January 2012: Intercontinental enhancement of particulate matter from NO_x and CO emissions

We show that anthropogenic emissions of nitrogen oxides (NO_x = 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 NO_x to nitrate. In fact, the intercontinental influences of NO_x 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, NO_x, and ammonia emissions and hence already high levels of PM. US NO_x 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 NO_x 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). For more information, see Leibensperger et al. [2011].