Perennial observations of molecular hydrogen (H2) at a suburban site in Switzerland

ARTICLE IN PRESS

Atmospheric Environment 41 (2007) 2111–2124 www.elsevier.com/locate/atmosenv

Perennial observations of molecular hydrogen ðH2Þ at a suburban site in Switzerland M. Steinbacher, A. Fischer, M.K. Vollmer, B. Buchmann, S. Reimann, C. Hueglin Empa, Swiss Federal Institute for Materials Testing and Research, Laboratory for Air Pollution/Environmental Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland Received 2 June 2006; received in revised form 27 October 2006; accepted 31 October 2006

Abstract Semi-continuous measurements of atmospheric molecular hydrogen ðH2 Þ and carbon monoxide (CO) were performed at a suburban sampling site in Switzerland from November 2002 to February 2005. The presented data provide information about time series, seasonal and diurnal cycles as well as sources and sinks in a suburban environment. Such records become increasingly important for the assessment of the effects of a possible future hydrogen economy on the environment. No significant trend of background H2 concentrations could be observed for the considered period. Seasonal cycles show the expected pattern with maxima in winter and minima in summer and autumn. When extracting the background H2 mixing ratios, the absolute levels as well as the seasonal amplitude agree well with literature data from semi-rural and remote sites. The H2 dry deposition velocity for summer days with low winds is estimated to be between 0:5  104 to 1  104 m s1 . Diurnal cycles are dominated by H2 emissions from nearby traffic. The influence of traffic-related emissions is also corroborated by a high H2 to CO correlation. Typical molar H2 to CO ratios from traffic were found to be 0.33 on a molar basis (ppb/ppb). A reduction of 37–62% in European anthropogenic H2 emissions is estimated for a period covering the last 25 years. Since the ambient H2 concentration did not decline during the same period, other simultaneous changes of sinks or sources of H2 must have compensated this reduction. r 2006 Elsevier Ltd. All rights reserved. Keywords: H2 ; Molecular hydrogen; H2 to CO ratio; Anthropogenic emissions; Deposition velocity

1. Introduction Molecular hydrogen (H2 ) is one of the most abundant trace gases in the atmosphere with a globally averaged mixing ratio of approximately 530 ppb (Novelli et al., 1999). However, limited attention has been paid to the sources and sinks as Corresponding author. Tel.: +41 44 823 4048;

fax: +41 44 821 6244. E-mail address: [email protected] (M. Steinbacher). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.10.075

well as the role that H2 plays in atmospheric chemistry. Interest has recently grown since H2 is regarded as a key component in the future energy chain. Consequently, several governments intensified their support of the development of hydrogen technology and infrastructure. The budget of H2 is mainly determined by its photochemically and biologically driven sources and sinks as well as by anthropogenic processes. Actual sources of H2 comprise combustion of fossil fuel, biomass burning, oxidation of methane and oxidation of non-methane hydrocarbons (mainly

ARTICLE IN PRESS 2112

M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

isoprene). Volcanoes, oceans and biogenic N2 fixation are additional minor emission sources (Conrad and Seiler, 1980; Seiler and Conrad, 1987; Novelli et al., 1999; Ehhalt, 1999; Warneck, 2000; Hauglustaine and Ehhalt, 2002; Sanderson et al., 2003; Rhee et al., 2006). Ambient air excess mixing ratios above background levels of H2 ðDH2 Þ and CO ðDCOÞ cover a molecular DH2 =DCO range from 0.15 to 0.6 depending on the characteristics of the sampling site and the season (Schmidt, 1974; Novelli et al., 1999; Simmonds et al., 2000; Barnes et al., 2003 and references therein). These results agree with expectations from the water–gas equilibrium ðCO þ H2 O2CO2 þ H2 Þ for conditions in automobile engines (Penner et al., 1977) and with car emission data (Vollmer et al., in preparation). Measurements on gasoline-fuelled cars on a chassis dynamometer revealed velocity-dependent H2 emissions of 17–720 mg km1 (Heeb et al., 2006). Soil uptake is the dominant sink for atmospheric H2 and is modelled to contribute approximately 80% to the total removal (Hauglustaine and Ehhalt, 2002; Sanderson et al., 2003; Rhee et al., 2006), with the remaining part being degraded by oxidation with OH-radicals. The predominant soil uptake is a unique feature among trace gases. Due to the larger continental area, the sink strength of the Northern Hemisphere (NH) is larger than that of the Southern Hemisphere (SH) resulting in higher mixing ratios in the latter. As a consequence, an interhemispheric H2 gradient of about 15–35 ppb was inferred from measurements (Schmidt, 1974, 1978; Khalil and Rasmussen, 1990; Novelli et al., 1999; Simmonds et al., 2000). The few existing approaches to model the global budget of atmospheric hydrogen were based on three-dimensional chemistry transport models and surface measurements (Novelli et al., 1999; Hauglustaine and Ehhalt, 2002; Sanderson et al., 2003). Although uncertainties with particular terms in the H2 budget were large (between 30% and 60%; Novelli et al., 1999; Ehhalt, 1999) it was generally concluded that sources and sinks are approximately balanced. The global annual H2 turnover is estimated at 70278 Tg yr1 (Novelli et al., 1999; Hauglustaine and Ehhalt, 2002; Sanderson et al., 2003) and 107 Tg yr1 (Rhee et al., 2006). This turnover, combined with the global atmospheric burden of H2 of 136–155 Tg (Novelli et al., 1999; Hauglustaine and Ehhalt, 2002), indicates a lifetime of 1:4–2:0 years. The model results from Sanderson et al. (2003)

