Emissions of the Refrigerants HFC-134a, HCFC-22 ... - ACS Publications

Feb 27, 2004 - Swiss Federal Laboratories for Materials Testing and Research (EMPA), Überlandstrasse 129, CH-8600 Dübendorf, Switzerland, and School...
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Environ. Sci. Technol. 2004, 38, 1998-2004

Emissions of the Refrigerants HFC-134a, HCFC-22, and CFC-12 from Road Traffic: Results from a Tunnel Study (Gubrist Tunnel, Switzerland) K O N R A D S T E M M L E R , * ,†,‡ SIMON O’DOHERTY,§ BRIGITTE BUCHMANN,† AND STEFAN REIMANN† Swiss Federal Laboratories for Materials Testing and Research (EMPA), U ¨ berlandstrasse 129, CH-8600 Du ¨ bendorf, Switzerland, and School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom

This study presents the quantification of the emissions of the refrigerants CFC-12 (CCl2F2), HCFC-22 (CHClF2), and HFC134a (CF3CH2F) from road traffic in Switzerland. These gases are used as refrigerants in car air conditioning systems (A/C-systems) and in cool aggregates for refrigeration transport. All three substances act as greenhouse gases, and CFC-12 and HCFC-22 are in addition stratospheric ozone depleting chemicals. The measurements have been performed in a highway tunnel in the area of Zu¨ rich and cover a large number of individual vehicles, which are thought to be representative of a typical European car fleet. The average emission rates per vehicle were found to be 1.0 ( 0.2 mg h-1 for CFC-12, 0.6 ( 0.4 mg h-1 for HCFC22, and 6.2 ( 0.8 mg h-1 for HFC-134a. These emission factors have been measured for driving vehicles and represent an average emission rate for all types of vehicles regardless of whether they are equipped with an A/C-unit or not. For an average vehicle equipped with an A/C-unit, these results translate into losses of about 14 mg h-1 for HFC134a and 20-30 mg h-1 for CFC-12, when the estimated distribution of HFC-134a-A/C-units (45%) and CFC-12-A/Cunits (3-5%) in the car fleet were taken into account. The emissions of CFC-12 and HFC-134a were mainly attributed to the losses from A/C-systems of passenger cars, whereas the emissions of HCFC-22 originate from losses of refrigeration systems of transport trucks. The observed emissions are discussed in respect to their environmental impact and compared to the overall greenhouse gas emissions of road traffic.

Introduction Volatile fluorinated hydrocarbons are widely used as fluids for refrigeration and air conditioning. When released to the atmosphere, these compounds are generally very stable * Corresponding author phone: +41 (0)56 310 43 01; fax: +41 (0)1 310 44 35; e-mail: [email protected]. † Swiss Federal Laboratories for Materials Testing and Research (EMPA). ‡ Present address: Paul Scherrer Instititute, CH-5232 Villigen PSI, Switzerland. § University of Bristol. 1998

