Pollutant Emissions of Mobile Air

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Environ. Sci. Technol. 2010, 44, 5277–5282

Fuel Consumption and CO2/Pollutant Emissions of Mobile Air Conditioning at Fleet Level - New Data and Model Comparison MARTIN F. WEILENMANN,* ROBERT ALVAREZ, AND MARIO KELLER ¨ berlandstrasse Empa, Materials Science and Technology, U 129, CH-8600 Du ¨ bendorf, Switzerland

Received December 2, 2009. Revised manuscript received May 5, 2010. Accepted May 13, 2010.

Mobile air conditioning (MAC) systems are the secondlargest energy consumers in cars after driving itself. While different measurement series are available to illustrate their behavior in hot ambient conditions, little data are available for lower temperatures. There are also no data available on diesel vehicles, despite these being quite common in Europe (up to 70% of the fleet in some countries). In the present study, six representative modern diesel passenger cars were tested. In combination with data from previous measurements on gasoline cars, a new model was developed - EEMAC ) Empa Emission model for Mobile Air Conditioning systems - to predict emissions from air conditioning. The measurements obtained show that A/C activity still occurs at temperatures below the desired interior temperature. The EEMAC model was applied to the average meteorological year of a central European region and compared with the US EPA MOBILE6 model. As temperatures in central Europe are often below 20 °C (the point below which the two models differ), the overall results differ clearly. The estimated average annual CO2 output according to EEMAC is six times higher than that of MOBILE6. EEMAC also indicates that around two-thirds of the fuel used for air conditioning could be saved by switching the MAC system off below 18 °C.

Introduction The impact of mobile air conditioning (MAC) systems on fuel consumption, CO2, and pollutant emissions of vehicles is an issue that has been discussed for nearly two decades (1-2). With the increasing significance of global warming and the high market penetration of MAC, the CO2 emissions of the latter are also highly relevant. In recent years, the main focus has been on improving the technical efficiency of MAC (3) (4) and on replacing refrigerants (5) - (7). Legislation (8) (9) stipulating maximum extra emissions for MACs and manufacturers developing more efficient technologies are both contributing to this process. However, despite MOBILE6 (10) monitoring and modeling the real-world emissions of MACs in 2000, few studies have since been published on these extra emissions. Many modellers currently use MOBILE6 all over the world, including Europe (11). It is also worth mentioning that the MAC submodel of the new MOVES (Motor Vehicle Emission Simulator) of US EPA (12) is based on MOBILE6. * Corresponding author phone: +41 44 823 46 79; fax: +41 44 823 40 44; e-mail: [email protected]. 10.1021/es903654t

