Environ. Sci. Technol. 1999, 33, 4134-4139
Nitrous Oxide (N2O) Emissions from Vehicles K. H. BECKER, J. C. LO ¨ RZER, R. KURTENBACH, AND P. WIESEN* Bergische Universita¨t-GH Wuppertal, Physikalische Chemie-FB9, Gauss-Strasse 20, D-42097 Wuppertal, Germany T. E. JENSEN AND T. J. WALLINGTON Ford Motor Company, 20000 Rotunda Drive, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053
Assessment of the impact of vehicle emissions on the global environment requires accurate data concerning nitrous oxide, N2O, emissions. We report herein “realworld” N2O emissions from road vehicles in a tunnel in Wuppertal, Germany, together with “laboratory” emission measurements conducted at the Ford Motor Company using a chassis dynamometer with a standard driving cycle for 22 different cars and trucks. Consistent results were obtained from both approaches, suggesting that a good approximation of the average emission factor (g of N2O/g of CO2) ) (6 ( 2) × 10-5. This corresponds to an emission rate of 16-8 mg of N2O/km for vehicles with fuel economies of 12-6 L/100 km (20-40 mi/U.S. gal). N2O emissions from vehicles have a global warming impact, which is 1-3% of that of the CO2 emissions from vehicles. We estimate that the global vehicle fleet emits 0.18 ( 0.06 Tg of N2O yr-1 (0.11 ( 0.04 Tg of N yr-1), which represents 2-6% of the atmospheric growth rate of this species. In addition to N2O, laboratory vehicle emission measurements of NH3, HONO, and HCN are reported.
Introduction Nitrous oxide, N2O, is a long-lived (130 years) trace constituent of the lower atmosphere present in a concentration that is currently 313 ppbv and increasing at a rate of 0.5-0.9 ppbv yr-1 (1, 2). As a result of its long lifetime and significant IR absorption, N2O is an important greenhouse gas believed to be responsible for 6% of anthropogenic radiative forcing (1). In addition to its importance as a greenhouse gas, N2O is transported through the troposphere into the stratosphere where it reacts with O(1D) atoms and is the source of stratospheric NOx. Quantification of the role of N2O in atmospheric chemistry and the greenhouse effect requires accurate data for its sources and sinks. Anthropogenic emissions of N2O are associated with biomass burning, fossil fuel combustion, industrial production of adipic and nitric acids, and the use of nitrogen fertilizer. Our interest here is the contribution of vehicular traffic to the global N2O inventory and its effect on potential global warming. It is well-established that vehicle exhaust contains N2O (3-9). Furthermore, it is widely accepted that new vehicles equipped with three-way catalysts generally emit more N2O than older vehicles without catalysts (7-11). Concern has been expressed that N2O emissions will rise substantially as * Corresponding author e-mail:
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the global fleet of old vehicles is replaced with modern vehicles equipped with three-way catalysts to reduce urban air pollution (8). However, the present and future vehicle contributions to the global inventory of N2O are unclear. In 1992, Berges et al. (8) studied N2O emissions from motor traffic through tunnels in Stockholm, Sweden, and Hamburg, Germany, and concluded that catalyst-equipped vehicles emit 106 mg of N2O/km (170 mg of N2O/mi). Extrapolation to the global car fleet led Berges et al. (8) to predict that global N2O emissions from vehicles could reach 6-32% of the atmospheric growth rate. Sjo¨din et al. (9) studied N2O emissions from motor traffic passing though a different tunnel in Sweden in 1992 (in Go¨teborg) and reported an average emission rate of 25 mg of N2O/km. Sjo¨din et al. (9) estimate that the traffic passing through the tunnel consisted of approximately 10% heavy-duty vehicles, 45% catalyst-, and 45% noncatalyst-equipped cars. If we take the extreme assumption that N2O emission from heavy-duty vehicles and noncatalyst-equipped cars is zero, then we arrive at a emission factor of approximately 56 mg of N2O/km (90 mg of N2O/mi) for catalyst vehicles; significantly less than that reported by Berges et al. (8). To better understand the environmental impact of vehicle exhaust, we have conducted field measurements of traffic passing through a highway tunnel in Wuppertal, Germany, and laboratory studies using a chassis dynamometer facility to measure N2O emissions from vehicles. Results are reported herein.
