Particle emissions from diesel passenger cars equipped with a particle trap in comparison to other technologies

Tail pipe particle emissions of passenger cars, with different engine and aftertreatment technologies, were determined with special focus on diesel engines equipped with a particle filter. The particle number measurements were performed, during transient tests, using a condensation particle counter. The measurement procedure complied with the draft Swiss ordinance, which is based on the findings of the UN/ECE particulate measurement program. In addition, particle mass emissions were measured by the legislated and a modified filter method. The results demonstrate the high efficiency of diesel particle filters (DPFs) in curtailing nonvolatile particle emissions over the entire size range. Higher emissions were observed during short periods of DPF regeneration and immediately afterward, when a soot cake has not yet formed on the filter surface. The gasoline vehicles exhibited higher emissions than the DPF equipped diesel vehicles but with a large variation depending on the technology and driving conditions. Although particle measurements were carried out during DPF regeneration, it was impossible to quantify their contribution to the overall emissions, due to the wide variation in intensity and frequency of regeneration. The numbers counting method demonstrated its clear superiority in sensitivity to the mass measurement. The results strongly suggest the application of the particle number counting to quantify future low tailpipe emissions.     

Carbonyl and nitrogen dioxide emissions from gasoline- and diesel-powered motor vehicles

Carbonyls can be toxic and highly reactive in the atmosphere. To quantify trends in carbonyl emissions from light-duty (LD) vehicles, measurements were made in a San Francisco Bay area highwaytunnel bore containing essentially all LD vehicles during the summers of 1999, 2001, and 2006. The LD vehicle emission factor for formaldehyde, the most abundant carbonyl, did not change between 1999 and 2001, then decreased by 61 +/- 7% between 2001 and 2006. This reduction was due to fleet turnover and the removal of MTBE from gasoline. Acetaldehyde emissions decreased by 19 +/- 2% between 1999 and 2001 and by the same amount between 2001 and 2006. Absent the increased use of ethanol in gasoline after 2003, acetaldehyde emissions would have further decreased by 2006. Carbonyl emission factors for medium- (MD) and heavy-duty (HD) diesel trucks were measured in 2006 in a separate mixed-traffic bore of the tunnel. Emission factors for diesel trucks were higher than those for LD vehicles for all reported carbonyls. Diesel engine exhaust dominates over gasoline engines as a direct source of carbonyl emissions in California. Carbonyl concentrations were also measured in liquid-gasoline samples and were found to be low (< 20 ppm). The gasoline brands that contained ethanol showed higher concentrations of acetaldehyde in unburned fuel versus gasoline that was formulated without ethanol. Measurements of NO2 showed a yearly rate of decrease for LD vehicle emissions similar to that of total NOx in this study. The observed NO2/NOx ratio was 1.2 +/- 0.3% and 3.7 +/- 0.3% for LD vehicles and diesel trucks, respectively.     

Are emissions of black carbon from gasoline vehicles underestimated? Insights from near and on-road measurements

Measurements of black carbon (BC) with a high-sensitivity laser-induced incandescence (HS-LII) instrument and a single particle soot photometer (SP2) were conducted upwind, downwind, and while driving on a highway dominated by gasoline vehicles. The results are used with concurrent CO(2) measurements to derive fuel-based BC emission factors for real-world average fleet and heavy-duty diesel vehicles separately. The derived emission factors from both instruments are compared, and a low SP2 bias (relative to the HS-LII) is found to be caused by a BC mass mode diameter less than 75 nm, that is most prominent with the gasoline fleet but is not present in the heavy-duty diesel vehicle exhaust on the highway. Results from both the LII and the SP2 demonstrate that the BC emission factors from gasoline vehicles are at least a factor of 2 higher than previous North American measurements, and a factor of 9 higher than currently used emission inventories in Canada, derived with the MOBILE 6.2C model. Conversely, the measured BC emission factor for heavy-duty diesel vehicles is in reasonable agreement with previous measurements. The results suggest that greater attention must be paid to black carbon from gasoline engines to obtain a full understanding of the impact of black carbon on air quality and climate and to devise appropriate mitigation strategies.     

