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Today’s emissions from mobile sources, including cars, trucks, and off-road vehicles, are estimated to be responsible for about 50% of nationwide NOx emissions, about 40-50% of HC (or VOC) emissions, and as much as 80-90% of CO emissions in urban areas. Similar contributions are seen in the USA and in other developed countries, including Europe and Japan. A large portion of these emissions comes from gasoline fueled cars and light trucks. However, the relative importance of NOx and PM emissions from diesel engines has been increasing. This increase has been caused by (1) the introduction of three-way catalyst technology for spark ignited engines in the 1980s, which lowered gasoline NOx emissions, and (2) increasing trends in diesel vehicle population and diesel fuel usage.
There is an increased penetration of diesel fueled vehicles in the passenger car market in Europe. With the exception of Sweden, where diesels almost disappeared, sales of diesel cars in most countries show an increasing trend. As illustrated in Table 1 , the proportion of diesel cars sold in 2002 in Austria, Belgium and France exceeded 60% of all new automobiles in these countries.
|Europe (weighted average)||7.2||15.6||13.9||22.1||39.3|
|* new car registrations January-June 2002|
Even though diesel passenger cars are not used in the USA (market penetration 0.03% in 1994), the consumption of diesel fuel relative to the total fuel consumption by all highway vehicles has been steadily increasing. The same increasing trend has been noted in Japan (Figure 1 ).
In the past, when diesel fuels contained high levels of sulfur, diesel engines were also important contributors to the sulfur dioxide (SO2) emission inventory. Lowering the fuel sulfur level from about 0.3-0.5% to 0.05%, which occurred in the mid-1990s in the USA and in Europe, was an effective means of controlling SO2 emissions from diesel fueled engines. In 1999, the contribution of highway diesel engines to the nationwide SO2 inventory in the USA is only 0.6%. However, the contribution of non-road engines, which still used high sulfur diesel fuel, amounted to 2.7% . The relative importance of diesels in SO2 pollution may be higher in countries which still use diesel fuels of high sulfur content.