Experimental Section Tunnel Study at Universita1 t Wuppertal. The emission measurements in the traffic tunnel were carried out using a fully automatic gas chromatographic analysis and data acquisition system. Nitrous oxide and CO2 were measured simultaneously using multi-port valves with a 2-m column (Porapak Q 80-100 mesh) with an electron capture detector (ECD) for N2O and a 2-m column (Porapak Q 80-100 mesh) with a thermal conductivity detector for CO2. To avoid contamination by higher hydrocarbons, a 1.3-m precolumn (Porapak Q 80-100 mesh) was used on which the slowly eluting components were backflushed. Air from the tunnel was pumped at 10 L/min through two 1-mL sample loops. The analysis time was 5 min. Chromatograms were recorded using the GC analysis program “BORWIN”. The GC system was calibrated every hour using certified calibration gas mixtures containing CO2 and N2O in quantities comparable to those expected in the tunnel air. The mixing ratios were determined by comparing the peak areas of the sample and standard. To provide a comparison with the tunnel air, a sampling line was installed from the instrument site in the tunnel out into the ambient environment to a site approximately 100 m from the tunnel exit. This sampling site in a forest is not influenced by the traffic emissions. Air was continuously sampled through this line and periodically (approximately hourly) analyzed to provide a comparison with the polluted tunnel air. Measurements were carried out in the Wuppertal “Kiesbergtunnel”. The tunnel has a length of 1.1 km and connects the freeway A46 between Du ¨ sseldorf and Wuppertal with the city of Elberfeld. The tunnel consists of two independent tubes in an east-west direction. Measurements were carried out in the lower (eastbound) tube through which the vehicles enter the city of Elberfeld. Vehicles entering the tunnel from the west encounter a 3.25% incline for 130 m that changes over a distance of 70 m to a 1.0% decline, which is uniform for the following 900 m. Ventilation in the tunnel is achieved 10.1021/es9903330 CCC: $18.00
1999 American Chemical Society Published on Web 10/12/1999
FIGURE 1. Typical weekly traffic volume through the east bound Kiesbergtunnel. by the physical movement of vehicles through the tunnel, which pushes the air mass through the tunnel. The sampling port was located 900 m from the entrance of the tunnel and 200 m before the exit. This site was selected as a compromise between the desire to monitor air as far as possible from the entrance, to sample the most polluted air, and the need to be sufficiently removed from the tunnel exit to avoid possible interference from outside air. It is reasonable to assume that the cars passing the sampling port are under warm driving conditions. The sampling probe was installed 4 m above the road surface and 2 m from the left-hand side of the tunnel wall. We assume here that the air in the tunnel at the monitoring site was well-mixed by the traffic flow, the validity of this assumption is discussed in the Results. In addition to the measurement of N2O and CO2 in the tunnel air, the number of cars passing the tunnel was counted using contact loops. For weekdays, the total number of cars passing through the tunnel was almost constant with a value of 13 317 ( 272 vehicles/day; during weekends, the traffic was lower (see Figure 1). Videorecording was conducted during the entire period of the study and was used to establish the contribution of heavy-duty trucks and commercial vans (10%), diesel-powered passenger cars (25%), gasolinepowered passenger cars (60%), and motorcycles (5%) to the traffic volume. The catalyst-equipped fraction of the passenger car traffic in the tunnel reflected the registration (78%) of gasoline-powered vehicles in the surrounding administrative district (Du ¨ sseldorf). The speed of the vehicles passing through the tunnel was typically 60-90 km h-1 except for congested traffic periods. Vehicle Emission