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

23 Anthropogenic emissions of nitrogen oxides (NOx ≡ NO + NO2) and carbon monoxide (CO) 24 affect particulate matter (PM) air quality on an intercontinental scale by changing background 25 concentrations of oxidants (OH, ozone, H2O2) and thus increasing the oxidation rate of sulfur 26 dioxide (SO2) to sulfate and NOx to nitrate. We conduct sensitivity simulations with the GEOS27 Chem chemical transport model and find that these intercontinental influences of NOx and CO 28 emissions on PM can be greater than those from SO2 emissions (a direct PM precursor). The 29 intercontinental impact of oxidant precursors is greatest in receptor regions with high domestic 30 SO2, NOx, and ammonia emissions and hence already high levels of PM. US NOx and CO 31 emissions increase annual mean PM in northern Europe and eastern China by up to 0.25 μg m -3 . 32 The increase in Europe is mostly as sulfate, whereas in China it is mostly as nitrate. East Asian 33 NOx and CO emissions have a weaker intercontinental influence (~0.2 μg m -3 in northern Europe, 34 ~ 0.1 μg m -3 in the eastern US). These intercontinental effects of NOx and CO emissions on PM 35 depend in a complex way on the chemical environment of receptor regions. Intercomparison of 36 results from different models would be of great interest. 37 38