designate a global mean hydrogen dry deposition velocity of 5:3  104 m s1 , which is lower than the previously derived value of 7  104 m s1 (Conrad and Seiler, 1980, 1985). The hydrogen dry deposition velocity has been shown to be strongly dependent on temperature (Schuler and Conrad, 1991; Yonemura et al., 1999, 2000). Microbiological CO and H2 consumption is limited by transport resistance in the soil and the in situ uptake rates may be limited by a higher soil moisture level (Yonemura et al., 2000). First measurements of atmospheric hydrogen were made in the 1950s (Glueckauf and Kitt, 1957). First measurements in polluted air masses revealed an average H2 concentration of 800 ppb for an industrial region in Mainz, Germany (Schmidt, 1974). Observations in Washington, DC near an inter-state highway showed H2 mixing ratios ranging from 600 ppb to more than 3000 ppb (Scranton et al., 1980). Khalil and Rasmussen (1990) made H2 observations at six locations at latitudes ranging from 711N to 901S during 1986–1989 and deduced an averaged annual increase of 3:2  0:5 ppb yr1 . Novelli et al. (1999) presented a large set of H2 flask sample measurements from the globally distributed NOAA/GMD cooperative air sampling network. They obtained a globally averaged H2 mixing ratio between 1991 and 1996 of 531 ppb and a downward trend of 2:7  0:2 ppb yr1 for the NH. In contrast, flask sampling observations at a network of globally distributed surface sites from 1992 to 1999 by CSIRO (Langenfelds et al., 2002) revealed a global mean growth rate of 1:4 ppb yr1 . Simmonds et al. (2000) inferred a slight positive trend of 1:2  0:8 ppb H2 yr1 from their measurements at Mace Head (Ireland) during the 1994–1998 period by extracting NH midlatitude baseline air masses. Continuous measurements of atmospheric H2 were carried out at Mt. Cimone, Italy from May 1994 to February 1996 (Bonasoni et al., 1997) with a mean H2 mixing ratio of 582 ppb for their measurements in 1994. Three years (1996–1998) observation of H2 above a forest canopy in a polluted environment at Harvard Forest, Massachusetts revealed an overall range of 414–917 ppb with a mean of 553 ppb (Barnes et al., 2003). H2 levels during pollution events were typically 100–200 ppb above the background seasonal cycle. H2 is often regarded as a key element in the future energy chain and hydrogen fuel cells are proposed as alternative energy source to conventional fossil fuels (Abraham, 2004; Science special issue, 2004).

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

A large-scale hydrogen economy might result in potential changes in the composition of the atmosphere on both regional and global scale. Reductions in soot, nitrogen oxides, sulphate, and greenhouse gases would be beneficial to air quality. However, the increased usage of H2 also bares some caveats and potential risks for the environment due to the possible increased emission of H2 from anthropogenic activities (Schultz et al., 2003; Prather, 2003; Warwick et al., 2004). Impacts could encompass an enhancement of the stratospheric water vapour (Tromp et al., 2003) and a decrease in the average OH, which consequently would enhance the tropospheric lifetime of greenhouse gases such as CH4 and amplify their radiative forcing (Warwick et al., 2004). Recent studies disagree about the potential environmental benefits and risks, largely due to the fact that the current atmospheric budget of H2 is not fully understood (Tromp et al., 2003; Schultz et al., 2003). Quasi-continuous measurements of H2 in Duebendorf, Switzerland were performed from 2002 to 2005 to provide a high-quality data set representative for suburban conditions. These are the first continuous measurements for over 30 years (Schmidt, 1974) from a site in Europe predominately influenced by automobile source. The presented data and their analyses provide information that is essential for an assessment of the effects of a possible future hydrogen economy on the environment. 2. Experimental 2.1. Sampling location Semi-continuous measurements of H2 and CO in ambient air have been performed in Duebendorf near Zurich from November 2002 to February 2005. The sampling site (47 240 N, 8 360 E; 433 m above sea level) is a station of the Swiss National Air Pollution Monitoring Network (NABEL) and it is classified as representative for suburban conditions. The sampling site is located in a broad and flat valley. Nearest small hills are found approximately 2 km southwest, 3 km north and 3 km northeast of the sampling site. The surrounding area of Duebendorf is densely populated and is characterized by a large variety of industrial activities. The air quality is dominated by traffic emissions with a main road passing 150 m south of the sampling site and a motorway running 750 m north of the station.

2113

2.2. Analytical method A commercial gas chromatograph (Reduction Gas Analyzer (RGA3), Trace Analytical, Inc., California, USA) was used to measure H2 and CO via reduction of mercuric oxide and detection of mercury by UV absorption. Synthetic air preconditioned by molecular sieve 5 A˚ and SOFNOCAT to remove H2 O and reactive impurities (CO and H2 ) is used as carrier gas. Aliquots of air samples are flushed with a rate of 20 ml min1 over a 1 ml sample loop. After equilibration, the sample volume is injected onto the columns. The chromatographic precolumn (Unibeads 1S mesh 60/80; 1/8 in OD x 30 in) is mainly used to remove CO2 , H2 O and hydrocarbons. Subsequently, H2 and CO are separated by the analytical column (molecular sieve 5 A˚ mesh 60/80; 1/8 in OD x 30 in) before being analysed by the mercury detector. The columns are kept at 75 1C; the oven is heated to 270 1C. In the beginning of the quasi-continuous observations, measurements were taken every 6 min. In May 2004 the interval was extended to 30 min. Precisions of 0.3% ð1sÞ and 0.4% ð1sÞ were determined from recurrent working standard analyses for H2 and CO, respectively. 2.3. Quality assurance The measurements of H2 and CO were alternated (every fifth sample) with calibration samples using real-air working standards with concentrations representative for ambient air concentration of the northern hemispheric troposphere. These working standards were referenced against the NOAA/GMD scale (Novelli et al., 1999). To account for the nonlinearity of the detector, two high concentration reference gases (50 and 100.2 ppm H2 ; Messer Schweiz, Switzerland) were dynamically diluted with zero air to the range of atmospheric concentrations by means of a dilution unit (MKAL diluter, Breitfuss Messtechnik GmbH, Harpstedt, Germany). The dilution unit was indirectly referenced against the primary gas flow standard of the Swiss Federal Office of Metrology. The different mixtures of the two high concentration standards showed an excellent agreement between each other and with the NOAA/GMD standards. The comparison of several cylinders of known H2 and CO concentrations with the AGAGE network showed a good agreement and provided an additional link to an independent scale.