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toward atmospheric oxidation and therefore accumulate in the atmosphere to high background concentration levels. The title compounds are the most abundant of the chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs) in the atmosphere. Historically, CFCs and HCFC-22 have been used as refrigerants, until it was recognized that these compounds have a severe impact on the stratospheric ozone balance. Therefore, a worldwide phase out of these compounds was decided in the Montreal Protocol (1987). Today, the CFCs have been replaced to a wide extent by chlorine free substitutes, such as the HFCs. Although the HFCs have no direct impact on the stratospheric ozone layer, they are still significant anthropogenic greenhouse gases. Therefore, the HFCs have been included in the Kyoto Protocol (1997) as target compounds to control the anthropogenic greenhouse gas emissions. At present, HFC-134a is the most widely used fluorinated refrigerant. It has been introduced into the market in the beginning of the 1990s, mainly as a substitute for CFC-12. Since then, its production and its release into the environment have been growing steadily (1). With a short time lag, the presence and accumulation of this compound were observed in the troposphere (2). Presently (2000-2002), the tropospheric background concentration of HFC-134a rises by about 4 ppt per year in the Northern Hemisphere and has reached 27 ppt in the beginning of the year 2003 (3). In this study, we focus on emissions of fluorinated refrigerants from road traffic (e.g., from air conditioning in cars and from refrigeration transport), which are among the most important application fields of these compounds. The measurements are performed in a highway tunnel close to Zu ¨ rich in Switzerland and are thought to be representative of typical European car fleets. The Swiss National greenhouse gas emission inventory (4) estimates the amount of HFC134a in car air conditioning (A/C) units as 950 tons for the year 2000, which accounts for roughly 40% of the total banked amount of HFC-134a in Switzerland. Therefore, a detailed understanding of the emission behavior of mobile A/C units is highly desirable to achieve precise emission estimates for these compounds. The former refrigerant used in mobile A/C systems, CFC12, is forbidden by the Swiss regulations in cars produced after October 1994. Newer cars therefore use exclusively HFC134a as a refrigerant in their AC-unit. Throughout this study, we refer to two previous investigations on refrigerant losses from mobile A/C-systems. The study of Siegl et al. (5) determined the emissions of HFC134a from 28 A/C-equipped cars in a stationary condition with their motor and A/C unit turned off. These experimental conditions are reasonable, as a car is parked on average for about 95% of the time. They found that all cars showed significant emissions of HFC-134a (0.4-15 mg h-1) and determined the average emission as 3 ( 3 mg h-1. The study of Schwarz and Harnisch (6) carefully investigated the refrigerant filling levels in 300 A/C-units of used passenger cars from three regions in Portugal, Germany, and Sweden. They quantified the average loss rates of HFC-134a as 6.05 ( 0.5 mg h-1 per A/C-unit. This figure represents an overall emission rate of the fugitive losses, taking into account the whole history of each vehicle. In a previous large field study, Schwarz (7) carefully analyzed the records of more than 1000 service operations on A/C-units in nine recovery/ recycling stations in Germany. Thus, he could additionally estimate the emission rates caused by malfunction or damage of the A/C-units and due to the service operations itself. The average 10.1021/es035324c CCC: $27.50

 2004 American Chemical Society Published on Web 02/27/2004

rate for these emission processes was approximated as 1.8 mg h-1 per A/C-unit. These relatively high refrigerant losses of automobile A/Csystems may be explained by the specific conditions in a vehicle, as the compressor part is driven by the engine of the vehicle and therefore cannot be hermetically sealed. Because of the exposure to vibration and heat, the subunits of an automotive A/C-unit must be connected by flexible hoses, which represent weak points in respect to refrigerant diffusion through the material and leaks in the seals of the tubing connectors.

