Intercontinental influence of NOx and CO emissions on particulate matter air quality

Anthropogenic emissions of nitrogen oxides (NOx ≡ NO + NO2) and carbon monoxide (CO) affect particulate matter (PM) air quality on an intercontinental scale by changing background concentrations of oxidants (OH, ozone, H2O2) and thus increasing the oxidation rate of sulfur dioxide (SO2) to sulfate and NOx to nitrate. We conduct sensitivity simulations with the GEOS–Chem chemical transport model and find that these intercontinental influences of NOx and CO emissions on PM can be greater than those from SO2 emissions (a direct PM precursor). The intercontinental impact of oxidant precursors is greatest in receptor regions with high domestic SO2, NOx, and ammonia emissions and hence already high levels of PM. US NOx and CO emissions increase annual mean PM in northern Europe and eastern China by up to 0.25 μg m−3. The increase in Europe is mostly as sulfate, whereas in China it is mostly as nitrate. East Asian NOx and CO emissions have a weaker intercontinental influence (∼0.2 μg m−3 in northern Europe, ∼0.1 μg m−3 in the eastern US). These intercontinental effects of NOx 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.