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

2114

3. Results and discussion Hourly averaged atmospheric H2 and CO mixing ratios at Duebendorf for the period from 19 November 2002 to 23 February 2005 are shown in Fig. 1. Gaps in the records are mostly due to the application of the instrument for measurements in other projects and due to instrument failures. Overall, a total of more than 68,000 chromatograms has been collected. The H2 mixing ratio range from a minimum of 453 ppb to a maximum of 1566 ppb with a median value of 550 ppb and a mean

( standard deviation) of 576  94 ppb. The respective numbers for CO are 102–2028 ppb, a median of 306 ppb and a mean of 385  240 ppb. The grey data in Fig. 1 represent the background mixing ratios that were determined similar to the method described by Reimann et al. (2004, 2005). The baseline mixing ratios were extracted iteratively by calculating the local linear regression of the concentrations using the ‘loess’ function and omitting data exceeding the 90-days running median of the air samples plus two standard deviations taken from the recurrent standard measurements. By this

a 1600

H2 [ppb]

1200

800 600

01/01/03

07/01/03

01/01/04

07/01/04

01/01/05

07/01/04

01/01/05

Date (mm/dd/yy)

b 2000

CO [ppb]

1500

1000

500

01/01/03

07/01/03

01/01/04 Date (mm/dd/yy)

Fig. 1. Time series of hourly averaged H2 (panel a) and CO (panel b) mixing ratios.

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

technique, all data are iteratively removed that deviate significantly (i.e. not coincidentally due to instrumental noise) from the median. The extracted background shows no distinct H2 trend of 0:16  1:21 ppb yr1 and a negative trend of 9:25  2:51 ppb yr1 for CO. Only data from 1 January 2003 to 1 January 2005 were considered for the trend determination since the seasonality might cause artefacts when using incomplete years. The trend determination for H2 at Duebendorf leads therefore to a result between the negative trend for the NH of 2:7  0:2 ppb yr1 reported by the NOAA/GMD cooperative air sampling network (Novelli et al., 1999) and the positive trend of 1:2  0:8 ppb yr1 for Mace Head, Ireland (Simmonds et al., 2000). However, these direct comparisons have to be considered with caution since H2 trends as function of latitude were reported (Novelli et al., 1999; Langenfelds et al., 2002) and significant interannual variations were observed e.g. due to largescale biomass burning events (Simmonds et al., 2004). 3.1. Seasonal cycles The mean seasonal cycle of H2 for the suburban site Duebendorf (Fig. 2, panel a) exhibits the lowest values in summer and autumn and maxima mixing ratios in winter, whereas the mean seasonal cycle of

a

581.3 621.8 583.7 549.2 566.8 560.4 552.5 569.5 535.3 574.3 625.9 580.5 502.5 514.4 497.5 499.9 524 522.1 514.2 518.8 480.1 485.6 508 502.4

2115

the background concentrations reveals a less distinct pattern. The lowest background concentrations could be found in autumn and seasonal background amplitudes of 45 ppb were observed in good agreement with previously reported studies at similar latitudes (Novelli et al., 1999; Simmonds et al., 2000; Barnes et al., 2003). Novelli et al. (1999) observed seasonal maxima and minima in the NH in late winter/early spring and in autumn, respectively. Barnes et al. (2003) reported for their long-term observations above a forest canopy in the eastern United States a broad H2 maximum from December to June and a short but distinct minimum in late autumn for the background seasonal cycles. Simmonds et al. (2000) investigated the seasonal H2 cycle of baseline NH air masses and also obtained highest concentrations during spring and lowest concentrations during late autumn. This pattern is consistent with the maximum summertime loss rates by OH oxidation and strongest rates of soil uptake in late summer and early autumn, when soils are at their driest (Yonemura et al., 2000; Barnes et al., 2003). Sanderson et al. (2003) simulated the seasonal H2 cycle by means of a global three-dimensional Lagrangian chemistry model and compared their results with data from the NOAA network. They showed that the dry deposition of H2 plays a major role in determining the seasonal cycle since an

b

343.5 551 486.5 338 291.1 311.2 290.1 332.5 324.5 367.5 509.9 436.2 194 235.8 214.3 192.4 182.3 185.1 167 157.3 151.6 173 207.5 178.5

600

all data

all data background data

background data

500

CO [ppb]

H2 [ppb]

600

550

400

300

500

200

J

F

M

A

M

J

J

Month

A

S

O

N

D

J

F

M

A

M

J

J

A

S

O

N

D

Month

Fig. 2. Annual cycles of H2 (panel a) and CO (panel b). Numbers on top denote the respective values. Uncertainty estimates represent 95% confidence intervals.

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

2116

updated dry deposition scheme resulted in a considerable improvement of the model-measurement comparison. In contrast to all of the abovementioned sampling sites, the Duebendorf station is located in a suburban environment with considerable primary anthropogenic H2 emissions nearby and with a limited quantity of natural soil surfaces due to a large amount of asphalt and concrete cover. However, the H2 depletion detected in semi-rural to rural environments is also observed at our sampling site. The stronger thermal convection in summer is leading to a more efficient mixing of air from the surface layer with air from the boundary layer above. This in turn prohibits a more distinct H2 depletion at the ground in summer resulting in a slightly higher background H2 mixing ratio than during other seasons. However, it is remarkable that the seasonal cycles of the background H2 mixing ratios in Duebendorf are in the same range and show similar minimum–maximum-amplitudes as the abovementioned data sets from semi-rural and rural sampling sites (Simmonds et al., 2000; Barnes et al., 2003). The seasonal CO pattern exhibits a broad minimum, spanning from late spring to autumn with averaged mixing ratios from 290 to 325 ppb and an amplitude of about 250 ppb. Background mixing ratios range from 150 ppb in autumn to about 235 ppb in winter. These numbers are higher

than the background concentrations at rural sites. This is related to the substantial influence of traffic emissions that leads to CO mixing ratios of up to 20 times background values (2 ppm compared to 100 ppb), whereas only a factor of three (1.6 ppm to 500 ppb) was observed for H2 . However, the CO background mixing ratios at Duebendorf are representative for background conditions of suburban locations. 3.2. Daily cycles The influence of anthropogenic processes on the short-term variability of H2 is well visible in Fig. 3 that illustrates four days of highly time resolved H2 and CO mixing ratios. Nearly every H2 peak coincides well with enhanced CO concentrations. The mixing ratios rise sharply during morning and evening rush hours, and enhanced concentrations were detected for both H2 and CO on Saturday to Sunday night due to traffic regarding to nightlife activities in the Greater Zurich area. Enhanced H2 and CO levels might also be favoured by low wind speeds on Saturday to Sunday night. However, the local meteorological conditions did not considerably vary compared to the previous nights. A more general picture is given in Fig. 4 that illustrates mean daily cycles for H2 (panel a) and CO (panel b) when distinguishing between working days, Saturdays and Sundays. All data are stored in

1000 H2

900

CO

700

1000

600 800

500

600

400 300

400

200 200

100 0

0 23.01.2003

24.01.2003

25.01.2003

26.01.2003

thu

fri

sat

sun

Fig. 3. Four-day period of high time resolution H2 and CO time series.

