Intercontinental influence of NO[subscript x] and CO emissions on particulate matter air quality

Abstract

Anthropogenic emissions of nitrogen oxides (NO[subscript x] ≡ NO + NO[subscript 2]) and carbon monoxide (CO) affect particulate matter (PM) air quality on an intercontinental scale by changing background concentrations of oxidants (OH, ozone, H[subscript 2]O[subscript 2]) and thus increasing the oxidation rate of sulfur dioxide (SO[subscript 2]) to sulfate and NOx to nitrate. We conduct sensitivity simulations with the GEOS–Chem chemical transport model and find that these intercontinental influences of NO[subscript x] and CO emissions on PM can be greater than those from SO[subscript 2] emissions (a direct PM precursor). The intercontinental impact of oxidant precursors is greatest in receptor regions with high domestic SO[subscript 2], NO[subscript x], and ammonia emissions and hence already high levels of PM. US NO[subscript x] and CO emissions increase annual mean PM in northern Europe and eastern China by up to 0.25 μg m[superscript −3]. The increase in Europe is mostly as sulfate, whereas in China it is mostly as nitrate. East Asian NO[subscript x] and CO emissions have a weaker intercontinental influence (~0.2 μg m[superscript −3] in northern Europe, ~0.1 μg m[superscript −3] in the eastern US). These intercontinental effects of NO[subscript x] and CO emissions on PM depend in a complex way on the chemical environment of receptor regions. Intercomparison of results from different models would be of great interest.