 2010 American Chemical Society

Published on Web 06/08/2010

The GREEN-MAC-LCCP model (13) aims to estimate the lifecycle emissions of individual vehicle MACs and thus cannot be directly compared with the fleet-and-tailpipeoriented EEMAC (Empa Emission model for Mobile Air Conditioning systems) discussed here. For Europe, with its different climates and varying vehicle technologies, most studies are based on data covering only a small temperature range or older vehicle technologies (14) (15). MOBILE6 and also the model used in ref 16 are based only on measurements above 22 °C and assume that MACs do not run at lower temperatures. The authors showed (17) that all systems in European cars, if left switched on, keep running except below 5 °C to prevent freezing. In this range of 5 °C-18 °C, it is not necessary to cool incoming air to maintain a comfortable temperature. However, cooling to about 3 °C dehumidifies the air for the rare case of windscreen misting. This load on the refrigerant circuit must be taken into account when calculating annual fleet emissions, which EEMAC does. In addition, no data are available for diesel vehicles, where the power for the MACs is delivered with a different efficiency than for gasoline cars. In this paper, the results of the subsequent investigation of ref 17 are presented and EEMAC is adapted to diesel vehicles. Finally, both EEMAC and MOBILE6 are applied to central European climatic conditions, with vast discrepancies in results. New Measurement Results for Diesel Cars. Technically, the European car fleet consists of smaller cars with smaller engines, smaller passenger compartments, and thus smaller MACs than the U.S. fleet (16). A high percentage of these systems are automatic, i.e. the driver only has to choose the desired temperature. Variable-stroke or electrically driven compressors are steadily replacing the belt-driven standard technology. In the context of mapping road traffic emissions for Europe, emissions were collected in tests using a chassis dynamometer in a climatic cell simulating different weather conditions. As it has been shown that the emissions from initially cooling down the overheated passenger compartment are negligible, since the fast warming-up of the engine compensates for the additional MAC-load, tests were carried out only with hot engines and a stabilized interior temperature (17). Test Setup. General Setup. To obtain representative results reflecting the situation on the roads of central Europe, six diesel passenger cars certified to Euro-4 standard were chosen based on official Swiss sales statistics regarding engine size and manufacturer distribution (Table S3 in SI (Supporting Information)). The individual vehicles, with an average mileage of 67,000 km, were borrowed from consenting private owners. A check was carried out as to whether the vehicles were in good working order (visual examination, no leaks, engine and MAC running correctly, no warning lights), but no maintenance took place. The climatic test cell used in this study permits temperatures between -20 and 50 °C ((1 °C). Relative humidity can be set at 5-95% ((5% above 10 °C). Solar lamps were installed to simulate solar radiation. For simplicity’s sake, they were installed to shine from the vehicle’s front end, where the largest window is typically located. An inclination angle of 45° was chosen as average for European latitudes. The solar lamps covered a surface area of 1.7 m2 with radiation of 800 W/m2. As the average car occupancy is 1.3 persons in Switzerland, only the driver was in the car for the test. VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Average extra CO2 emissions from MAC of the diesel test sample in the CADC cycle at different temperatures and humidities. Values are given in % of CO2 released for driving. Various sensors were installed to monitor the behavior of the MAC. The main gauge was mounted on the driver’s headrest and used to obtain the main information on the internal temperature. Additional thin film thermocouples were installed at other passenger positions, in the ventilation ducts, and along the lines of the refrigerant systems. These sensors attached to the lines were only intended to provide qualitative information on the operation of the refrigerant system. Compressor activity (i.e., the compressor-on fraction of time for systems with a clutch) was monitored by collecting the clutch signal, where available. In the case of variable displacement compressors, activity was estimated according to the temperature difference on the refrigerant circuit. All of these signals plus exhaust emissions were recorded at 10 samples/s. The exhaust emissions were also measured in bags in accordance with European Council Directive 70/220/EEC for passenger cars (4). The values for extra emissions generated by MACs were determined as the difference between a test with the MAC off and one with the MAC on. For automatic MACs, the temperature knob was set such that the true temperature at the driver’s head was 23 °C. For manual systems, a good setting for cooling and ventilation was determined before the test. The blower was set at 30-50% of fan speed (e.g., position 2 of 4). The engine was already completely warmed up at the start of the test (oil temperature >80 °C). The test cycle used was CADC, which originates from the EU - ARTEMIS project and represents real-world driving in Europe (18) - (21) (SI Figure S2 and Table S2). It consists of three phases that depict urban, rural, and motorway driving. These tests were run at ambient temperatures of 13, 23, 30, and 37 °C and at relative humidities of 20%, 50%, and 80% as shown in Table S4 in the SI. The lowest test temperature of 13 °C was intentionally chosen to study the situations where the MAC is only needed for windshield defogging.

Results All emission values obtained in the tests are listed in the SI. CO2 and Fuel Consumption. For the tests at 23 °C, the difference between the vehicles is highlighted in Figure S3. The reference tests show the variations in vehicle fuel consumption due to differences in vehicle mass and engine sizes. The MACs are turned off. The differences between the reference tests and those with the MAC on highlight the varying degrees of efficiency of the MAC systems. In order to keep the interior cool, the MACs in vehicles 1, 3, and 5 consume about twice as much extra fuel as the others. Note that for the purposes of the following discussion, fuel consumption and CO2 emissions are considered to be proportional and are treated as synonymous in relative comparisons (0.3148 g fuel corresponds to 1 g CO2), since CO and HC emissions are comparatively low. Figure 1 shows the average extra CO2 emissions generated by MACs at different ambient conditions. The relative difference from the tests with MAC on and off is shown. Similar results to those in ref 17 can be derived: 5278