Experimental Procedures Description of the Tunnel. The Gubrist tunnel is located on the highway by-passing the town area of Zu ¨ rich, Switzerland, in the northwest direction. It is located about 9 km from the city center. The tunnel is a two-bore tunnel with two lanes in each direction and has a length of 3270 m. Each tube has a cross-section of 48 m2. The traffic speed is limited to 100 km h-1. During the measurement period, the measured mean traffic speed was 90 km h-1. On average, about 1740 passenger cars and 165 trucks were passing the tunnel per hour. No forced ventilation was in action during the study; therefore, a natural ventilation was caused by the traffic flow. The measured mean air velocity was 5.7 m s-1. The Environmental Agency of Zu¨rich (AWEL) and the regional air quality association Ostluft perform every other year a measurement campaign in the Gubrist tunnel to detect the development of the emission factors of the major car exhaust gases. In 1993, additionally intensive studies of organic exhaust gases and aerosols were performed at this site (8-12). Sampling and Analysis. This study was performed between September 17 and October 20, 2002. During this period, the measurement was continuously running for 27 days (17 working days and 10 holidays). The sampling position was located about 10 m before the tunnel exit. A 7 m copper tubing and a 5 µm steel filter plate were used as the inlet system. The analyses were performed in situ with an automated gas chromatograph, equipped with a mass-spectrometer detector (GC-MS, Agilent 5793N). This type of instrument has been in operation at Mace Head (Ireland) and Cape Grim (Tasmania) in the Advanced Global Atmospheric Gases Experiment (AGAGE) network for several years (13). Prior to the analysis, substances from 1 L of air are preconcentrated with the help of an automated adsorption-desorption system (ADS) developed at the University of Bristol (14). The concentration unit consists of a multibed adsorbent microtrap. During adsorption, the trap is cooled to about -50 °C by means of thermoelectric coolers. The substances are desorbed by rapid heating (rate ∼58 °C/s) of the microtrap to 235 °C and are transferred into the gas chromatograph without further cryofocusing. The chromatographic separation is achieved with a 120 m × 0.32 mm CP-SIL 5CB capillary column (Chrompack) with a 5 µm film thickness and with a temperature program starting at 37 °C and ramping to 180 °C within 51 min. The individual compounds are detected by single ion mass spectrometry of selected characteristic ions. The selected fragment ions for the main target compounds are m/z ) 69 and 83 for HFC134a; m/z ) 85 and 87 for CFC-12; and m/z ) 51, 67, and 69 for HCFC-22. The sampling period was 20 min. By this analytical method, in total about 20 individual halogenated hydrocarbons and some C4-C8 hydrocarbons are automatically analyzed every 2 h. Each ambient air measurement is bracketed with measurements of a field air standard in the concentration range as present in the tunnel air. The real air standards are prepared by pumping ambient air at Du¨bendorf, a town in the agglomeration of Zu ¨ rich, with a specially cleaned scuba diving pump (RIX Industries, SA-3) into 35 L

SUMMA treated canisters (Essex). These field standards are either calibrated against the standard scale from the Scripps Institution of Oceanography (SIO98) for the halocarbons CFC12 and HCFC-22, against a diluted standard from Bristol University, propagated from a gravimetrically prepared primary standard with a stated accuracy of (1% (Linde Gases) for HFC-134a, or against a 30-component standard containing benzene and toluene in the lower ppb range (National Physical Laboratory, NPL). At the entrance of the tunnel, no automated analyses of the halocarbons were performed. Therefore, air samples have been taken by pumping air at a constant rate through a mass flow controller into air sampling canisters. The sampling position was about 200 m inside from the entrance portal of the tunnel. A total of 16 air samples with individual sampling periods between 20 min and 5.6 days have been taken. Nine of these tanks had a sampling time of 20 min and were taken simultaneously to individual GC-MS measurements at the tunnel exit, and seven tanks have been sampled for longer periods (6 and 10 h and 1.0, 2.7, 2.9, 2.9, and 5.6 days) to obtain average refrigerant concentrations at the tunnel entrance. The total sampling time of all canisters was 16 days. The air samples have been analyzed offline in the laboratory by the GC-MS method previously described. Characterization of the Swiss Car Fleet. Switzerland’s car fleet currently comprises about 3.7 million passenger cars and about 300 000 delivery vans, trucks, and buses. The age pattern of the passenger car fleet shows that around 41% of the passenger cars are older than 8 years (15). For delivery vans, trucks, and buses, around 45% are older than 8 years. These figures represent the fractions of the cars, which were set in operation before 1994, when a national legal rule, which placed a ban on the import of cars equipped with CFC-12 air conditioning systems, went in force. For automotive A/Cunits using CFC-12 as a refrigerant, refilling and servicing is still allowed until 2004 in Switzerland; therefore, many of the cars built before 1994 still use CFC-12 as a refrigerant. On the basis of the turnover rate of the car fleet and the fraction of the new released cars equipped with an A/Csystem, the Swiss greenhouse gas inventory (4) estimates the fraction of passenger cars equipped with an A/C-system as about 45% in the year 2002. Most of these A/C-units use HFC-134a as a refrigerant, as A/C-units became a standard car equipment in the second half of the 1990s, when HFC134a was the uniquely used refrigerant. Using the turnover rates of the individual vehicle categories and estimations for the fraction equipped with A/C-units in the years before 1994, it can be estimated that only about 3-5% of the passenger cars still use CFC-12 as a refrigerant in the year 2002. This estimation assumes that the conversion or replacement of the older A/C-units to the HFC-134a units is negligible. A study from Germany (16), which found that only 7% of the CFC-12 units in cars have been converted or replaced during a 30 month interim regulation period after the prohibition of CFC-12 in automotive A/C-units, confirms this assumption.