CO mixing ratio [ppb]

1200

800 H2 mixing ratio [ppb]

1400

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

620

500 450

CO [ppb]

600 H2 [ppb]

2117

580

400 350

560 300 540

250

mon to fri sat sun

520 0

3

6

mon to fri sat sun

200 9

12

15

18

21

0

3

6

9

12

15

18

21

Hour

Hour

Fig. 4. Daily cycles of H2 (panel a) and CO (panel b) for working days (Monday to Friday), Saturdays, and Sundays.

local winter time ðGMT þ 1 hÞ. H2 concentrations decrease from midnight to approximately 5 AM due to the H2 uptake by the soil and dilution effects due to vertical mixing in the nocturnal boundary layer. After 5 AM, H2 levels rise due to exhaust emissions from commuter traffic. The H2 morning peak is therefore only present on working days and slightly visible on Saturdays. Later on when traffic decreases, the H2 mixing ratios reduce again due to dilution with less H2 containing air masses and soil uptake. When the evening rush hour starts, H2 mixing ratios begin to rise again and remain on an elevated level since H2 is emitted into the shallow nocturnal boundary. When the traffic reduces during the night, H2 levels start to drop due to the abovementioned processes. A rather analogous pattern is observed for CO since the source and sink processes of H2 and CO are nearly identical and especially the sources are even similar in their relative importance (Ehhalt, 1999; Warneck, 2000). However, the afternoon minimum for CO is more pronounced than the early morning minimum before the rush hour starts, whereas the early morning minimum for H2 is nearly as distinct as its afternoon minimum. The stronger CO depletion in the afternoon compared to the early morning minimum is due to the lower background for CO. As a result, the dilution with air masses from above due to thermal convection leads to the observed stronger decrease for CO. Consequently, the dy-

namic range of the diurnal cycle of CO is larger than for H2 since the CO background is lower and the peak values of CO during the rush hours can be even higher than the H2 concentrations (see Fig. 3). 3.3. H2 to CO ratios Fig. 5 shows a H2 versus CO scatterplot of all available data. It corroborates the close correlation between the ambient air CO and H2 mixing ratios at the Duebendorf site. A slight non-linear relation can be seen with a rather linear H2 =CO-relation up to 800 ppb H2 and 1000 ppb CO and somewhat augmented H2 mixing ratios during more polluted periods. Most of the data points at the upper end were observed in winter during high-pressure situations with a stable boundary layer and foggy conditions at the ground. Exemplarily, some periods are highlighted. Black squares represent data from 20 to 27, February 2003. This period was characterized by sunny and mild conditions in the Alps with fog up to 600–800 m and low temperatures within the foggy regions. The restricted air mass exchange due to the temperature inversion resulted in an accumulation of air pollutants in the boundary layer. Similar conditions prevailed from 4 to 6 November 2003 (open circles) and from 20 to 21 November 2003 (stars). It remains so far unexplained why enhanced pollution levels coincides with a shift to higher H2 to CO ratios. Burning

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

2118

to this process. Thus, an explanation cannot so far be given.

1600 1400

3.4. Estimation of H2 emissions from traffic

1000 800 600 400 500

1000

1500

2000

CO [ppb] Fig. 5. H2 versus CO scatterplot of all available data at Duebendorf. Selected periods with high levels of air pollutants are highlighted with different symbols. See text for further explanations.

of wood in residential furnaces can be excluded as an additional H2 and CO source with a high H2 to CO ratio. Canister samples taken in a region with frequent and widespread usage of residential wood furnaces (Roveredo, southern Switzerland) showed a molecular H2 =CO of about 0.25, which is lower than the ratio from the exhaust of fossil-fuel vehicles (see the following section). Since the methane oxidation is another major source of H2 and CO and the CH4 levels also rose during these winterly inversions, this process has to be considered as well when trying to explain the observed non-linearity, although photochemical activity is low during these winter inversions. Reported H2 and CO yields of the CH4 oxidation (via the formation of formaldehyde) result in a H2 to CO formation ratio of 0.25 (Barnes et al., 2003) to 0.33 (Novelli et al., 1999). Thus, the degradation of CH4 and HCHO cannot cause the observed bias towards higher H2 levels under high pollution episodes since the inferred H2 to CO production ratio from anthropogenic sources in Duebendorf is rather similar (see the following section). Vollmer et al. (in preparation) observed a similar non-linear H2 =CO-relation in a highway tunnel and concluded that vehicle engine emissions are biased towards higher H2 =CO ratios under oxygen-deficient conditions, i.e. accelerating periods. Since most of the periods with enhanced H2 /CO ratios in Duebendorf were observed under certain meteorological conditions, the nonlinear relation cannot be straightforward related

For the determination of a characteristic H2 to CO ratio from traffic, only morning hours at working days were considered (see Fig. 6). We assume that the nearby traffic is by far the most dominant H2 and CO source and no significant dilution takes place during these early morning hours. A reduced major axis regression analysis yields a slope of ð0:30  0:02ð2Þ0:33  0:01Þ ppb H2 per ppb CO. The winter data cover a broader concentration range and show a higher coefficient of determination ðR2winter ¼ 0:90; R2summer ¼ 0:68Þ. This might be due to the onset of photochemical processes and thermal convection during the considered period in summer. Therefore, our abovementioned assumptions are more justified in winter so that our best estimate of a typical H2 =CO ratio from traffic is 0.33 ppb/ppb (or 0.024 g H2 per g CO). Our ratio is lower than the one reported by Novelli et al. (1999) above a busy intersection in Boulder, Colorado (0.6 ppb/ppb), but higher than the ratio in polluted air masses derived from measurements in Mace Head (0.15 ppb/pbb; Simmonds et al., 2000) and agrees well with the reported data from the Harvard Forest, when only considering the pollution plumes

900

summer regression: y = 467.1 + 0.295 x winter regression: y = 426.3 + 0.331 x

800 H2 [ppb]

H2 [ppb]

1200

700

600

500 200

400

600

800

1000

1200

CO [ppb] Fig. 6. H2 versus CO scatterplot during winter (November to February) and summer (June to August) morning hours at working days. Three hour periods were selected (winter: 5–7 o’clock local winter time; summer: 6–8 o’clock local winter time).

