Can the EPA's call for the toughest smog standards since 1997 be traced to concerns about global warming?

No

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by Justin Barnes

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Photochemical smog and greenhouse gas emissions, while they often occur together from the same sources, are two separate environmental issues. Carbon dioxide (CO2), the most important greenhouse gas, has nothing to do with smog formation, nor do methane (CH3), nitrous oxide (N2O), HFCs, and PFCs, which are more minor contributors.

Smog is a secondary pollutant formed from the chemical interaction of volatile organic compounds (VOCs), nitrogen oxides (NOx), and sunlight. Nitrogen oxides, while they may sound similar to nitrous oxide, are different compounds with different chemical properties. When smog forms, it is primarily the result of fuel evaporation (a source of VOCs) and NOx emissions from automobiles. This interaction forms ground level ozone, the primary component of photochemical smog.

To combat this problem, fuels in smog prone areas are designed to be less evaporative, particularly during the summer months. When you hear talk of summer blend fuels, this is what they are talking about. NOx emissions are reduced through the use of catalytic converters, which, among other things, convert NOx to N2O. While this perhaps bears some relation to global warming since N2O is a greenhouse gas, ultimately it means very little in the overall scheme of things because N20 is a minor contributor to greenhouse gas forcing and because N20 emissions from other sources (e.g., fertilizer application) are much larger than those from motor vehicles. In any case stricter smog standards would seem to increase, not decrease, the emissions of the N2O from motor vehicles while leaving carbon dioxide emissions unchanged.

In addition, catalytic converters also help reduce carbon monoxide (CO) emissions by converting it to carbon dioxide (CO2). Thus using catalytic converters to reduce NOx and CO emissions actually increases greenhouse gas emissions. It is a trade-off between pollutants like CO and NOx which can cause serious harm to human health and CO2, the main source of greenhouse forcing. Again however, automobiles would produce carbon dioxide with or without catalytic converters and the overall contribution of catalytic converter generated CO2 is small compared to overall emissions. The only common solution to these problems is greater automobile fuel efficiency, which is presently being debated as a separate issue.

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