Results and Discussion Figure 1 shows the measured mixing ratios of HFC-134a, CFC-12, HCFC-22, and benzene at the exit of the Gubrist tunnel from Friday, September 20 to Monday, October 1. The presented compounds have been selected as they are used as refrigerants in mobile A/C-systems (CFC-12 and HFC134a) and for transport refrigeration (HCFC-22). Benzene and toluene are included in the discussion, as these gases are representative car exhaust gases and therefore allow a comparison of the results with other traffic emission studies. It is obvious that all gases show very variable mixing ratios with usually higher concentrations during daytime than during night and early morning. This behavior is quite opposite to the observed occurrence of these compounds VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Time series (black line) of refrigerant and benzene mixing ratios in the Gubrist tunnel exit from September 20-30. The dashed line represents the mixing ratios normalized to the mean air flow through the tunnel. The bold horizontal lines represent concentrations from tanks sampled at the entrance of the tunnel during the indicated time. Panel A: (black line) mixing ratios of HFC-134a; (gray line) total vehicle number per 5 min. The mixing ratio beyond the scale on September 25 is 823 ppt. Panel B: (black line) mixing ratios of CFC-12; (gray line) air velocity in the tunnel. Panel C: (black line) mixing ratios of HCFC-22; (gray line) number of heavy duty vehicles per 5 min. The mixing ratios beyond the scale on September 20, 25, and 26 are 1010, 558, and 398 ppt, respectively. Panel D: (black line) mixing ratios of benzene; (gray line) number of passenger cars per 5 min.

TABLE 1. Average Emission Factors for Refrigerants and for Aromatic Compounds Per Vehicle and Mean Mixing Ratios of the Compounds in the Tunnel entrancee compound

emission factora,b [mg h-1]

emission factora,b [mg km-1]

HFC-134a CFC-12 HCFC-22 benzene toluene

6.2 ( 0.8 1.0 ( 0.2 0.6 ( 0.4 240 ( 10 590 ( 20

0.069 ( 0.009 0.011 ( 0.002 0.006 ( 0.004 2.7 ( 0.1 6.4 ( 0.3

exitf

mean mixing ratio in the tunnelc [ppt] 84 559 209 1440 3760

emission factor derived from correlation methoda,d [mg km-1] 0.07 ( 0.01 (R2 ) 0.22) 0.016 ( 0.005 (R2 ) 0.13) -0.1 ( 0.2 (R2 ) 0.00) 3.5 ( 0.2 (R2 ) 0.86) 8.4 ( 0.5 (R2 ) 0.79)

186 572 219 6600 14400

a Error limits are for 95% confidence intervals. b Results from more than 180 GC-MS measurements (∼150 000 cars). c The current tropospheric background mixing ratios are as follows (3): HFC-134a: 24.5 ppt; CFC-12: 536 ppt; and HCFC-22: 160 ppt. d Results from more than 300 GC-MS measurements (∼240 000 cars). e Canister measurements. f Online GC-MS measurements.

during ambient air field studies in the Zu¨rich area, where the compounds showed enhanced concentrations during night and early morning due to the meteorological conditions over this type of terrain. For benzene, a reasonably good correlation of the concentration and the traffic density in the tunnel was observed, whereas the correlations for HFC-134a, HCFC-22, and CFC-12 are weaker. Taking HFC-134a as an example, it can be seen that occasionally very high concentrations of the compound occur (e.g., September 25, see figure caption for the exact concentration), which do not correlate with the simultaneous car density. The most likely explanation is that one or a few cars passing during a sampling period (on average 790 ( 470 different vehicles were recorded per sample) showed extraordinary high refrigerant losses, likely exceeding the average emission per car by a factor of up to an order of a thousand. This implies that the refrigerant emissions of road traffic are highly influenced by single cars having extraordinary leaking refrigerant circuits. On the other hand, Siegl et al. (5) showed on an ensemble of 28 cars that significant refrigerant losses are likely to occur from every single A/C-system. Average emission factors can be calculated from each sampling period, on the basis of the differences in the concentrations at the exit and the entrance of the tunnel, the measured air flow in the tunnel, and the traffic density using eq 1:

EFmass/distance )

∆CpVairtsampleAtunnel nvehiclesltunnel

(1)

where EFmass/distance is the mean emission factor per car for a given compound expressed as emitted mass per traveled distance, ∆Cp is the difference in concentration of the pollutant (exit-entrance), Vair is the air velocity in the tunnel, Atunnel is the tunnel cross-section, nvehicles is the number of vehicles crossing the tunnel per time, tsample is the air sampling period for the chemical analyses, and ltunnel is the length of the tunnel. While for exhaust gases the expression of emissions per traveled distance is meaningful, for refrigerants the losses per time (eq 2) seem favorable:

EFmass/time ) VtrafficEFmass/distance

(2)

where Vtraffic is the simultaneous determined mean traffic speed in the tunnel. In a first approach, the results from the canister measurements at the entrance of the tunnel and the mean values of simultaneous online GC-MS measurements at the exit are used to calculate the emission factors. The resulting average emission factors for the whole car fleet are shown in Table 1, columns 2 and 3. In a second step, a correlation model was applied. This method uses daily mean values for the pollutant concentration at the tunnel exit and of the traffic density as bases and

correlates the deviation of each single measurement period from these base values as given in eq 3:

{( ) (

CP(t) - Cp ) EFmass/distanceR

)}

nvehicles nvehicles (t) Vair Vair

(3)

where

R)

ltunnel ) 0.0528 m-1 s-1 Atunneltsample

(4)

and Cp is the daily mean concentration of the pollutant at the tunnel exit, and (nvehicles/Vair) is the daily mean of the given ratio. The aim of this model is to achieve a higher time resolution than when using the canister measurements at the tunnel entry, to be able to detect the correlation of the measured concentrations with the traffic density and with secondary parameters, as vehicle classes and meteorological parameters. Daily means are used as base values, as the analysis of the GC-MS measurements for halocarbons not emitted by cars and the data from three air pollution monitoring sites lying within 10 km distance from the Gubrist tunnel (NABEL-network (17)) showed two periods with increased air pollution, each lasting for several days, during this campaign, which then can be filtered out by this method. This is a crucial point, as the mean concentrations of the refrigerants in the tunnel (Table 1) are only slightly enhanced as compared to the concentrations we found during measurement campaigns in Zu ¨ rich city center and its agglomeration. This is also evident when comparing the mean concentrations at the tunnel entry and exit, respectively (see Table 1). The disadvantage of the method is that the emissions are overestimated, as also traffic related emissions from outside the tunnel entrance portal are partly included in the calculated emission factors. The overestimation can be calculated from the more precise emission factors for benzene and toluene as about 30%. Figure 2 shows the correlations corresponding to eq 3 for the refrigerants. Table 1, column 6 summarizes the resulting emission factors. For the refrigerants HFC-134a and CFC-12, the correlations with the traffic density are weak, but significant trends were observed for both compounds. For HCFC-22, no correlation with the total vehicle numbers was observed (see Table 1). Figure 2 shows additionally the correlation for the car exhaust gas benzene. As this compound shows a reasonably strong correlation with the traffic density, it is likely that the weaker correlations for the refrigerants are caused by a much broader distribution of the emissions rates for the individual cars. This finding is partly explicable, as only a part of the cars is equipped with air conditioners or refrigeration units. Especially for HCFC-22, it is likely that VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Correlation plots of the measured concentrations of HFC-134a, CFC-12, and benzene with the traffic density and with the air flow through the tunnel. The data are plotted according to eq 3. The value given in parentheses in the equation for the linear regression is the 95% CI interval of the slope. only a negligible fraction of the passenger cars use mixtures containing HCFC-22 as refrigerants in their A/C-system (socalled drop in refrigerants). For HCFC-22, the main traffic related sources are expected to be refrigeration transport vehicles. But from the whole truck fleet in Switzerland, only about 7% are presently registered as refrigeration transporters. Furthermore, the refrigerants used for these vehicles are quite disperse. Although a big fraction may still use HCFC22 or blends of it, newer cooling units use HFC blends or HFC-134a, and likely the refrigerant CFC-12 is also still used in some older vehicles. The highest emission factors of the refrigerants were observed for HFC-134a. This is plausible as a comparatively large fraction of the Swiss car fleet is equipped with HFC134a-A/C-units. In comparison, the average emission factor for CFC-12 is lower by approximately a factor of 6. When taking into account the limited extent of the use of CFC-12 in the Swiss car fleet (e.g., 3-5% of the cars, as estimated previously), we estimate the average loss from a single vehicle with a CFC-12-A/C system as 20-30 mg per hour driving. For an HFC-134a-unit (when 45% of the cars are equipped with a HFC-134a unit, as estimated previously), this loss is estimated as about 14 mg per hour driving (i.e., 6.2 mg h-1 × 100%/45% ) 14 mg h-1). The probably higher losses from 2002