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

from the New York City–Washington DC corridor (0.31–0.41 ppb/ppb in winter; Barnes et al., 2003). Remarkably, our figure compares reasonably well with the ratio of 0.43 ppb/ppb derived from measurements in the industrial area of Mainz, Germany in 1972 (Schmidt, 1974) (i.e. in a time when no catalytic converter systems were in use). A direct comparison is only feasible with the ratios from Boulder and Mainz because transport processes, photochemical aging of the air masses during the transport and depletion due to dry deposition make it hardly possible to derive a pure H2 to CO signal from traffic at rural or remote sites. Since no information is given by Schmidt (1974) about the type of reference standards, CO calibration scales changed within the last 25 years and reliable reference standards are a challenging task especially for H2 , the ratio for Mainz has to be considered as a rough indication for the pre-catalyst era. The discrepancy between our ratio and the ratio of Novelli et al. (1999) might be due to a different traffic fleet (number of diesel and gasoline vehicles) or fuel composition. The low ratio reported by Simmonds et al. (2000) might be due to H2 depletion during transport from the sources to the sampling site even if they estimate a local deposition velocity at the Mace Head station of 0:26 mm s1 that is lower than most of the deposition velocities reported in the literature. Our results also compare reasonably well with H2 to CO ratios extracted from measurements in a highway tunnel close to Zurich (Vollmer et al., in preparation) that revealed a H2 to CO ratio of 0.48 ppb H2 per ppb CO. 3.5. Local sink estimation Typical nocturnal decays (from 11 PM to 3 AM) of H2 and CO were used to estimate the sink strength of H2 due to dry deposition. A selection of working days with maximum temperature above 25 1C and wind speed below 2 m s1 was applied since these conditions usually resulted in distinct nocturnal decays both for H2 and CO. As OH concentrations typically are up to 2 orders of magnitude smaller in the night than during the day (e.g. Plass-Du¨lmer et al., 1998), only soil uptake and entrainment of less H2 -containing air are considered as relevant processes during the night. Moreover, it is assumed that anthropogenic H2 and CO emissions are negligible between 11 PM and 3 AM. The nocturnal change of the H2 and CO concentrations with time can then be described by

(adapted from Seinfeld and Pandis, 1998)  bg  dX X ðtÞ X  X ðtÞ dH ¼ vx  dt HðtÞ HðtÞ dt

2119

(1)

with X  H2 or CO, the deposition velocity v, the boundary layer height H and the background concentrations X bg . H2 ðtÞ, COðtÞ and HðtÞ are approximated by a linear interpolation between the initial time t0 and the time t ¼ t0 þ Dt according to X ðt0 Þ þ X ðt ¼ t0 þ DtÞ 2 X ðt0 Þ þ ðX ðt0 Þ þ ðdX =dtÞDtÞ ¼ 2 2X ðt0 Þ þ ðdX =dTÞDt . ð2Þ ¼ 2 The temporal change of the boundary layer height HðtÞ (that can be considered as a measure for the entrainment of upper air masses) is determined by means of Eq. (1) and by using values for CO. A range for vCO of 0:4  104 to 4  104 m s1 was chosen (Simmonds et al., 2000; Moxley and Cape, 1997 and references therein). The observed mean CO mixing ratio between 11 PM and midnight (437 ppb CO) for the selected days was chosen as COðt0 Þ. The background mixing ratio was estimated as the average afternoon minimum for the selected days (208 ppb CO). A critical parameter for calculating the deposition velocity according to Eq. (1) is the nocturnal boundary layer height. An approach similar to Garland and Derwent (1979) and Moxley and Cape (1997) was applied to estimate the boundary layer height. Initially, the logarithmic wind profile equation (see e.g. Stull, 1988) is used to estimate the friction velocity u   u z uðzÞ ¼ ln (3) z0 k X ðtÞ ¼

with the wind speed u at z ¼ 10 m height, the von Karman constant k ¼ 0:4 and the roughness length ðz0 Þ typical for suburban areas of 0.4 (Stull, 1988). Once the friction velocity has been calculated, it is possible to determine the boundary layer height by  2 1=3 g T u4 1 H¼ (4) f k g dT=dt with the Zilitinkevich constant g ¼ 0:4, the mean temperature T in Kelvin, the coriolis parameter for Duebendorf f, the acceleration of gravity g and the mean cooling rate dT=dt (Moxley and Cape, 1997).