Studies at Ford Motor Company. The emissions measurement sequence consisted of a series of 2-8 U. S. EPA Urban Dynamometer Driving Schedule (UDDS or Federal Test Proceedure-75) tests carried out on nearly consecutive days. Vehicle tests were conducted at the Ford Research Laboratory (Vehicle Emissions Research Laboratory, VERL 1) in Dearborn, MI, on a 48-in., single-roll (BurkePorter, Grand Rapids, MI) chassis dynamometer. The tailpipe of the vehicle was connected to a dilution tube through an insulated, heated transfer line. The dilution tunnel flow rate was 10 m3 min-1 for gasoline-fueled vehicles, 20 m3 min-1 for alternate-fueled vehicles, and 30 m3 min-1 for dieselfueled vehicles. The regulated emissions (non-methane hydrocarbon (NMHC), CO, and NOx) were measured by standard procedures (12). A range of both prototype and production vehicles were tested over a time period covering two years (1996-1997). The cars tested were (in alphabetical order) Ford Crown Victoria, Ford Mondeo (2 vehicles), Ford Taurus, Honda Accord, Lincoln Continental, Lincoln Grand Marquis, Lincoln Mark VIII, Mercury Sable (2 vehicles), Lincoln Town Car (2 vehicles), and Volkswagen Passat. The trucks tested were Ford Explorer (3 vehicles), Ford F-150, Ford Windstar (4
vehicles), and a competitor’s vehicle. All vehicles were equipped with modern three-way catalyst systems. The emission levels of the regulated (NOx, CO, and hydrocarbon) pollutants for these vehicles were generally between those required for compliance with U.S. Tier 1 and California ULEV regulations. For the class of vehicles considered here (passenger vehicles and light-duty trucks), Tier 1 corresponds to 400 mg/mi NOx and 250 mg/mi NMHC emission rates. For ULEV vehicles, the NOx and NMHC emission rates would be below 200 and 40 mg/mi, respectively. Results for testing with five fuels will be reported here, although individual vehicles were tested with only one fuel. They are U.S. Certification fuel (Cert), California Reformulated fuel (CRF), compressed natural gas (CNG), M-85 (85% methanol/15% gasoline) and U.S no. 2 diesel fuel. The gas-phase emission rates of 20 species, including NO, NO2, HONO, HCN, NH3, and N2O were measured in realtime by Fourier transform infrared spectroscopy (FTIR). HONO and HCN were not observed in the emissions of these vehicle above the instrumental limit of detection of 9 and 2 mg/km, respectively. The diluted exhaust sample was filtered through a 142-mm quartz fiber filter (Palliflex, Tissuquartz 2500QAO) with backing in a stainless steel holder at room temperature. A new filter was used for each four UDDS series. The 1.27cm (0.5-in.) o.d. sample transfer lines were either stainless steel or Teflon. A rotary vane pump (Gast 0822) pulled the diluted exhaust sample through the FTIR gas cell at 30 L/min with 93.3 kPa absolute pressure in the gas cell. The background spectra were recorded before and after the sample acquisition. The average of the two background spectra was used to correct the sample data. The high-resolution FTIR spectrometer (Mattson Instruments, Nova-Cygni 120) was equipped with a water-cooled, glow bar source and a liquid-nitrogen cooled, narrow band Hg-CdTe (MCT) detector with linearized preamplifier. The spectrometer was operated at a resolution of 0.25 cm-1, and the interferograms were zero-filled to an effective resolution of 0.125 cm-1. The variable path length, multipass gas cell (Wilks, 9020) with KBr windows was used in the 14th order, resulting in an effective path length of 21.75 m. Data acquisition and processing were controlled by a computer equipped with an array processor (Concurrent, 5450). The system was used to co-add, transform, and analyze spectra for 20 compounds in 3-s intervals. The sample component concentrations were determined from a linear relationship to the spectral line strength of standard reference spectra. A more complete description of this analytical system is available elsewhere (13).