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CFC-12-systems are not unexpected, as experts (18, 19) point out that sealing techniques improved and losses decreased since the introduction of HFC-134a as a replacement for CFC-12. To our knowledge, no measurements of the loss rates for CFC-12 from mobile A/C-units have been published. For HFC-134a, such measurements exist. Siegl et al. (5) quantified the emissions from 28 parked light duty vehicles. Although the selection of the vehicles, comprised of Japanese and American cars, is not very representative for midEuropean car fleets, we like to compare these results with our data under the driving condition. As pointed out by Siegl et al. (5), increased refrigerant pressures by a factor of approximately 4 during A/C-compressor operation, and as pointed out by Schwarz (7), heat evolution and vibrations during driving are likely to enhance the leaking rate of the A/C-units. The emission rates per A/C-unit estimated from this study are about a factor of 5 higher than that found by Siegl et al. for the parked cars (3 mg h-1). When considering these two emission rates, one concludes that the emissions during parking dominate the overall emissions, as vehicles are parked for about 95% of their time. Schwarz and Harnisch (6) received from their investigations of the refrigerant levels in A/C-units of 300 used cars an average emission rate of 6.05 ( 0.5 mg h-1 per A/C-unit for HFC-134a. The selection of the passenger cars corresponds to typical European car fleets and is therefore comparable to the car fleet investigated in the present study. Their average emission rate, which is again likely dominated by fugitive emissions during parking, is by a factor 2.3 lower than the emission estimated per A/C-unit from this study. Beneath the expectations (5, 7) that refrigerant emissions during driving are higher than from stationary vehicles, the incorporation of malfunctioning A/C-units, which have been excluded from the other studies, might be also a reason for the higher emissions found in this study. Furthermore, this study measured the emissions of a car fleet with an average vehicle age of about 10 years (15), whereas the other studies excluded the first generation HFC-134a-A/C-units from their analyses. To gain an insight into the type of sources of the refrigerants, the traffic counts (loop detector) were split into two categories, cars with a total length shorter than 6 m referred later on as passenger cars and vehicles with a length above 6 m categorized as heavy duty vehicles. A multiple linear regression model was used to split the emission factors for the entire fleet (EFfleet) in the two categories, passenger cars (EFpc), and heavy duty vehicles (EFhdv).