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• The extra CO2 at 13 °C is 4%, 2.5%, and 1% for urban, rural, and motorway driving, respectively. In ref 17, it was also shown that this extra CO2 is independent of solar radiation. • The values clearly rise with temperature. They are highest in urban and lowest in motorway driving, as it takes longer to cover a given distance in urban areas. • The extra CO2 emissions rise in a roughly linear pattern from 23-37 °C. Pollutant Emissions Results. The relation between MAC activity and CO (carbon monoxide), HC (sum of hydrocarbons), NOx (nitrogen oxides), NO2 (nitrogen dioxide), PM (particulate matter), and PN (particulate number) was also analyzed. Since the absolute values of these pollutants are significantly lower than the CO2 emissions, the scatter of the data is considerably larger. For the CO, HC, and NO2 of all diesels and PM and PN for vehicles with diesel particulate filters (DPF), the scatter is such that no trend can be derived from temperature or humidity. Data are thus only discussed here for NOx (all vehicles) and PM and PN for non-DPF vehicles. The NOx emissions are shown in Figure S4 in the SI. The extra emissions rise exponentially as a function of temperature. In contrast, there is a clear trend toward lower NOx as humidity increases. The reduction in NOx occurs concurrently with increasing humidity since moist air has a higher specific heat capacity than dry air, which induces lower engine temperatures and less NOx emissions are thus produced (22). In contrast, PM and PN data show clear trends toward higher emissions at higher humidities and higher temperatures (Figures S5 and S6 in the SI). EEMAC Applied to Diesel Cars. The model presented in ref 17 for gasoline cars was adapted to the data for diesel cars described above. The rise in CO2 over temperature is suggested as a straight line interpolating the data points at 23°, 30°, and 37° for the temperature range where the interior really needs to be cooled and adding a horizontal line through the data point at 13° (Figure 2, left) for the range where the MAC only runs for demisting purposes. The lower end of this horizontal line is put conservatively at 5 °C, as most MACs have a set point of 3 °C, meaning they run until the evaporator output temperature reaches 3 °C to prevent freezing. Intuitively, it would appear to make more sense if, at lower temperatures, this line were also to rise with temperature. However, there are contradictory arguments: first, the emission values at 23° are not much higher than those at 13°, thus the gradient of that line would be rather small (Figure 1). Second, at these low temperatures, the compressor-on fraction for compressors with a clutch is low (below 20%), meaning that between running phases there are prolonged interruptions where both the refrigerant circuit and the air ducts may reheat to the temperature of the engine compartment. The refrigerant circuit thus mainly runs in a transient start condition (i.e., cooling itself down) rather than in a stabilized mode as at higher temperatures (23). Nevertheless, a sensitivity analysis was performed with different gradients

FIGURE 2. Left: Proposed model structure of EEMAC for extra emissions (relative, in %) due to MAC activity as a function of temperature. Right: Theoretical relationship between relative CO2 emissions and humidity at 30° from ref 17 (blue line), measurement data from diesel cars (asterisk) and proposed model (red line). Note: data are scaled to 50% humidity.

FIGURE 3. EEMAC for extra CO2 emissions caused by A/C activity. for this line at lower temperatures, which yielded little impact (see next section) on the results. The influence of humidity in ref 17 was taken into account by physical reasoning: when warm humid air is cooled to 3 °C, most of the water condenses, which leads to additional thermal load on the refrigerant circuit and the compressor. This additional load is calculated using the classic Mollier diagram. This method was validated with the diesel measurements (Figure 2, right), which demonstrated high agreement. The remaining difference between ref 17 and the diesel data may be caused by different engine and MAC efficiencies of the two fleets of cars. Linear interpolation between the data points at 20%, 50%, and 80% humidity is proposed in order to keep the model simple and as close to measured data as possible. For high ambient temperatures and humidity, the load on the MAC and thus the CO2 emissions are limited, as the refrigerant circuit will run at full load, no longer being able to keep the passenger compartment cool. This limit was estimated by analyzing online data and comparing com-