Results Tunnel Study at Universita1 t Wuppertal. Continuous N2O and CO2 measurements were carried out in the Kiesbergtunnel during several weeks in 1997. Figure 2A shows the diurnal variation of the CO2 concentration for two days in March 1997 as an example. As expected a strong correlation between the CO2 concentration in the tunnel air and the number of cars passing through the tunnel was observed. Accordingly, during rush hours in the morning and in the late afternoon, there was a significant increase in the CO2 concentration in the tunnel. Anomalous behavior was observed during several monitoring nights with higher than expected N2O and CO2 concentrations recorded in the tunnel. As described in the Experimental Section, the ventilation in the tunnel is achieved by the traffic flow pushing air through the tunnel. During the night with very low traffic volumes, the ventilation in the tunnel is poorly characterized (or perhaps nonexistent), and the measurements become sensitive to emission sources other than traffic. By comparison with measurements of N2O and CO2 in the ambient air outside the tunnel, we are able to exclude the possibility that the VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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observed in the tunnel during the nighttime hours, we will only consider the time periods between 06:00 and 21:00 where the levels of both CO2 and N2O are correlated with the traffic flow (see Figure 2). Figure 3 shows a plot of ∆N2O versus ∆CO2 for the time period March 20-26, 1997; the line through the data is a linear least-squares fit that gives the emission factor (g of N2O/g of CO2) ) (6.1 ( 1.2) × 10-5. This result can be compared directly to the analogous emission factors of (1.4 ( 0.9) × 10-4 and (6 ( 3) × 10-5 measured for traffic flowing though the Klaratunnel in Sweden in April/May 1992 and through the Elbtunnel in Germany in October 1992 (8), respectively. The emission factor measured in the Kiesbergtunnel in 1997 is indistinguishable from that measured in Elbtunnel in 1992 but is substantially lower than that measured in the Klaratunnel in 1992. To the best of our knowledge, there are no substantial differences in the vehicle operating conditions (vehicle speeds, road grade, temperature, etc.) in the Kiesbergtunnel and those in the Klaratunnel and the Elbtunnel. In Sweden in 1992, passenger cars equipped with catalytic converters made up 33% of the fleet; in Germany in 1992, this fraction was 22%. The tunnel study reported here was performed in 1997 with 78% of the passenger cars equipped with catalysts. It is interesting that there is no discernible difference between the N2O emissions from traffic passing though the Kiesbergtunnel in 1997 and previous measurements in Germany in 1992. This observation goes against the prevailing wisdom that the introduction of catalyst-equipped vehicles should lead to a substantial and measurable increase in N2O emissions from the total vehicle fleet.
FIGURE 2. Measured levels of CO2 (A) and N2O (B) in the Kiesbergtunnel during March 20 and 21, 1997; open symbols show the traffic volume. observed increases are caused by N2O and CO2 sources outside the tunnel. It should be noted that conversion of NO2 via HONO into N2O by heterogeneous processes on a variety of different surfaces is a well-documented phenomenon in laboratory experiments (14, 15). Preliminary analysis of the temporal variation of NO2, HONO, and N2O levels in the tunnel during the night when ventilation in the tunnel is poor suggests that N2O may be formed in the tunnel by such heterogeneous processes. Figure 2B shows the N2O measurements in the tunnel on March 20 and 21, 1997. When comparing the data scatter in Figure 2, panels A and B, it should be realized that the emission of CO2 from vehicles is approximately 4 orders of magnitude greater than that of N2O. Hence, the concentration units are ppmv in panel A and ppbv in panel B, and the observed CO2 levels are typically 20-40% greater than ambient whereas only 2-5% greater for N2O. Accordingly, taking into account the accuracy of the