CO2 emission benefit of diesel (versus gasoline) powered vehicles

Concerns regarding global warming have increased the pressure on automobile manufacturers to decrease emissions of CO2 from vehicles. Diesel vehicles have higher fuel economy and lower CO2 emissions than their gasoline counterparts. Increased penetration of diesel powered vehicles into the market is a possible transition strategy toward a more sustainable transportation system. To facilitate discussions regarding the relative merits of diesel vehicles it is important to have a clear understanding of their CO2 emission benefits. Based on European diesel and gasoline certification data, this report quantifies such CO2 reduction opportunities for cars and light duty trucks in today's vehicles and those in the year 2015. Overall, on a well-to-wheels per vehicle per mile basis, the CO2 reduction opportunity for today's vehicles is approximately 24-33%. We anticipate that the gap between diesel and gasoline well-to-wheel vehicle CO2 emissions will decrease to approximately 14-27% by the year 2015.     

Quinone emissions from gasoline and diesel motor vehicles

Gas- and particle-phase emissions from gasoline and diesel vehicles operated on chassis dynamometers were collected using annular denuders, quartz filters, and PUF substrates. Quinone species were measured using O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine derivatization in conjunction with gas chromatography-mass spectrometry and high-performance liquid chromatography-mass spectrometry. Nine quinones were observed, ranging from C6 to C16. New species identified in motor vehicle exhaust include methyl-1,4-benzoquinone, 2-methyl-1,4-naphthoquinone (MNQN), and aceanthrenequinone. Gas-phase motor vehicle emissions of quinones are also reported for the first time. Six gas-phase quinones were quantified with emission rates of 2-28 000 microg L(-1) fuel consumed. The most abundant gas-phase quinones were 1,4-benzoquinone (BON) and MNQN. The gas-phase fraction was > or = 69% of quinone mass for light-duty gasoline emissions, and > or = 84% for heavy-duty diesel emissions. Eight particle-phase quinones were observed between 2 and 1600 microg L(-1), with BQN the most abundant species followed by 9,10-phenanthrenequinone and 1,2-naphthoquinone. Current particle-phase quinone measurements agree well with the few available previous results. Further research is needed concerning the gas-particle partitioning behavior of quinones in ambient and combustion source conditions.     

The effects of the catalytic converter and fuel sulfur level on motor vehicle particulate matter emissions: gasoline vehicles

Scanning mobility and electrical low-pressure impactor particle size measurements conducted during chassis dynamometer testing reveal that neither the catalytic converter nor the fuel sulfur content has a significant effect on gasoline vehicle tailpipe particulate matter (PM) emissions. For current technology, port fuel injection, gasoline engines, particle number emissions are < or = 2 times higher from vehicles equipped with blank monoliths as compared to active catalysts, insignificant in contrast to the 90+% removal of hydrocarbons. PM mass emission rates derived from the size distributions are equal within the experimental uncertainty of 50-100%. Gravimetric measurements exhibit a 3-10-fold PM mass increase when the active catalyst is omitted, which is attributed to gaseous hydrocarbons adsorbing onto the filter medium. Both particle number and gravimetric measurements show that gasoline vehicle tailpipe PM emissions are independent (within 2 mg/mi) of fuel sulfur content over the 30-990 ppm concentration range. Nuclei mode sulfate aerosol is not observed in either test cell measurements or during wind tunnel testing. For three-way catalyst equipped vehicles, the principal sulfur emission is SO2; however a sulfur balance is not obtained over the drive cycle. Instead, sulfur is stored on the catalyst during moderate driving and then partially removed during high speed/load operation.     

Measurements of particle number and mass concentrations and size distributions in a tunnel environment

Particulate matter emissions were measured in two bores of the Caldecott Tunnel in Northern California during August and September 2004. One bore (Bore 1) is open to both heavy- and light-duty vehicles while heavy-duty vehicles are prohibited from entering the second bore (Bore 2). Particulate matter number and mass size distributions, chemical composition, and gaseous copollutants were recorded for four consecutive days near the entrance and exit of each bore. Size-resolved emission factors were determined for particle number, particle mass, elemental carbon, organic carbon (OC), sulfate, nitrate, and selected elements. The size distributions in both the bores showed a single large mode at roughly 15-20 nm in mobility diameter, with occasional smaller modes around 100 nm. The PM10 mass emission factor for heavy-duty vehicles was 14.5 times higher than that of light-duty vehicles. The particles derived from diesel are more abundant in elemental carbon, 70.9% of PM10 emissions, as compared to the light-duty vehicles. Conversely, a greater percentage of OC was found in light-duty emissions than heavy-duty emissions. In comparison to previous studies at the Caldecott Tunnel, less particle mass but more particle numbers are emitted by vehicles than was the case 7 years ago.     

