bars) was undertaken by means of the speed function determined according to equ.(3). Clear differences in comparison with the emission factors at actual speed occur only in the work period. The reduction of the specific emission in the roadworks period is only partly attributable to the reduced speed. In addition, because of the progressive reduction in speed, the vehicles passed through the measurement cress-section mainly in coasting mode. A smaller reduction in emission would have been obtained according to the mean speed function. NOx emission factor (g/kin) 30 25 20 15 10
0 87•04
87107
87110
88101
88/04
88/07
88/10
89/01
year/month priv. car V-125 km/h
~
lorry
Fig. 9. Mean monthly values of the NO x emission factors of private car traffic (calculated for V=125 km/h) and commercial vehicle traffic (lorries) Fig. 9 shows the emission factors of commercial traffic on a monthly basis (dark bars). A far stronger variation occurs here. This is partly explainable by the above-mentioned classification difficulties in identifying types of vehicle. In addition, the composition of the commercial vehicle group is far more heterogeneous than that of private car traffic, with a band width in the emissions which can amount to a factor as high as 10 from the empty small lorry up to the fully-loaded juggernaut. The emissions in winter are far higher than in summer, which confirms the suspicion that in summer the number of "false" lorries is much higher. In the raodworks period the lorry emissions are also lower, although the mean speed remained practically unaffected (Fig. 7). Here again the roadworks with a diversion in the area of the measurement cross-section seems to have caused the lorry drivers to go steady on the accelerator, thus causing less emissions due to the thrust mode. In addition, however, there seems to be a slight tendency for the specific lorry emission rate to rise. This cannot be explained at present. A rise in average loads plus a move towards heavier commercial vehicles - a reflection of the improved economic climate - are two possible reasons. In order to illustrate absolute differences between private car and lorry emissions, the private car emission factors have once again been included in this diagram (hatched bars).
348 NOx emission factor (g/km)
30 25 20 15
10 ¸ 5
87/2
87/3
87/4
88/1
88/2
88/3
88/4
89/1
year/quarter priv. car V-125 km/h ~ lorry Fig. 10. Mean quarterly values of the NO~ emission factors of private car traffic (calculated for V=125 km/h) and commercial vehicle traffm (lorries) In Fig. 10 the calculated emission factors for both vehicle groups are shown by quarters. Here again we see both a winter increase from the last quarter of 1988 onwards an a rise in the lorry emission rate. The private car emission factors show in the quarterly representation only a small scatter, with a slightly downward trend. CONCLUSION AND PROSPECTS Analysis of the data has not yet been completed. Some additional corrections will undoubtedly still have to be made to the lorry counts in the light of data on the different traffic breakdowns at holiday periods and on the various weekdays. This could lead to reduced scatter. No major changes in the findings are expected however. Recent developments in the traffic measurement techniques could undoubtedly improve the results of such measurements through a better and more finely grades distinction of vehicle types. The increase in the number of low-pollutant vehicles seems to be having the first effect on the emissions. Emissions from commercial vehicles, on the other hand, seem to exhibit a slightly upward trend. REFERENCES: 1. The exhaust gas mission behaviour of private cars in the Federal Republic of Germany in the reference year 1985 (in german), UBA-Berichte 7/87, Erich Schmidt Verlag, Berlin, 1987. 2. Vereinigung der Technischen Uberwachungsvereine (Editor): Large-scale experiment for investigating the effects of a speed limit on the exhaust gas emission behaviour of private cars on motorways (in german), Verlag TUV Rheinland, Cologne, 1986. 3. The exhaust gas emission behaviour of commercial vehicles in the Federal Republic of Germany in the reference year 1980 (in german), UBA-Berichte 11/83, Erich Schmidt Verlag, Berlin, 1983. 4. J.A. Bullin et.al., Determination of vehicle emission rates from roadways by mass balance techniques, Environ. Sol. & Techn. 15 (1980), pp. 700-705. 5. J.S. Hooghiemstra, Emissieschattigen weegverkeer d.m.v, een massabalas, Einsverlag project LB 135, May 1985, Landbauhochschule Wageningen (NL). 6. P. Leisen et.al., Immission measurements for emission balancing, in (2), (in german). 7. P.Leisen, Determination of motor vehicle emission factors by means of immission measurements (in german), VDI-Berichte No. 608 (1987), pp. 505-535.