analytical technique used, the error in the measured N2O concentration is larger than for CO2. However, as shown in Figure 2B, a good correlation between the N2O concentration in the tunnel air and the traffic density was observed for rush hours in the morning and in the late afternoon. While a significant decay of the N2O concentration and the number of cars passing the tunnel was observed in the late evening hours, the N2O concentration often increased again during nighttime although the traffic density remained low. If one assumes that the excess in the CO2 and N2O concentration, ∆CO2 and ∆N2O, in the tunnel air in comparison with that measured in the ambient air outside the tunnel during weekdays is proportional to the emission strength of the corresponding species, an emission ratio can be easily calculated. To avoid complications associated with the interpretation of the anomalously high N2O levels 4136
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Jimenez et al. (16) have reported an emission factor of (9.7 ( 1.9) × 10-5 (g of N2O/g CO2) from real-world cross road tunable infrared diode laser measurements of 1391 catalyst-equipped gasoline cars in Los Angeles in 1996. Heavyduty and noncatalyst vehicles emit less N2O than catalystequipped gasoline cars. It is reasonable that the emission factor of (6.1 ( 1.2) × 10-5 measured here for total traffic flow is lower than that measured for catalyst-equipped gasoline cars. Vehicle Emission Studies at Ford Motor Company. The results of the vehicle emission studies are given in Table 1. This data set lists the NMHC, CO, CO2, NOx, NO, NO2, N2O, and NH3 emissions from a wide variety of different vehicles operated using several different fuels. In addition to the species listed in Table 1, IR features attributable to HONO and HCN were searched for, but were not observed above the instrumental limits of detection of 9 and 2 mg/mi, respectively. The data in Table 1 provides a comprehensive picture of the emissions of N-containing compounds from modern vehicles. The emissions of the oxides of nitrogen for this group of vehicles, while the vehicles are exercised to the Federal Test Procedure portion of the Urban Dynamometer Driving Schedule, are generally similar for cars and trucks, as seen in Table 1. Nitrous oxide, N2O, is not measured by the chemiluminescent detector used to quantify vehicle emissions of nitrogen oxides (NOx). The FTIR provides an independent measure of N2O. The contribution of N2O to the total oxides of nitrogen emissions is about 10% for both cars and lightduty trucks for the vehicles and fuels reported here. N2O can reach a maximum of 25% of the total oxides of nitrogen and go as low as 5%. For diesel, it is only about 1% of the oxides of nitrogen emitted from the vehicle tested. This lower emission for diesel is probably related to differences in the oxygen availability and the combustion process. The N2O emissions from the 22 vehicles (all modern and equipped with catalytic converters) studied here fall in the range of 2-32 mg km-1 and are of a magnitude consistent with previous dynamometer measurements (3, 5, 7, 17).
FIGURE 3. Excess N2O versus excess CO2 concentration in the tunnel air (March 20-26, 1997). The line is a linear least-squares fit.
TABLE 1. Vehicle Tailpipe Emission Data vehicle identifier
fuela
car A car B car C car D car E car F car G car H car I car J car K car L car M truck A truck B truck C truck D truck E truck F truck G truck H truck I
Cert Cert CRF CRF CRF CRF CRF CRF CRF CNG M-85 diesel diesel Cert Cert Cert CRF CRF CRF CRF CRF CNG
car truck
average average
NMHC (mg/km)
CO (g/km)
CO2 (g/km)
NOx (mg/km)
NO (mg/km)
NO2 (mg/km)
N2O (mg/km)
NH3 (mg/km)
63 ( 16 67 ( 11 85 ( 2 66 ( 2 97 ( 6 41 ( 2 94 ( 6 31 ( 3 14 ( 3 8(1 18 ( 2 107 ( 19 116 ( 86 54 ( 11 61 ( 2 51 ( 3 83 ( 3 43 ( 2 83 ( 6 78 ( 8 61 ( 3 18 ( 2
0.84 ( 0.07 0.23 ( 0.02 1.16 ( 0.02 0.68 ( 0.02 0.59 ( 0.04 0.53 ( 0.02 0.86 ( 0.02 0.23 ( 0.02 0.15 ( 0.05 0.23 ( 0.02 0.59 ( 0.05 0.30 ( 0.02 0.53 ( 0.23 0.44 ( 0.06 1.16 ( 0.07 0.44 ( 0.03 2.16 ( 0.06 0.48 ( 0.04 0.69 ( 0.05 0.97 ( 0.06 1.36 ( 0.09 0.32 + 0.09
