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TDI vs. gasoline - The TDi emissions levels are among the lowest ever for Diesel powered engines. All TDi powered Volkswagens sold in the US meet so-called "Tier 1" emission limits. The TDi is often "cleaner" overall than gasoline powered cars. CO2 emissions are 25% less than a conventional gasoline powered engine. CO, HC and NOx emissions are less than previous Volkswagen Diesels. Diesel fuel has lower evaporative emissions than gasoline. Diesel fuel also requires less energy intensive refining than gasoline.

Diesel engines generally emit higher amounts of NOx and particles than equivalent gasoline powered cars, even though CO and HC emissions may be lower, and total emissions are lower due to much better fuel consumption. The current TDI Volkswagens typically emit slightly somewhat lower than the Tier 1 limits for NOx and particles (around 0.052 g/mi of particulate matter [PM] and 0.82 g/mi of NOx per EPA data), but the CO and HC emissions are far below the Tier 1 limits and well below the emissions of the equivalent gasoline engine.

Furthermore, most of the unregulated toxic gaseous emissions tend to be lower for diesel engines. For example, benzene (which is a known carcinogen) is lower in diesels by nearly an order of magnitude (i.e., factor of ten) than an equivalent gasoline engine. Diesels also tend to be significantly lower in emissions of alkenes (e.g., ethene), carbonyls (e.g., formaldehyde), and semivolatiles like polyaromatic hydrocarbons (PAHs, many of which are known or suspected carcinogens).

PM has always been regulated by mass (e.g., grams per mile). However, very recent studies show that particle number may be the more important aspect of PM emissions. According to a "real world vehicle testing report" by University of Minnesota renowned combustion particle scientists, new data show that PM number emissions from modern gasoline cars may equal or exceed diesel PM levels. It goes on to discuss gasoline PM emissions and that fact that gasoline engines may need a particulate filter much like that of a diesel. The University of Minnesota study showed that newer and older gasoline vehicles matched or exceeded diesel PM number emissions at high speed/load . It appears that diesel engines equipped with diesel particulate filters (DPFs), as many are now in Europe, will have a significant advantage in PM emissions over gasoline engines. Other recent studies are suggesting that gasoline PM is generally more toxic that diesel PM.

The emission levels from diesel engines tend to remain more-or-less constant throughout the useful life of the engine, whereas gasoline engines have many more emission-related components which deteriorate and lead to higher and higher emissions as the engine gets older.

Volkswagen has made continuous progress on emissions through the years, and 2000-model TDI engines emit far less than the 1996 models first available here. Further progress has been made in Europe with new fuel-injection and emission-control technology, but for various technical and market-related reasons, this technology is not available here yet, but will likely be arriving within the next few years.
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Bio-Diesel fuel is available in limited geographical areas. The fuel is produced from a recently grown crop of vegetation (often soybean ) and is produced without the partial fossilization and the passage of millennia. The emission levels of the bio-diesel is lower than the more common petroleum Diesel.
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California and New York issues - The state of California places limits on the "fleet average" emissions of auto manufacturers. Currently, a manufacturer can only sell a certain proportion of "Tier 1" vehicles in relation to the number of "LEV" or low-emission vehicles. Certain other states have copied the California legislation. Volkswagen has stated that the withdrawal from sale of 2000-model TDI vehicles from certain states is due to these reasons. Furthermore, California has declared diesel exhaust to be a toxic air contaminant, although other studies dispute this conclusion. The situation in New York is the same.

NOx and volatile organic compounds (VOCs) are involved in complex photochemical reactions to form ground-level ozone. Recent U.S. Department of Energy-supported studies have strongly suggested that the strict control of NOx emissions may have the unintended consequence of making ambient ground-level ozone worse. The California Air Resources Board (CARB) has put increasingly strict emission limits on NOx emissions in an attempt to reduce ground-level ozone (ozone is an inhalation health concern), for which Southern California is notorious. However, many previous studies have shown that ozone levels are actually higher on weekends (WE) than on weekday (WD) (WD/WE effect) when diesel truck traffic decreases relatively much more than automobile traffic (which mostly are powered by gasoline engines). The DOE studies have confirmed that NOx is reduced significantly more than VOC (HC) and as a result ozone levels increase. It has been discovered that most large urban areas in the U.S. have similar conditions in which ambient ozone levels rise with decreasing ambient NOx levels and that NOx controls in Southern California (and other urban U.S. locations) are generally counterproductive for reducing ambient ozone, they actually increase ambient ozone levels. Were it not for large concurrent HC emission reductions on weekends, weekend ozone would be even higher than it is, and the weekend/weekday ozone difference would be even larger. DOE concludes that gasoline exhaust and gasoline vapor account for ~80 percent of ambient NMHC (VOCs) in on-road samples and at regional air monitoring locations suggesting that gasoline emissions are responsible for the majority of ozone found in the SoCAB. Whether these recent findings change CARB's (or even EPA's) ozone control strategy remain to be seen.

In addition, other recent studies are suggesting that carbon monoxide (CO) emissions (most of which are from gasoline engine vehicles) are becoming more and more responsible for generating ground-level ozone. The National Research Council's (NRC's) report confirmed the importance of carbon monoxide in the formation of urban ozone, concluding that more than 20 percent of vehicle-related ozone pollution comes from carbon monoxide. The Council also notes that carbon monoxide emissions will play an even larger role in ozone formation as volatile organic compound emissions from vehicles continue to decrease. Another source concludes that CO emissions may be responsible for as much as 35% of the ground-level ozone.

CARB (and to a slightly lesser extent EPA) focuses on reducing NOx. But atmospheric scientist Gary Whitten of ICF Consulting notes that if the tradeoff of reducing NOx is to increase hydrocarbon and carbon monoxide emissions, the environment would be poorly served. The reason, according to Whitten, is that a reduction in hydrocarbon and carbon monoxide emissions has a much greater beneficial impact on ozone formation than an equivalent reduction in NOx. Whitten concludes, "The effectiveness of THC for reducing ozone in these simulations must be as much as 8 times better than NOx reductions on an equal percentage of the mobile emissions basis." Since diesel engines tend to have significantly lower emissions of CO and HC (VOCs), while generally higher emissions of NOx, one could conclude based on these recent studies that an increasing market share of diesel-powered cars and light trucks will have a positive impact on ground level ozone rather that the negative impact which has always been assumed.
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