The effect on photochemical smog of converting the U.S. fleet of gasoline vehicles to modern diesel vehicles

With the increased use of particle traps and nitrogen oxide (NO x ) control devices to reduce air pollution, “modern” diesel vehicles are being encouraged over gasoline vehicles globally as a central method of slowing global warming. Data to date, though, suggest that the NO 2 :NO ratio from modern diesel may exceed that of gasoline, and it is difficult to reduce diesel NO x below gasoline NO x without increasing particle emissions. Here, it is calculated that, unless the diesel NO 2 :NO ratio and total NO x are reduced to those of gasoline, modern diesel, which should have lower hydrocarbon (HC) and carbon monoxide (CO) emissions than gasoline, may still enhance photochemical smog at the surface and aloft, on average, over the U.S. relative to gasoline. The reason is that vehicle‐produced smog in the U.S. depends more on NO x and the NO 2 :NO ratio than on HCs or CO. It is also found that vehicle NO x controls may be more effective than NO 2 :NO ratio controls at reducing ozone.