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Science  20 Feb 2004:
Vol. 303, Issue 5661, pp. 1106b
DOI: 10.1126/science.303.5661.1106b

Automobiles emit approximately one-third of the CO2 produced by fossil fuel burning, along with huge quantities of carbon monoxide and hydrocarbons. Because diesel engines can be made to emit less than similar gasoline models, it would seem that replacing gasoline with diesel vehicles could help reduce air pollution and slow global warming. This is, in fact, a strategy being pursued or discussed in much of Europe and in California.

Jacobson et al. present a study showing that nitrogen emissions from diesel engines may still enhance photochemical smog. The culprits in this inconvenient and previously neglected complication are NOx (NO and NO2) and the ratio of NO2 to NO in the emissions. Diesel vehicles emit 4 to 30 times more NOx than gasoline vehicles and have NO2/NO as much as 16 times higher. Because these two factors are critical in the production of smog, unless ways can be found to reduce them in the exhaust, large-scale conversion to diesel could worsen air pollution. Moreover, increased production of ozone caused by high NOx and higher particulate emissions make the choice of diesel more problematic than may be suggested if considering only CO, CO2, and hydrocarbons. — HJS

Geophys. Res. Lett. 31, L02116 (2004).

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