EFfleet ) EFpc fpc + EFhdv fhdv

(5)

where fpc and fhdv are the fractions of passenger cars and heavy duty vehicles, respectively, passing the tunnel in a given measurement interval. From previous studies at this site (8-10, 12) and other highway tunnels (20), it is known that due to the low average proportion of heavy duty vehicles, the information on the latter category is too sparse to obtain firm emission factors. The data given in Table 2 for EFhdv are therefore often not significantly (95% CI level) differing from zero. Nevertheless, the values can give an impression on the importance of the heavy duty vehicles for the observed emission factors. For the refrigerant HFC-134a, there seems to be a substantial emission factor for heavy duty vehicles. These emissions stem very likely from refrigeration trucks. For CFC12, we could not find clear evidence for emissions from heavy duty vehicles. Possibly, road transport companies have already changed to refrigerants, which are more readily available. For HCFC-22, the result from the multiple regressions suggests that the major contribution to the emissions stem from heavy duty vehicles. As only a small fraction of

TABLE 2. Average Emission Factors for Refrigerants and for Aromatic Compounds Attributed to Passenger Cars and Heavy Duty Vehiclesa passenger cars [mg km-1] compound HFC-134a CFC-12 HCFC-22 benzene toluene a

heavy duty vehicles [mg km-1]

emission factors derived in this study 0.06 ( 0.02 0.016 ( 0.005 -0.01 ( 0.01 0c 3.7 ( 0.2 9.1 ( 0.6

Errors correspond to 95% CI levels.

0.2 ( 0.1 0.01 ( 0.04 0.09 ( 0.10 0.07 ( 0.09c 1.7 ( 1.6 0.50 ( 4.5 b

passenger cars [mg km-1]

heavy duty vehicles [mg km-1]

emission factors derived from Gubrist tunnel study in 1993b

15 ( 2 29 ( 5

2 ( 10 -2 ( 20

From Staehelin et al. (9). c In this approach, the emission factor for passenger cars was set to zero.

the heavy duty vehicles are refrigeration transporters and not all of them use HCFC-22 as a refrigerant, the emission factor for HCFC-22 is highly uncertain, and a much longer measuring period is required to obtain precise results. For benzene and toluene, the emissions of heavy duty vehicles are significantly lower than for the passenger cars. This finding corresponds to study of Staehelin et al. (9) performed in the same tunnel in 1993, who related the effect to the difference that passenger cars are mainly powered by gasoline engines, while the heavy duty vehicles are powered by diesel engines. The emission factors for benzene and toluene derived from the Gubrist study in 1993 are given in Table 2 for comparison. The much lower emission factors for passenger cars found in this study were expected because of technical improvements (catalysts, gasoline formulation) in the car fleet (21). This distinct reduction of the organic exhaust gases is indeed noteworthy, but this topic will be investigated separately in a tunnel study at this site in 2005. In addition to the three refrigerant gases discussed, we measured the concentrations of a broad range of halogenated hydrocarbons that potentially might be used in technical car equipment parts (e.g., SF6, HCFC-141b, HCFC-142b, CFC113, CFC-114, CFC-115 CH3CCl3, and HFC-152a) or might be produced from fuel combustion (CH3Br, CH3Cl, CH2Cl2, and CHCl3). None of these compounds exhibited enhanced concentrations at the tunnel exit or any correlation with the traffic. Sporadic concentration peaks were, however, detected for a broad range of the technical gases (notably for HFC-125 and CH2Cl2), but these peaks were thought to occur rather from highly specialized equipment or transport goods and cannot be attributed to any type of vehicle. The data for CFC-11 and Halon-1211 were not exploitable, as we found emissions from permanent installations in the tunnel. Environmental Implications. The discussed refrigerants have two major effects on the environment. First, they are powerful greenhouse gases, which interfere with the radiative balance of the earth (22). Second, the older refrigerants CFC12 and HCFC-22 are long-lived chlorine-containing compounds, which enhance the chlorine loading of the stratosphere and lead to ozone loss by known catalytic cycles (23). With respect to the first issue, the most important greenhouse gases emitted from motor vehicles are carbon dioxide, methane, nitrous oxide, and the refrigerant gases reported in this study. The efficiency of the individual gases in respect to global warming is often reported relative to CO2. These relative global warming potentials (direct GWP; 100 years time horizon (22)) are estimated as 1 for CO2, 21 for CH4, 310 for N2O, 8200 for CFC-12, 1300 for HFC-134a, and 1500 for HCFC-22. Because of its high emissions, CO2 is by far the most important greenhouse gas. With the present fuel consumption, Swiss CO2 emissions from road traffic are presently estimated as 13 700 ktons per year (4). When applying the overall fugitive emissions found in this study to the whole Swiss vehicle fleet, the annual emissions of HFC-134a would account for 280 ( 40 ktons of CO2