293 ( 2 288 ( 5 239 ( 1 258 ( 2 285 ( 1 295 ( 3 286 ( 2 305 ( 4 217 ( 2 247 ( 2 244 ( 9 193 ( 11 161 ( 2 394 ( 3 289 ( 4 293 ( 4 259 ( 2 268 ( 2 339 ( 2 345 ( 2 381 ( 40 329 + 4
134 ( 11 146 ( 30 205 ( 13 91 ( 6 24 ( 4 74 ( 8 124 ( 4 44 ( 2 55 ( 3 6(2 86 ( 6 475 ( 25 804 ( 8 69 ( 12 262 ( 32 147 ( 22 430 ( 6 59 ( 9 77 ( 12 77 ( 5 210 ( 8 102 ( 10
92 ( 8 71 ( 18 157 ( 8 57 ( 6 15 ( 3 50 ( 5 76 ( 4 29 ( 3 33 ( 4 8(2 17 ( 4 315 ( 15 492 ( 56 45 ( 4 177 ( 21 91 ( 14 298 ( 8 36 ( 5 68 ( 14 52 ( 3 130 ( 2 69 ( 14
ndb 5(2 3(1 2(1 2(1 nd 3(1 nd 2(1 4(1 nd 17 ( 2 90 ( 78 nd 3(1 2(1 4(1 2(1 3(1 3(1 2(1 nd
21 ( 2 11 ( 2 25 ( 2 8(1 5(2 10 ( 1 32 ( 2 9(2 4(1 2(1 4(1 4(1 14 ( 9 6(1 22 ( 3 7(1 27 ( 1 4(2 11 ( 2 17 ( 2 17 ( 2 24 ( 4
49 ( 6 3(3 65 ( 4 3(1 81 ( 3 10 ( 4 31 ( 3 28 ( 1 11 ( 2 44 ( 4 29 ( 4 nd nd 11 ( 2 59 ( 3 13 ( 2 48 ( 3 19 ( 1 104 ( 18 43 ( 4 44 ( 2 84 ( 11
0.53 0.89
255 322
174 160
109 107
14 3
11 15
62 59
32 47
a Fuel identification: Cert, U.S. certification fuel; CRF, California reformulated fuel; M-85, a mixture of 85% methanol and 15% gasoline (by volume): diesel, U.S. no. 2 diesel fuel; CNG, compressed natural gas. b nd, not determined.
The best correlation between N2O emissions and any of the parameters listed in Table 1 was between N2O and CO emissions (see Figure 4A). A linear least-squares fit gives N2O emission rate ) (0.017 ( 0.004) × CO emission rate. The correlation between N2O and CO may simply reflect the oxygen availability on the catalyst. The aim of the present work is to provide an assessment of the global contribution of vehicle exhaust to N2O emissions. While the correlation between N2O and CO emissions is interesting, it is not easy to provide an accurate global estimate of CO emissions from
vehicles. In contrast, it is relatively straightforward to calculate the global CO2 emission from vehicles. Hence, even though there are clearly large differences between individual vehicles, it is more useful to quote an average N2O emission versus CO2 emission rate. Figure 4B shows a plot of N2O emission versus CO2 emission rates. Linear least-squares analysis (forced through the origin) gives N2O emission rate ) (4.5 ( 1.1) × 10-5 × CO2 emission rate, quoted uncertainties are 2 standard deviations. This emission rate measured under laboratory conditions with well-maintained vehicles is VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Vehicle emissions of N2O versus CO (A), and CO2 (B)s see Table 1. indistinguishable from the emission factor of (6.1 ( 1.2) × 10-5 measured under real-world conditions in the Kiesbergtunnel.
Conclusions Real-world N2O emission measurements from traffic passing through a tunnel in Wuppertal, Germany (13 000 vehicles/ day), together with laboratory chassis dynamometer emission measurements at the Ford Motor Company using 22 different cars and trucks provide a consistent picture of the emission of N2O from modern vehicles. The emission factor (g of N2O/g of CO2) measured in the real-world was (6.1 ( 1.2) × 10-5 while that measured in the laboratory was (4.5 ( 1.1) × 10-5. In view of the data scatter and uncertainties inherent in the measurements, we recommend the use of an emission factor of (6 ( 2) × 10-5 when calculating traffic-related N2O emissions for emissions inventory purposes. This recommended range is consistent with the overall emission factor measured by Berges et al. (8) in 1992 in a tunnel study in Germany of (6 ( 3) × 10-5 but is substantially lower than the range of (1.4 ( 0.9) × 10-4 derived from a similar study in Sweden (8). It is useful to place these emission rates into perspective in terms of vehicle contributions to the global N2O budget and to radiative forcing of climate change. The contribution of global vehicular traffic to the global N2O budget can be estimated using two different approaches. First, we could assume that the N2O observed in the tunnel studies is attributable solely to catalyst-equipped passenger cars, calculate the emission factor for such vehicles, and combine this result with an estimate of the global fuel consumption of catalyst-equipped passenger cars. Second, we could assume that the vehicle mix traveling through the Wuppertal tunnel is representative of the global vehicle population and 4138