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

Since we restricted our selection to conditions with low wind speeds, we got a mean friction velocity of 0:05 m s1 and thus a mean boundary layer height of 16 m. We are aware that this is a simple approach that is based on several assumptions and does not consider the influence of the atmospheric stability. As we restricted our analysis to situations with high daily maximum temperatures (i.e. fair weather periods) and low wind speeds during the night, we tend to favour the consideration of strongly stable conditions. In consequence, we might slightly overestimate the friction velocity and in turn the nocturnal boundary layer height since the friction velocity is usually lower under stable than under neutral conditions. However, reported values for the nocturnal boundary layer height range from 20 to 30 m (Mahrt and Vickers, 2002), up to 50–250 m (Arya, 1981; Salmond and McKendry, 2002) and from about 100 to 500 m before sunrise (Stull, 1988). Therefore we chose an Hðt0 Þ from 10 to 150 m to partly span the wide and highly variable range reported in literature. The resulting boundary layer height changes are between 2 and 37 m h1 depending on vCO and Hðt0 Þ. Inserting the calculated dH=dt values in Eq. (1) for H2 , we can calculate vH2 as a function of the initial boundary layer height and the CO deposition velocity. Initial and background H2 mixing ratios were determined in analogy to the CO mixing ratios (H2 ðt0 Þ ¼ 586 ppb; Hbg 2 ¼ 540 ppb). Subsequently, we estimate vH2 between 0.5 and 9  104 m s1 for the whole selected range of boundary layer heights and CO deposition velocities (see Fig. 7). Considering the abovementioned mean boundary layer height of 16 m as the best estimate, we therefore claim the most reliable vH2 at our sampling site to values between 0.5 and 1  104 m s1 (see Fig. 7). This range is valid for the assumed range for vCO of 0.4–4  104 m s1 . For CO, the entrainment from above is the dominant factor that is affecting the nocturnal depletion. Therefore, the estimated rate of entrainment and in turn the H2 sink due to deposition does not strongly depend on varying CO deposition velocities but strongly depends on the assumed initial boundary layer height. In spite of the uncertainties associated with our approach, our estimated range of vH2 is in agreement with the literature values of 0–10  104 m s1 (Yonemura et al., 1999) but is lower than the 7  104 m s1 (Conrad and Seiler, 1980, 1985), 2:6  104 m s1 (Simmonds et al., 2000), 4:8  1:3  104 m s1 (Gerst and Quay, 2001) and the recent model results

H2 deposition velocity [10-4 m/s] 4 CO deposition velocity [10-4 m/s]

2120

3

2

1

0 20

40

60

80

100

120

140

boundary layer height [m] Fig. 7. Estimated H2 deposition velocities as a function of the boundary layer height at midnight and the CO deposition velocity.

of Sanderson et al. (2003) of 5:3  104 m s1 . Our presented estimate might be representative for a suburban environment and our selection criteria, i.e. days with maximum temperature above 25 1C and low winds. Since these days might favour reduced soil humidity and dry soils leads to a stronger soil uptake, our estimate could represent an upper limit for the sink strength in Duebendorf. 3.6. ‘Bottom-up’ estimation of anthropogenic H2 emissions in Europe A straightforward approach is applied to qualitatively estimate the anthropogenic H2 emissions in Europe and their progression with time. For this purpose, we use DH2 =DCO ratios and multiply them with the reported CO Expert Emissions used in EMEP models for EU-25 (EMEP, 2005) (see Fig. 8a, b). For the year 2003, the up-scaling results in European-wide (EU-25) H2 emissions from traffic between 370 and 480 Gg and total anthropogenic H2 emissions between 750 and 950 Gg when applying our DH2 =DCO ratio (see Section 3.3) and the DH2 =DCO reported by Schmidt (1974), respectively, for suburban conditions in Mainz, Germany in the 1970s (see Fig. 8d). The ratio from Schmidt (1974) acts as a surrogate for the conditions before the widespread use of vehicle catalysts became more

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

2121

Fig. 8. Temporal evolution of CO and H2 emissions in Europe (EU-25); panel (a) CO emissions from all anthropogenic sectors; panel (b) CO emissions from road transport; panel (c) estimated H2 emissions from all anthropogenic sectors; panel (d) H2 emissions from road transport. CO emissions taken from the EMEP expert emissions (EMEP, 2005). H2 to CO ratios from Schmidt (1974) and the present study were taken for H2 emission estimation. The shaded areas illustrate the range between the two curves.

and more common. Both the present and the ancient ratio were used to illustrate the uncertainty of our approach and to cover the maximum potential range of anthropogenic H2 emissions. Doing the same for the oldest reported EMEP data, we derive former H2 emissions from traffic between 800 and 1040 Gg and 1650 and 2150 Gg from all anthropogenic activities (see Fig. 8c). Therefore, our estimate yields a decrease of the anthropogenic H2 emissions of 37–62%. The shaded area in Fig. 8 highlights the uncertainty of our approach and reveals that the decrease of the anthropogenic H2 emissions is unquestionably existent, despite the large uncertainties of our method. On a global scale, anthropogenic emissions are responsible for 14–28% of the total sources (Novelli et al., 1999; Hauglustaine and Ehhalt, 2002 and references therein; Rhee et al., 2006). When we apply the estimated decrease for Europe to the whole globe, we obtain a reduction of H2 emissions between 5% and 17% within the last 25 years. It raises the important question why the observed

ambient air H2 mixing ratios did not significantly decline during this period. Schmidt (1974) showed with static flux chamber experiments for different temperatures and soils an exponential decrease of the H2 concentration in the chamber. This leads to the conclusion that the uptake obeys first-order kinetics as observed for other soil–atmosphere interactions resulting in lower uptake rates under the presence of lower ambient concentrations. Since the other major sink for H2 , the reaction of H2 with OH radicals, also obeys (pseudo)-first-order kinetics, this sink term could also compensate changes in the source budget. As the OH radical concentration does not reveal a global trend within the last 25 years (Prinn et al., 2005) we do not assume that this source term significantly changed its strength. Another reason might be a compensation due to an increase in H2 emissions from biomass burning. Reliable and precise statistics concerning the trends of changing fire occurrence are still insufficient, the trends are not uniform and a high inter-annual and national