pressor activity up to 100% and indoor temperatures rising with CO2 emissions. Finally, the EEMAC appears as in Figure 3. The calculation procedure and the parameters of these models are stated in the SI. As the measurement data for the pollutants is much more scattered than for CO2, as these emissions are several orders of magnitude smaller, it was decided to model the extra emissions of the following pollutants only as a function of ambient temperature and not humidity: CO and HC for gasoline vehicles, NOx for all diesels, PM and PN for diesels without DPF (Figures S7 in the SI). Model Comparison. EEMAC was compared to MOBILE6 (10) with regard to two different aspects. First, the two methodologies are discussed, focusing on their advantages and drawbacks. Second, they are applied to a typical European climate situation to compare annual average emissions. To make this comparison meaningful, MOBILE6 was adapted to the two vehicle fleets discussed here. This is VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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necessary, as these two fleets have different characteristics (engine displacement, vehicle mass, etc.) to those of the original MOBILE6 fleet. This adaptation is as most European modellers apply MOBILE6: the full-load points of the compressor activity functions as in MOBILE6 (according to Table 2 and Figure 9 in ref 10, see SI, Figures S8 and S9) are combined with the full-load CO2 and pollutant values of one’s own measurements. Since the emissions due to MAC activity are discussed in relative values, i.e. as a % of the emissions caused by driving, the results are not biased due to this adaptation. Comparison of Model Approaches. MOBILE6 was developed as follows: compressor activity, ambient conditions, and time of day were monitored for a large number of vehicles for several weeks. Compressor activity curves were derived based on ambient conditions. As compressor activity correlates better with the heat index (10) than with temperature, humidity is automatically taken into account. Compressor activity changed with the time of day (solar radiation), thus leading to different activity curves (Figure S9). Next, a set of cars was measured twice on chassis dynamometers, once with the MAC off and once with the MAC operating at full load. Finally, the percentage of compressor activity in a given meteorological situation was multiplied by the emission difference in these tests. This approach offers several benefits: as recordings were carried out in real traffic, genuine human behavior was automatically accounted for. Furthermore, several cars were monitored over several weeks, meaning that the data are statistically robust. Nonetheless, it has certain drawbacks or assumptions that are misleading for the situation in Europe. For instance, as all data originate from ambient temperatures above 20 °C, the effect of MACs running at lower temperatures is disregarded. In combining compressor activity with emission data from full-load tests, it is assumed that a compressor running at 100% puts an equal load on the engine in all weather conditions. The opposite was found in measurements from ref 17, which becomes clearly apparent with regard to the refrigerant circuit: the lower temperature is close to the set point and is thus constant, while the temperatures in the high-pressure range depend on those of the intake air. As the pressure difference follows the temperature difference, the load varies as well. Further, it is assumed that engine efficiency is constant with or without MAC activity. Finally, as this model deals only with simple on-off compressors, it may overestimate the emissions of others such as variable stroke or electric compressors. EEMAC has the following advantages: since emissions are measured directly in tests under different ambient conditions, the intermediate variable of compressor activity is not needed. It therefore works for any technology. Furthermore, as in ref 17 Figure 3, in the case of gasoline vehicles fuel consumption without MAC activity drops as the ambient temperature rises, which is pertinent due to the dethrottling effect at lower air densities. Establishing the difference between a test without MAC activity at one temperature and a test with MAC activity at another temperature would thus yield a biased result, which was avoided here. As tests were performed at 13 °C, the model may be parametrized for lower ambient temperatures. It must be noted as a drawback that driver behavior was simulated, assuming all drivers prefer a temperature of 23 °C, as this is postulated as optimal for working in a sitting position (24). As both fleets of cars used in EEMAC include only six vehicles, the model results are statistically less robust than the MOBILE6 output, although the authors attempted to counterbalance this by selecting the cars representatively, taking into account vehicle and engine size, manufacturer, MAC technology (manual vs automatic, constant stroke vs variable stroke vs electrically driven, etc). As the trends of the 5280