Size-resolved polycyclic aromatic hydrocarbon emission factors from on-road gasoline and diesel vehicles: temperature effect on the nuclei-mode

Motor vehicles are a major source of polycyclic aromatic hydrocarbon (PAH) emissions in urban areas. Motor vehicle emission control strategies have included improvements in engine design, exhaust emission control, and fuel reformulation. Therefore, an updated assessment of the effects of the shifts in fuels and vehicle technologies on PAH vehicular emission factors (EFs) is needed. We have evaluated the effects of ambient temperature on the size-resolved EFs of nine US EPA Priority Pollutant PAH, down to 10 nm diameter, from on-road California gasoline light-duty vehicles with spark ignition (SI) and heavy-duty diesels with compression ignition (CI) in summer 2004 and winter 2005. During the winter, for the target PAH with the lowest subcooled equilibrium vapor pressure --benzo[a]pyrene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene-- the mass in the nucleation mode, defined here as particles with dp <32 nm, ranged between 14 and 38% for SI vehicles and 29 and 64% for CI vehicles. Our observations of the effect of temperature on the mass of PAH in the nucleation mode are similar to the observed effect of temperature on the number concentration of diesel exhaust particles in the nucleation mode in a previous report.     

Separation of fine particulate matter emitted from gasoline and diesel vehicles using chemical mass balancing techniques

Samples of fine particulate matter were collected in a roadway tunnel near Houston, TX over a period of 4 days during two separate sampling periods: one sampling period from 1200 to 1400 local time and another sampling period from 1600 to 1800 local time. During the two sampling periods, the tunnel traffic contained roughly equivalent numbers of heavy-duty diesel trucks. However, during the late afternoon sampling period, the tunnel contained twice as many light-duty gasoline-powered vehicles. The effect of this shift in the vehicle fleet affects the overall emission index (grams pollutant emitted per kilogram carbon in fuel) for fine particles and fine particulate elemental carbon. Additionally, this shift in the fraction of diesel vehicles in the tunnel is used to determine if the chemical mass balancing techniques used to track emissions from gasoline-powered and diesel-powered emissions accurately separates these two emission categories. The results show that the chemical mass balancing calculations apportion roughly equal amounts of the particulate matter measured to diesel vehicles between the two periods and attribute almost twice as much particulate matter in the late afternoon sampling period to gasoline vehicles. Both of these results are consistent with the traffic volume of gasoline and diesel vehicles in the tunnel in the two separate periods and validate the ability for chemical mass balancing techniques to separate these two primary sources of fine particles.     

Size-resolved emissions of organic tracers from light- and heavy-duty vehicles measured in a California roadway tunnel

Individual organic compounds found in particulate emissions from vehicles have proven useful in source apportionment of ambient particulate matter. Species of interest include the hopanes, originating in lube oil, and selected PAHs generated via combustion. Most efforts to date have focused on emissions and apportionment PM10 or PM2.5 However, examining how these compounds are segregated by particle size in both emissions and ambient samples will help efforts to apportion size-resolved PM, especially ultrafine particles which have been shown to be more potent toxicologically. To this end, high volume size-resolved (coarse, accumulation, and ultrafine) PM samples were collected inside the Caldecott tunnel in Orinda, California to determine the relative emission factors for these compounds in different size ranges. Sampling occurred in two bores, one off-limits to heavy-duty diesel vehicles, which allows determination of the different emissions profiles for diesel and gasoline vehicles. Although tunnel measurements do not measure emissions over a full engine duty cycle, they do provide an average emissions profile over thousands of vehicles that can be considered characteristic of "freeway" emissions. Results include size-fractionated emission rates for hopanes, PAHs, elemental carbon, and other potential organic markers apportioned to diesel and gasoline vehicles. The results are compared to previously conducted PM2.5 emissions testing using dynamometer facilities and othertunnel environments.     