ARTICLE IN PRESS 2122

M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

variability exists (FAO, 2001; Scholes et al., 2003). Since we applied the estimated trend for Europe to the whole globe, changes in anthropogenic emissions in regions outside Europe may have compensated the H2 emission reduction. Therefore, the missing decrease in ambient H2 mixing ratios might be a combination of reduced soil uptake and enhanced emissions due to biomass burning or anthropogenic emissions in regions outside Europe. However, further investigations to understand the H2 budget are required. 4. Conclusions Quasi-continuous measurements of H2 and CO were performed at a suburban sampling site in Switzerland from November 2002 to February 2005. These are the first long-term measurements from a site in Europe predominately influenced by automobile source for over 30 years. Investigations in urban and suburban environments and in emissions from anthropogenic processes become more and more important before the hydrogen economy becomes eventually reality. The analysis of extracted background mixing ratios showed a good agreement with literature data from semi-rural and remote sites. This reveals that the determination of H2 background mixing ratios is also possible for time series in suburban environments. Nevertheless, H2 mixing ratios are predominantly driven by H2 emissions from nearby roads. A H2 versus CO scatterplot identified a slightly non-linear relation with augmented H2 concentration above the linear H2 =CO relationship during more polluted periods. It remained so far unexplained why enhanced pollution levels were biased towards higher H2 =CO ratios even if preliminary results from road tunnel measurements indicate different H2 =CO ratios under various engine conditions (Vollmer et al., in preparation). We extracted molar H2 to CO ratios from traffic from morning rush hour periods in winter that are not very different from measurements in suburban conditions in Germany in the 1970s before catalyst were implemented in vehicles (Schmidt, 1974). Subsequently, the influence of three-way catalysts on the H2 =CO ratio is still an open question even if recent chassis dynamometer measurements revealed large H2 emissions before catalyst light-off (Heeb et al., 2006). A ‘bottom-up’ approach to estimate the temporal evolution of anthropogenic emissions in Europe revealed—despite its uncertainties associated with the simplicity

of the approach—a considerable estimated reduction of anthropogenic H2 emissions during the last three decades. As the ambient H2 concentrations did not simultaneously decline, we suppose that the reduced emissions were either compensated by lowered soil uptake or enhanced emissions due to biomass burning or anthropogenic emissions in regions outside Europe. An evaluation of the local dry deposition potential during the night was performed for warm periods with low wind speeds. Our best estimate ranges at the lower end of the H2 deposition velocities reported in the literature. However, in contrast to previously published studies, our flux calculations were conducted in a suburban environment with partly sealed soil surfaces due to asphalt and concrete cover in the vicinity. Nevertheless, such conditions are representative for frequently prevalent anthropogenically modified environments. This study provides results that can be helpful for assessing the impacts of the potential use of H2 as an energy carrier. However, due to the large number of uncertainties and the expected role that H2 might play in the future energy chain, we claim that further detailed investigations on the H2 budget and the effects of H2 on the environment are required. Acknowledgements This work was financially supported by the Swiss Federal Office for the Environment (FOEN). This work was carried out under the auspices of the 6th EU framework program # FP6-2005-Global-4 ‘EUROHYDROS—A European Network for Atmospheric Hydrogen Observations and Studies)’ and the ‘Competence Center for Energy and Mobility, Switzerland (CCEM-CH)’. We thank Matthias Hill for his technical support. References Abraham, S., 2004. The Bush administration’s approach to climate change. Science 305, 616–617. Arya, S.P.S., 1981. Parameterizing the height of the stable atmospheric boundary layer. Journal of Applied Meteorology 20, 1192–1202. Barnes D.H., Wofsy S.C., Fehlau B.P., Gottlieb E.W., Elkins J.W., Dutton G.S., Novelli P.C., 2003. Hydrogen in the atmosphere: observations above a forest canopy in a polluted environment. Journal of Geophysical Research 108, art.-nr.: 4197, 10.1029/2001JD001199. Bonasoni, P., Calzolari, F., Colombo, T., Corazza, E., Santaguida, R., Tesi, G., 1997. Continuous CO and H2 measurements at

ARTICLE IN PRESS M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124 Mt. Cimone (Italy): preliminary results. Atmospheric Environment 31, 959–967. Conrad, R., Seiler, W., 1980. Contribution of hydrogen production by biological nitrogen fixation to the global hydrogen budget. Journal of Geophysical Research 85, 5493–5498. Conrad, R., Seiler, W., 1985. Influence of temperature, moisture, and organic carbon on the flux of H2 and CO between soil and atmosphere: field studies in subtropical regions. Journal of Geophysical Research 90, 5699–5709. Ehhalt D.H., 1999. Gas phase chemistry of the troposphere. In: R. Zellner (Ed.), Global Aspects of Atmospheric Chemistry. Steinkopff, Springer, Darmstadt, pp. 21–109. EMEP, 2005. Expert emissions used in EMEP models; hhttp:// webdab.emep.int/i. FAO, 2001. Global forest fire assessment. Forest resources assessment programme, Working Paper 55, pp. 495, Rome. Garland, J.A., Derwent, R.G., 1979. Destruction at the ground and the diurnal cycle of concentration of ozone and other gases. Quarterly Journal of the Royal Meteorological Society 105, 169–183. Gerst, S., Quay, P., 2001. Deuterium component of the global molecular hydrogen cycle. Journal of Geophysical Research 106, 5021–5031. Glueckauf, E., Kitt, G.P., 1957. The hydrogen content of atmospheric air at ground level. Quarterly Journal of the Royal Meteorological Society 83, 522–528. Hauglustaine, D.A., Ehhalt, D.H., 2002. A three-dimensional model of molecular hydrogen in the troposphere. Journal of Geophysical Research 107, art-nr. 4330, doi:10.1029/ 2001JD001156. Heeb, N.V., Saxer, C.J., Forss, A.-M., Bru¨hlmann, S., 2006. Correlation of hydrogen, ammonia and nitrogen monoxide (nitric oxide) emissions of gasoline-fueled Euro-3 passenger cars at transient driving. Atmospheric Environment 40, 3750–3763. Khalil, M.A.K., Rasmussen, R.A., 1990. Global increase of atmospheric molecular hydrogen. Nature 347, 743–745. Langenfelds, R.L., Francey, R.J., Pak, B.C., Steele, L.P., Lloyd, J., Trudinger, C.M., Allison, C.E., 2002. Interannual growth rate variations of atmospheric CO2 and its d13 C, H2 , CH4 and CO between 1992 and 1999 linked to biomass burning. Global Biogeochemical Cycles 16, 1048 doi: 10.1029/2001GB001466. Mahrt, L., Vickers, D., 2002. Contrasting vertical structures of nocturnal boundary layers. Boundary-Layer Meteorology 105, 351–363. Moxley, J.M., Cape, J.N., 1997. Depletion of carbon dioxide from the nocturnal boundary layer. Atmospheric Environment 31, 1147–1155. Novelli, P.C., Lang, P.M., Masarie, K.A., Hurst, D.F., Myers, R., Elkins, J.W., 1999. Molecular hydrogen in the troposphere: global distribution and budget. Journal of Geophysical Research 104, 30427–30444. Penner, J.E., McElroy, M.B., Wofsy, S.C., 1977. Sources and sinks for atmospheric H2 : a current analysis with projections for the influence of anthropogenic activity. Planetary Space Science 25, 521–540. Plass-Du¨lmer, C., Brauers, T., Rudolph, J., 1998. POPCORN: a field study of photochemistry in north-eastern Germany. Journal of Atmospheric Chemistry 31, 5–31. Prather, M.J., 2003. An environmental experiment with H2 ? Science 302, 581–582.