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individual emissions are similar to the average trend obtained in this study, this indicates that the tolerance range is acceptable. Due to the limited number of data points, the decision was taken to use straight lines in the model to avoid implying too great a level of accuracy. The model maps for the MOBILE6 “Morning-Afternoon” case are shown in Figure S10 in the SI. In Figure S11, this case is compared with the EEMAC from Figure 3. For motorway driving, the two models agree closely except for the 20% humidity (rare in Europe). For rural driving, the models agree well for temperatures above 25 °C. However, for 20-25 °C, EEMAC provides higher values. This discrepancy is even greater in urban driving. This difference may be explained as follows: EEMAC data show that the compressor-on fraction rises from motorway to urban. For example, at 23 °C, the average compressor-on fractions are 70%, 56%, and 45% for urban, rural, and motorway situations, respectively (with average velocities of 17, 60, and 120 km/h). The refrigerant circuit is thus less effective when the engine is running at a lower rpm, and the engine runs less efficiently. In MOBILE6, the measured trend of compressor-on fraction over average speed for 31-35 °C coincides very well with the aforementioned percentages but was considered insignificant for the final model. Both models thus show comparable CO2 emissions in the temperature range that formed the main focus of MOBILE6 (above 30 °C). However, at intermediate temperatures down to 20 °C, the discrepancies rise to a factor of 2. Below 20 °C, EEMAC describes notable emissions, while in MOBILE6 activity is assumed to be zero. Comparison of the Fleets Measured. Both models were then applied to the following reference case: the Design Reference Year (DRY), EN ISO 15927-4:2005 of the Swiss Association of Engineers and Architects for the city of Berne, which consists of hourly values for humidity and temperature. Its frequency distribution is shown in Figure S12. For a substantial number of hours (95%), the ambient temperatures are below 20 °C and humidities are around 80%. This data were combined with the average diurnal traffic activity for Switzerland (1), Figure S12. While MOBILE6 was applied as described above, EEMAC was applied twice: “EEMAC full” assumes that everyone with a MAC leaves it running for the whole year. This is quite realistic for automatic systems, as they provide a comfortable environment in both cold and hot weather with the same setting, and the driver therefore sees no reason to change the settings. Furthermore, unbeknownst to the driver, many systems automatically turn the MAC on at ignition under all ambient conditions. “EEMAC reduced” assumes that the driver turns the A/C off as soon as temperatures drop below 18°. This value is chosen as a compromise: without direct solar radiation hitting the vehicle’s occupants, ventilation is sufficient up to 20 °C to keep the internal temperature of the vehicle at 23 °C, whereas with direct solar radiation, cooling down to 16 °C may be necessary. This is confirmed by theory: an outside temperature of 18 °C and, in the worst case, 100% humidity results in values of 23 °C and 70% humidity in the passenger compartment, which is the upper limit of the comfort range (24). Not using the MAC in this case results in interior temperatures of 23 °C and thus does not cause any higher risk of tiredness or discomfort. The small number of cases when the MAC needs to be turned on below 18 °C due to windscreen misting is neglected, as this is fairly seldom. These two extreme approaches are chosen to stress the possible savings of fuel and emissions. The results are shown in Figure 4. Huge discrepancies are evident between the results of the two models for the gasoline vehicles, owing to the differences between the two models in the midto-low ambient temperature range common in

FIGURE 4. Annual average extra CO2 emissions for the different models, applied to the region of Berne.

FIGURE 5. Grand total of relative extra emissions due to MAC activity for the Swiss fleet in 2010. Central Europe. The application of MOBILE6 to European gasoline fleets therefore leads to a clear underestimation of extra emissions caused by MAC. There is also a discrepancy when comparing the EEMAC-reduced approach with MOBILE6, which is mainly due to MOBILE6 neglecting the fact that the extra emissions caused by a MAC depend on the vehicle’s average velocity. For the driver, however, the main message of this finding is that two-thirds of CO2 and fuel consumption from MAC activity could be saved without discomfort by switching off the MAC below 18 °C. The saving would be 7, 1.5, and 0.75% of overall fuel consumption for urban, rural, and motorway driving, respectively. For urban driving, where open windows do not notably increase drag and noise is also acceptable, the MAC could be turned off even up to 23-25 °C with only minor discomfort. This would lead to MAC emissions of only 0.6%, and the saving would rise to 9.3% of annual fuel consumption. For diesel vehicles, MAC emissions are also clearly underestimated by MOBILE6. MOBILE6 and EEMAC-reduced show quite similar values. This indicates that the energy efficiency constant assumed by MOBILE6 holds well for diesel cars, as their part load efficiencies are much closer to full load efficiency than for gasoline cars. Here 3.9, 1.5, and 0.8% of fuel could be saved by switching off the MAC below 18 °C. As indicated above, a drawback of EEMAC is the assumption that MAC emissions are constant over temperatures ranging from 5 °C to approximately 20 °C. To verify the impact of the assumed gradient in this range on the results, the slope of this line was varied once such that the activity at 5 °C was half that of 13 °C, and once such that the activity at 5 °C was zero. In this last (unrealistic) case, the slope becomes steeper than that of the line valid from 23-37 °C. The resulting emissions vary less than 5% as shown in Figure S13; therefore, the sensitivity to that gradient is low. Comparison of Total Fleet. To obtain values for the extra emissions generated by MACs for the actual fleet of a country or region, different additional data (or assumptions) about the composition of the fleet are required. The figures used