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simply multiply the measured emission factor by the global vehicle fuel consumption. For simplicity and because the tunnel measurements do not support the assumption that the N2O is attributable solely to catalyst-equipped passenger cars, we choose to adopt the second approach. Using values of 964 Tg (1 Tg ) 1012 g) for the annual global vehicle fuel consumption in 1995 (637 Tg of gasoline, 327 Tg of automotive diesel (18)), 0.855 for the average carbon content of gasoline by mass (19), 44 for the molecular weights of N2O and CO2, 12 for the atomic weight of carbon, and (6 ( 2) × 10-5 for the emission factor of N2O, we arrive at an estimate for the contribution of vehicular traffic to the global N2O budget of 964 × 0.855 × (44/12) × (6 ( 2) × 10-5 ) (0.18 ( 0.06) Tg as an upper limit. Atmospheric levels of N2O are increasing at a rate of 4.7 ( 0.9 Tg yr-1 (3.0 ( 0.6 Tg of N yr-1) (8). Hence, emissions from the global vehicle fleet represent approximately 2-6% of the atmospheric growth rate of N2O. The global warming potential of N2O is 330 times that of CO2 (1). Using an emission factor (g of N2O/g of CO2) of (6 ( 2) × 10-5, it follows that N2O emissions from vehicles have a global warming impact that is 1-3% of that of the CO2 emitted from vehicles. We conclude that N2O emissions from vehicles make minor (though nonnegligible) contributions to the global atmospheric N2O budget and to anthropogenic radiative forcing of global climate change. These findings are in disagreement with the conclusions of the previous study by Berges et al. (8), which concluded that if the entire fleet of cars were to be equipped with catalysts the global N2O emissions from vehicles would double and reach 6-32% of the atmospheric growth rate. Berges et al. (8) measured N2O emission rates of (1.4 ( 0.9) × 10-4 and (6 ( 3) × 10-5 for traffic flowing though the Klaratunnel in Sweden in April/ May 1992 and the Elbtunnel in Germany in October 1992, respectively. In 1992, catalyst-equipped passenger cars represented only a small fraction (20-30%) of the total traffic. In 1997, catalyst-equipped passenger cars dominate the traffic volume. Berges et al. (8) assumed that N2O emissions from noncatalyst cars and trucks were zero and derived emission factors by attributing all of the measured N2O to the relatively small number of catalyst-equipped cars. The results from the present work show that assumption is not valid and that N2O emissions from catalyst cars operating under real-world conditions are substantially lower than previously reported. We recommend use of an emission factor of (6 ( 2) × 10-5 in emission inventory calculations for the modern vehicle fleet.
Acknowledgments Financial support for the tunnel study in Wuppertal by the Deutsche Bundesstiftung Umwelt (DBU) under Contract 07352 is gratefully acknowledged. We thank Steve Japar and Steve Pezda (both Ford) for their helpful comments. In addition, we thank the “Landschaftsverband Rheinland/ Rheinisches Strassenbauamt Essen” and the City of Wuppertal for their support. T.J.W. thanks the Alexander von Humboldt Stiftung (AvH) for an AvH fellowship.
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(15) Kleffmann, J.; Becker, K. H.; Wiesen, P. J. Chem. Soc., Faraday Trans. 1998, 94, 3289. (16) Jimenez, J. L.; Nelson, D. D.; Zahniser, M. S.; McManus, J. B.; Kolb, C. E.; Koplow, M. D.; Schmidt, S. E. Proceedings of the 7th CRC On-Road Vehicle Emissions Workshop, San Diego, CA, 1997. (17) Michaels, H. Emission of Nitrous Oxide from Highway Mobile Sources; U.S. EPA Report EPA420-R-98-009; U.S. EPA: Washington, DC, 1998. (18) World Gasoline and Diesel Fuel Survey; Associated Octel: London, 1995. (19) Marland, G.; Rotty, R. M. Tellus 1984, 36B, 232.
Received for review March 24, 1999. Revised manuscript received August 26, 1999. Accepted September 1, 1999. ES9903330
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