UC Berkeley

The concentration of nitrogen oxides (NO_x ≡ 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 NO_x and its oxidation products controls the concentration and transport of NO_x and affects the spatial extent of air pollution. Previous studies have shown that permanent loss of NO_x 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 NO_x 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 NO_x to the atmosphere and by aerosol-phase hydrolysis to form HNO₃. In contrast, observations of HNO₃ and NO_x 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 NO_x oxidation products affects the lifetime of NO_x and the production of O₃ in different environments. I show that, in general, the formation of short-lived NO_x reservoirs extends the lifetime of NO_x by sequestering NO_x in less reactive forms. Furthermore, I show that in areas where the formation of RONO₂ is the dominant pathway for NO_x 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, NO_x chemistry has shifted in the United States towards a regime where RONO₂ chemistry plays a greater role in the loss of NO_x.