in this case are from ref 1, based on official Swiss statistics. They are shown in the SI p S11. Applying these data results in Figure 5. The decision to switch off the MAC below 18 °C has an effect of 4.4% in urban driving. For rural and motorway, the figures are 1% and 0.2%, resulting in 2% for all traffic. The discrepancy between the two models remains. Pollutant Emissions. The annual surplus in pollutant emissions due to MAC activity for the two vehicle fleets discussed above is shown in Figure S14 in the SI. In terms of the CO produced by gasoline, the extra emissions in % are slightly higher than those for CO2. As the EEMAC-full and EEMAC-reduced approaches show similar values, most of the extra CO arises from temperatures above 20 °C. The picture is similar, but the figures are somewhat lower for HC. For diesel vehicles, NOx emissions react more strongly to greater compressor activity than CO2. As with CO2, a large proportion of these extra emissions comes from operating the MAC at lower temperatures and could thus be avoided by turning MACs off below 18 °C. This also holds true for the particles in the non-DPF subgroup. Reductions of 7-15% in urban and 3-5% in rural driving can be achieved.

Discussion New data on the impact of mobile air conditioning (MAC) activity on CO2, fuel consumption, and pollutant emissions of diesel cars representing the European fleet are presented in this paper. Tests were conducted with and without the MAC running at different humidities and temperatures to calculate extra emissions as a function of temperature and humidity. The results obtained confirm the findings of ref 17 for gasoline cars, i.e. that unless manually switched off, MACs remain running at ambient temperatures far below the desired temperature of the passenger compartment. This dehumidifies intake air to prevent possible windscreen misting, although this is seldom. A simple model has been derived to calculate extra emissions and fuel consumption resulting from MAC activity as a function of ambient VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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temperature and humidity. This model takes these emissions into account at temperatures from 5-20 °C. This is in contrast to the MOBILE6 model (10). When the present EEMAC model for gasoline cars is applied to activity and meteorological data for Central Europe, annual average extra emissions of 9.9, 2.8, and 1.25% of CO2 result for urban, rural, and motorway driving, respectively. Moreover, up to two-thirds of these emissions are caused by operating the MAC at temperatures below 18 °C, where mere ventilation is sufficient, and the MAC could thus be switched off without any discomfort to the vehicle occupants. When the MOBILE6 model is applied, the resulting extra emissions are lower by a factor of 3 to 10 and thus greatly underestimate the European situation. If the actual fleet of a country is modeled, the overall influence of MAC activity on relative emissions is somewhat smaller, as the total fleet is not equipped with a MAC system. However, the discrepancies between the two models remain, and 2% of CO2 and fuel could be saved without any discomfort to the vehicle occupants. This is twice the amount of fuel than is saved annually by renewing the fleet. The authors suggest that these findings are used to inform the public but also propose discussions with manufacturers to optimize new MACs such that they turn off completely if no cooling is needed and turn on only when the windscreen mists. Legislation could also support this process. The emissions of those chemicals that show significant trends in relation to MAC activity rise by the same order of magnitude. In the case of NOx for diesel cars, and particulate matter (PM) and particle number (PN) for diesel cars without DPF, turning off the MAC at ambient temperatures below 18 °C would also cut emissions by 4-10%.

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Acknowledgments This work was carried out as part of the European Research on Mobile Emission Sources (ERMES) cooperation project between Germany, Austria, Switzerland, Netherlands, Sweden, Spain, Norway, France, and EU-JRC. The authors wish to thank FOEN, ASTRA, and BFE for funding.

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Supporting Information Available Tables with detailed vehicle and test descriptions, CO2 and pollutant emission values of test series, and model parameters for CO2 and extra emissions caused by MAC activity. This material is available free of charge via the Internet at http:// pubs.acs.org.

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