equivalents. For CFC-12 analogously, a total annual emission of 270 ( 60 ktons of CO2 equivalents results, and for HCFC22, the emissions would account for 29 ( 19 ktons of CO2 equivalents per year. Therefore, the fugitive refrigerant emissions could equal roughly 3.5-5% of the CO2 emission of the Swiss car fleet in respect to global warming impact. This estimation represents an upper limit, as the investigation of Schwarz and Harnisch (6) showed that the overall losses of HFC-134a are lower than the emissions observed in this study from driving cars. Their results applied to the Swiss car fleet (about 1.8 million A/C-units) would result in a total annual emission of HFC-134a accounting for about 120 ktons of CO2 equivalents. For comparison, in 1997 a highway tunnel study from Germany (24) quantified the global warming impact of the N2O emission from cars as about 1-3% of that of the CO2 emitted by the vehicle fleet. CH4 emission data, while sparse (25), show that the CH4 emissions from road traffic are of minor importance. The major road traffic emissions leading directly to the depletion of the stratospheric ozone are restricted to the chlorinated fluorocarbons CFC-12 and HCFC-22. The impact of the different gases on the ozone layer is often quantified by calculating their ozone depletion potential relative to that of CFC 11 (23). The relative ozone depletion potentials are estimated as about 1 for CFC-12 and about 0.05 for HCFC22. Therefore, it appears that CFC-12 is by far the most important ozone depleting chemical emitted from the Swiss car fleet. When applying emission rates found in this study on the Swiss car fleet, a total annual emission of 33 tons of CFC-12 per year can be estimated, which again represents an upper limit, as the emissions rates during driving are likely exceeding the emissions of stationary cars. From three measurement campaigns in 2002 in Zu ¨ rich, in the Zu ¨ rich city agglomeration (Du ¨ bendorf), and at a rural site (Rigi mountain, 1030 m above sea level), which is exposed to a wider region of the populated Swiss plateau, we can roughly estimate the total CFC-12 emissions relative to the emissions of carbon monoxide, benzene, or HFC-134a, for which the Swiss Agency for the Environment, Forests and Landscape (SAEFL) keeps emission inventories (389.5 ktons of CO, 1400 tons of benzene, and 260 tons of HFC-134a for whole Switzerland) (4). By this way, we estimate the CFC-12 emissions from all sources to be in the range of 50-150 tons per year in Switzerland. For comparison, Switzerland’s import statistics (26) declared an average annual import of about 50 tons of CFCs for refrigeration applications in the years 19982000. These imported CFCs are likely used to replace part of the CFCs, which have been emitted from refrigeration units during these years. The traffic emissions therefore likely account for a significant fraction of the total CFC-12 emissions in Switzerland. The presently measured CFC-12 emissions from traffic are expected to decay according to the turnover rate of the Swiss car fleet. From the present age distribution of the vehicle fleet, a half-life of the CFC-12 emissions in the VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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order of 3-5 years can be expected. A strict prohibition (27) of the refilling of CFC-12 in A/C-units, which comes in force in 2004, is expected to accelerate this decrease.

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Acknowledgments The authors thank the personnel of OSTLUFT c/o Amt fu ¨r Abfall, Wasser, Energie und Luft des Kantons Zu¨rich (AWEL), for giving us the opportunity to join their traffic emission study and for providing us with the data on traffic and air flow in the Gubrist tunnel and the Swiss Agency for the Environment, Forests and Landscape for helpful discussions.

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Received for review November 27, 2003. Revised manuscript received January 13, 2004. Accepted January 13, 2004. ES035324C