2123

Prinn, R.G., Huang, J., Weiss, R.F., Cunnold, D.M., Fraser, P.J., Simmonds, P.G., McCulloch, A., Harth, C., Reimann, S., Salameh, P., O’Doherty, S., Wang, R.H.J., Porter, L.W., Miller, B.R., Krummel, P.B., 2005. Evidence for variability of atmospheric hydroxyl radicals over the past quarter century. Geophysical Research Letters 32, L07809 doi: 10.1029/ 2004GL022228. Reimann, S., Schaub, D., Stemmler, K., Folini, D., Hill, M., Hofer, P., Buchmann, B., Simmonds, P.G., Greally, B.R., O’Doherty, S., 2004. Halogenated greenhouse gases at the Swiss High Alpine Site of Jungfraujoch (3580 m asl): continuous measurements and their use for regional European source allocation. Journal of Geophysical Research 109, D05307 doi: 10.1029/2003JD003923. Reimann, S., Manning, A.J., Simmonds, P.G., Cunnold, D.M., Wang, R.H.J., Li, J., McCulloch, A., Prinn, R.G., Huang, J., Weiss, R.F., Fraser, P.J., O’Doherty, S., Greally, B.R., Stemmler, K., Hill, M., Folini, D., 2005. Low European methyl chloroform emissions inferred from long-term atmospheric measurements. Nature 433, 506–508. Rhee, T.S., Brenninkmeijer, C.A.M., Ro¨ckmann, T., 2006. The overwhelming role of soils in the global atmospheric hydrogen cycle. Atmospheric Chemistry and Physics 6, 1611–1625. Salmond, J.A., McKendry, I.G., 2002. Secondary ozone maxima in a very stable nocturnal boundary layer: observations from the Lower Fraser Valley, BC. Atmospheric Environment 36, 5771–5782. Sanderson, M.G., Collins, W.J., Derwent, R.G., Johnson, C.E., 2003. Simulation of global hydrogen levels using a Lagrangian three-dimensional model. Journal of Atmospheric Chemistry 46, 15–28. Schmidt, U., 1974. Molecular hydrogen in the atmosphere. Tellus 26, 78–90. Schmidt, U., 1978. The latitudinal and vertical distribution of molecular hydrogen in the troposphere. Journal of Geophysical Research 83, 941–946. Scholes, M.C., Matrai, P.A., Andreae, M.O., Smith, K.A., Manning, M.R., 2003. Biosphere–Atmosphere interactions. In: Brasseur, G.P., Prinn, R.G., Pszenny, A.A.P. (Eds.), Atmospheric Chemistry in a Changing World. Springer, Berlin, pp. 19–71. Schuler, S., Conrad, R., 1991. Hydrogen oxidation activities in soil as influenced by pH, temperature, moisture, and season. Biology and Fertility of Soils 12, 127–130. Schultz, M.G., Diehl, T., Brasseur, G.P., Zittel, W., 2003. Air pollution and climate-forcing impacts of a global hydrogen economy. Science 302, 624–627. Science special issue, 2004. Towards a hydrogen economy. Science 305, 957–976. Scranton, M.I., Barger, W.R., Herr, F.L., 1980. Molecular hydrogen in the urban troposphere: measurements of seasonal variability. Journal of Geophysical Research 85, 5575–5580. Seiler, W., Conrad, R., 1987. Contribution of tropical ecosystems to the global budget of trace gases, especially CH4 , H2 , CO, and N2 O. In: Dickerson, R.E. (Ed.), The Geophysiology of Amazonia: Vegetation and Climate Interactions. Wiley, New York, pp. 33–62. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley Inc., New York, pp. 1326. Simmonds, P.G., Derwent, R.G., O’Doherty, S., Ryall, D.B., Steele, L.P., Langenfelds, R.L., Salameh, P., Wang, H.J.,

ARTICLE IN PRESS 2124

M. Steinbacher et al. / Atmospheric Environment 41 (2007) 2111–2124

Dimmer, C.H., Hudson, L.E., 2000. Continuous highfrequency observations of hydrogen at the Mace Head baseline atmospheric monitoring station over the 1994–1998 period. Journal of Geophysical Research 105, 12105–12121. Simmonds, P.G., Derwent, R.G., Manning, A.L., Spain, G., 2004. Significant growth in surface ozone at Mace Head, Ireland, 1987–2003. Atmospheric Environment 38, 4769–4778. Stull, R.B., 1988. An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, Dordrecht, p. 666. Tromp, T.K., Shia, R.-L., Allen, M., Eiler, J.M., Yung, Y.L., 2003. Potential environmental impact of a hydrogen economy on the stratosphere. Science 300, 1740–1742.

Warneck, P., 2000. Chemistry of the Natural Atmosphere. Academic Press, San Diego, p. 927. Warwick, N.J., Bekki, S., Nisbet, E.G., Pyle, J.A., 2004. Impact of a hydrogen economy on the stratosphere and troposphere studied in a 2-D model. Geophysical Research Letters 31, L05107 doi:10.1029/2003GL019224. Yonemura, S., Kawashima, S., Tsuruta, H., 1999. Continuous measurements of CO and H2 deposition velocities onto an andisol: uptake control by soil moisture. Tellus 51B, 688–700. Yonemura, S., Yokozawa, M., Kawashima, S., Tsuruta, H., 2000. Model analysis of the influence of gas diffusivity in soil on CO and H2 uptake. Tellus 52B, 919–933.