The concentration of nitrogen oxides (NOx ≡ NO + NO2) regulates the concentrations of all major atmospheric oxidants and the formation of secondary air pollutants, with consequences for climate, human health, and ecosystems. The chemical cycling between NOx and its oxidation products controls the concentration and transport of NOx and affects the spatial extent of air pollution. Previous studies have shown that permanent loss of NOx occurs both through production of HNO3 and through production of alkyl and multifunctional nitrates (RONO2). Despite their importance to atmospheric chemistry, significant uncertainties remain in the relative importance of these pathways and in the fates of RONO2 and HNO3 in the atmosphere.
I use observations from two intensive field studies, the Southern Oxidant and Aerosol Study (SOAS) in Centreville Alabama and the Korea-United States Air Quality Study (KORUS-AQ) over South Korea, to provide new constraints on the lifetimes and fates of NOx oxidation products. I show that RONO2 compounds produced from isoprene oxidation have a lifetime under three hours and are lost both by gas-phase chemistry to re-release NOx to the atmosphere and by aerosol-phase hydrolysis to form HNO3. In contrast, observations of HNO3 and NOx over the Yellow Sea between China and South Korea confirm that HNO3 is nearly chemically inert in the troposphere. This finding contradicts recent proposals that HNO3 undergoes photolysis extremely rapidly in the aerosol phase.
I apply these findings to investigate how the chemistry of NOx oxidation products affects the lifetime of NOx and the production of O3 in different environments. I show that, in general, the formation of short-lived NOx reservoirs extends the lifetime of NOx by sequestering NOx in less reactive forms. Furthermore, I show that in areas where the formation of RONO2 is the dominant pathway for NOx loss, ozone production efficiency no longer increases with temperature, changing the response of air quality to meteorology. Finally, I show that over the past decade, NOx chemistry has shifted in the United States towards a regime where RONO2 chemistry plays a greater role in the loss of NOx.