The Lifetime of Nitrogen Oxides in an Isoprene Dominated Forest

The lifetime of NO.sub.x (NO.sub.x ≡ NO + NO.sub.2) affects the concentration and distribution of NO.sub.x and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NO.sub.x is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NO.sub.x and NO.sub.x reservoirs. We find that the lifetime of NO.sub.x during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (ΣANs). During SOAS, ΣAN production was rapid, averaging 90 ppt hr.sup.−1 during the day, and occurred predominantly during isoprene oxidation. Analysis of the ΣAN and HNO.sub.3 budgets indicate that ΣANs have an average lifetime of under 2 hours, and that approximately 45 % of the ΣANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that ΣAN hydrolysis is the largest source of HNO.sub.3 and the primary pathway to permanent removal of NO.sub.x from the boundary layer in this location. Using these new constraints on the fate of ΣANs, we find that the NO.sub.x lifetime is 11 ± 5 hours under typical midday conditions. The lifetime is extended by storage of NO.sub.x in temporary reservoirs, including acyl peroxy nitrates and ΣANs.