Electric vehicles in China: emissions and health impacts

E-bikes in China are the single largest adoption of alternative fuel vehicles in history, with more than 100 million e-bikes purchased in the past decade and vehicle ownership about 2× larger for e-bikes as for conventional cars; e-car sales, too, are rapidly growing. We compare emissions (CO(2), PM(2.5), NO(X), HC) and environmental health impacts (primary PM(2.5)) from the use of conventional vehicles (CVs) and electric vehicles (EVs) in 34 major cities in China. CO(2) emissions (g km(-1)) vary and are an order of magnitude greater for e-cars (135-274) and CVs (150-180) than for e-bikes (14-27). PM(2.5) emission factors generally are lower for CVs (gasoline or diesel) than comparable EVs. However, intake fraction is often greater for CVs than for EVs because combustion emissions are generally closer to population centers for CVs (tailpipe emissions) than for EVs (power plant emissions). For most cities, the net result is that primary PM(2.5) environmental health impacts per passenger-km are greater for e-cars than for gasoline cars (3.6× on average), lower than for diesel cars (2.5× on average), and equal to diesel buses. In contrast, e-bikes yield lower environmental health impacts per passenger-km than the three CVs investigated: gasoline cars (2×), diesel cars (10×), and diesel buses (5×). Our findings highlight the importance of considering exposures, and especially the proximity of emissions to people, when evaluating environmental health impacts for EVs.     

Vehicle traffic as a source of particulate polycyclic aromatic hydrocarbon exposure in the Mexico City metropolitan area

Surface properties of aerosols in the Mexico City metropolitan area have been measured in a variety of exposure scenarios related to vehicle emissions in 2002, using continuous, real-time instruments. The objective of these experiments is to describe ambient and occupational particulate polycyclic aromatic hydrocarbon (PAH) concentrations associated with vehicular traffic and facilities using diesel vehicles. Median total particulate PAH concentrations along Mexico City's roadways range from 60 to 910 ng m(-3), averaged over a minimum of 1 h. These levels are approximately 5 times higher than concentrations measured in the United States and among the highest measured ambient values reported in the literature. The ratio of particulate PAH concentration to aerosol active surface area is much higher along roadways and in other areas of fresh vehicle emissions, compared to ratios measured at sites influenced more by aged emissions or noncombustion sources. For particles freshly emitted by vehicles, PAH and elemental carbon (EC) concentrations are correlated because they both originate during the combustion process. Comparison of PAH versus EC and active surface area concentrations at different locations suggests that surface PAH concentrations may diminish with particle aging. These results indicate that exposure to vehicle-related PAH emissions on Mexico City's roadways may present an important public health risk.     

Lubricating oil and fuel contributions to particulate matter emissions from light-duty gasoline and heavy-duty diesel vehicles

Size-resolved particulate matter emissions from heavy-duty diesel vehicles (HDDVs) and light-duty gasoline vehicles (LDGVs) operated under realistic driving cycles were analyzed for elemental carbon (EC), organic carbon (OC), hopanes, steranes, and polycyclic aromatic hydrocarbons. Measured hopane and sterane size distributions did not match the total carbon size distribution in most cases, suggesting that lubricating oil was not the dominant source of particulate carbon in the vehicle exhaust. A regression analysis using 17alpha(H)-21beta(H)-29-norhopane as a tracer for lubricating oil and benzo[ghi/perylene as a tracer for gasoline showed that gasoline fuel and lubricating oil both make significant contributions to particulate EC and OC emissions from LDGVs. A similar regression analysis performed using 17alpha(H)-21beta(H)-29-norhopane as a tracer for lubricating oil and flouranthene as a tracerfor diesel fuel was able to explain the size distribution of particulate EC and OC emissions from HDDVs. The analysis showed that EC emitted from all HDDVs operated under relatively high load conditions was dominated by diesel fuel contributions with little EC attributed to lubricating oil. Particulate OC emitted from HDDVs was more evenly apportioned between fuel and oil contributions. EC emitted from LDGVs operated underfuel-rich conditions was dominated by gasoline fuel contributions. OC emitted from visibly smoking LDGVs was mostly associated with lubricating oil, but OC emitted from all other categories of LDGVs was dominated by gasoline fuel. The current study clearly illustrates that fuel and lubricating oil make separate and distinct contributions to particulate matter emissions from motor vehicles. These particles should be tracked separately during ambient source apportionment studies since the atmospheric evolution and ultimate health effects of these particles may be different. The source profiles for fuel and lubricating oil contributions to EC and OC emissions derived in this study provide a foundation for future source apportionment calculations.     

Influence of diesel engine combustion parameters on primary soot particle diameter

Effects of engine operating parameters and fuel composition on both primary soot particle diameter and particle number size distribution in the exhaust of a direct-injected heavy-duty diesel engine were studied in detail. An electrostatic sampler was developed to deposit particles directly on transmission electron microscopy (TEM) grids. Using TEM, the projected area equivalent diameter of primary soot particles was determined. A scanning mobility particle sizer (SMPS) was used for the measurement of the particle number size distribution. Variations in the main engine operating parameters (fuel injection system, air management, and fuel properties) were made to investigate soot formation and oxidation processes. Primary soot particle diameters determined by TEM measurements ranged from 17.5 to 32.5 nm for the diesel fuel and from 24.1 to 27.2 nm for the water-diesel emulsion fuel depending on the engine settings. For constant fuel energy flow rate, the primary particle size from the water-diesel emulsion fuel was slightly larger than that from the diesel fuel. A reduction in primary soot particle diameter was registered when increasing the fuel injection pressure (IP) or advancing the start of injection (SOI). Larger primary soot particle diameters were measured while the engine was operating with exhaust gas recirculation (EGR). Heat release rate analysis of the combustion process revealed that the primary soot particle diameter decreased when the maximum flame temperature increased for the diesel fuel.     

Determination of single particle mass spectral signatures from light-duty vehicle emissions

In this study, 28 light-duty gasoline vehicles (LDV) were operated on a chassis dynamometer at the California Air Resources Board Haagen-Smit Facility in El Monte, CA. The mass spectra of individual particles emitted from these vehicles were measured using aerosol time-of-flight mass spectrometry (ATOFMS). A primary goal of this study involves determining representative size-resolved single particle mass spectral signatures that can be used in future ambient particulate matter source apportionment studies. Different cycles were used to simulate urban driving conditions including the federal testing procedure (FTP), unified cycle (UC), and the correction cycle (CC). The vehicles were selected to span a range of catalytic converter (three-way, oxidation, and no catalysts) and engine technologies (vehicles models from 1953 to 2003). Exhaust particles were sampled directly from a dilution and residence chamber system using particle sizing instruments and an ATOFMS equipped with an aerodynamic lens (UF-ATOFMS) analyzing particles between 50 and 300 nm. On the basis of chemical composition, 10 unique chemical types describe the majority of the particles with distinct size and temporal characteristics. In the ultrafine size range (between 50 and 100 nm), three elemental carbon (EC) particle types dominated, all showing distinct EC signatures combined with Ca, phosphate, sulfate, and a lower abundance of organic carbon (OC). The relative fraction of EC particle types decreased as particle size increased with OC particles becoming more prevalent above 100 nm. Depending on the vehicle and cycle, several distinct OC particle types produced distinct ion patterns, including substituted aromatic compounds and polycyclic aromatic hydrocarbons (PAH), coupled with other chemical species including ammonium, EC, nitrate, sulfate, phosphate, V, and Ca. The most likely source of the Ca and phosphate in the particles is attributed to the lubricating oil. Significant variability was observed in the chemical composition of particles emitted within the different car categories as well as for the same car operating under different driving conditions. Two-minute temporal resolution measurements provide information on the chemical classes as they evolved during the FTP cycle. The first two minutes of the cold start produced more than 5 times the number of particles than any other portion of the cycle, with one class of ultrafine particles (EC coupled with Ca, OC, and phosphate) preferentially produced. By number, the three EC with Ca classes (which also contained OC, phosphate, and sulfate) were the most abundant classes produced by the nonsmoking vehicles. The smoker category produced the highest number of particles, with the dominant classes being OC comprised of substituted monoaromatic compounds and PAHs, coupled with Ca and phosphate, thus suggesting used lubricating oil was associated with many of these particles. These studies show, by number, EC particles dominate gasoline emissions in the ultrafine size range particularlyforthe lowest emitting newer vehicles, suggesting the EC signature alone cannot be used as a unique tracer for diesels. This represents the first report of high time- and size-resolved chemical composition data showing the mixing state of nonrefractory elements in particles such as EC for vehicle emissions during dynamometer source testing.     

Apportionment of motor vehicle emissions from fast changes in number concentration and chemical composition of ultrafine particles near a roadway intersection

High frequency spikes in ultrafine number concentration near a roadway intersection arise from motor vehicles that accelerate after a red light turns green. The present work describes a method to determine the contribution of motor vehicles to the total ambient ultrafine particle mass by correlating these number concentration spikes with fast changes in ultrafine particle chemical composition measured with the nano aerosol mass spectrometer, NAMS. Measurements were performed at an urban air quality monitoring site in Wilmington, Delaware during the summer and winter of 2009. Motor vehicles were found to contribute 48% of the ultrafine particle mass in the winter measurement period, but only 16% of the ultrafine particle mass in the summer period. Chemical composition profiles and contributions to the ultrafine particle mass of spark vs diesel vehicles were estimated by correlating still camera images, chemical composition and spike contribution at each time interval.. The spark and diesel contributions were roughly equal, but the uncertainty in the split was large. The distribution of emissions from individual vehicles was determined by correlating camera images with the spike contribution to particle number concentration at each time interval. A small percentage of motor vehicles were found to emit a disproportionally large concentration of ultrafine particles, and these high emitters included both spark ignition and diesel vehicles.     

Effect of engine load on diesel soot particles

This study concentrates on characterization of nonvolatile fraction of diesel particles. These particles have an impact on earth's radiation balance as well as on health effects of vehicle emissions. In addition to composition and size distribution of particles, an important factor affecting their health effects and properties and lifetimes in the atmosphere is their morphology. The effect of engine parameters on soot particle size distributions and also on particle morphology has been studied. It was found that the shape of the size distribution and also the structure of diesel particles depend on engine load. The number distributions were found to obey log-normal assumption. The width of the distribution increased with increasing engine load. The geometric standard deviations of measured distributions varied from 1.7 to 2.1. Simultaneously, the fractal dimension of particles decreased with increasing engine load. The values for mass fractal dimensions based on sealing of particle mass and mobility size were between 2.6 and 2.8. Both electron microscopy and measurements of aerodynamic size versus mobility size suggest that the morphology of particles in different size regimes vary, with the large particles being less compact than the small ones.     

A comparative investigation of ultrafine particle number and mass emissions from a fleet of on-road diesel and CNG buses

Particle number, particle mass, and CO2 concentrations were measured on the curb of a busy urban busway used entirely by a mix of diesel and CNG operated buses. With the passage of each bus, the ratio of particle number concentration and particle mass concentration to CO2 concentration in the diluted exhaust plume were used as measures of the particle number and mass emission factors, respectively. With all buses accelerating pastthe monitoring point, the results showed that the median particle mass emission from CNG buses was less than 9% of that from diesel buses. However, the median particle number emission from CNG buses was 6 times higher than the diesel buses, and the particles from the CNG buses were mainly in the nanoparticle size range. Using a thermodenuder to remove the volatile material from the sampled emissions showed that the majority of particles from the CNG buses, but not from the diesel buses, were volatile. Approximately, 82% of the particles from the CNG buses and 38% from the diesel buses were removed by heating the emissions to 300 degrees C.     

Effect of fuel injection pressure on a heavy-duty diesel engine nonvolatile particle emission

The effects of the fuel injection pressure on a heavy-duty diesel engine exhaust particle emissions were studied. Nonvolatile particle size distributions and gaseous emissions were measured at steady-state engine conditions while the fuel injection pressure was changed. An increase in the injection pressure resulted in an increase in the nonvolatile nucleation mode (core) emission at medium and at high loads. At low loads, the core was not detected. Simultaneously, a decrease in soot mode number concentration and size and an increase in the soot mode distribution width were detected at all loads. Interestingly, the emission of the core was independent of the soot mode concentration at load conditions below 50%. Depending on engine load conditions, growth of the geometric mean diameter of the core mode was also detected with increasing injection pressure. The core mode emission and also the size of the mode increased with increasing NOx emission while the soot mode size and emission decreased simultaneously.     

Climate effects of emission standards: the case for gasoline and diesel cars

Passenger transport affects climate through various mechanisms involving both long-lived and short-lived climate forcers. Because diesel cars generally emit less CO(2) than gasoline cars, CO(2) emission taxes for vehicle registrations and fuels enhance the consumer preference for diesel cars over gasoline cars. However, with the non-CO(2) components, which have been changed and will be changed under the previous and upcoming vehicle emission standards, what does the shift from gasoline to diesel cars mean for the climate mitigation? By using a simple climate model, we demonstrate that, under the earlier emissions standards (EURO 3 and 4), a diesel car causes a larger warming up to a decade after the emissions than a similar gasoline car due to the higher emissions of black carbon and NO(X) (enhancing the O(3) production). Beyond a decade, the warming caused by a diesel car becomes, however, weaker because of the lower CO(2) emissions. As the latter emissions standards (EURO 5 and 6) are phased in, the short-term warming due to a diesel car becomes smaller primarily due to the lower black carbon emissions. Thus, although results are subject to restrictive assumptions and uncertainties, the switch from gasoline to diesel cars encouraged by CO(2) taxes does not contradict with the climate mitigation focusing on long-term consequences.