Nocturnal isoprene declines in a semi-urban environment

Abstract

In environments dominated by biogenic hydrocarbon emissions, the transition from daytime to nighttime is an important period because ambient isoprene levels rapidly decrease. In sub-urban environments with abundant sources of nitrogen oxides, the nighttime isoprene chemistry has implications for the regional nitrogen oxide budget. Given the substantial production of alkyl nitrates from isoprene reactions during the nighttime, nitrogen oxides can then be transported long distances in the form of nitrates. The factors that influence nighttime chemistry of isoprene in environments with abundant sources of nitrogen oxides are not well known. Therefore, the objective of this study was to understand the processes controlling isoprene levels just before and after sunset, in an environment under the influences of moderate concentrations of ambient nitrogen oxides. Utilizing in-situ Proton-Transfer-Reaction Mass Spectrometer measurements, isoprene levels were studied during July to August 2011 in a sub-urban environment outside the metropolis of Washington, D.C., USA. Numerical modeling investigations were also pursued to determine yields of alkyl nitrates. Pre-sunset isoprene rises, observed on eight measured days, were likely related to the cessation of atmospheric turbulence and stabilization of the surface layer. After sunset, nocturnal declines of isoprene, while not fully explained by chemical reactions, depended on the amounts of nitrate radicals in the lower atmosphere. Furthermore, isoprene destruction rates and associated production of alkyl nitrates depended on the amounts of nitrogen oxides. Compared with cases with ambient nitrogen oxides lower than 10 parts per billion, model results showed that twice the amounts of isoprene were destroyed when average nitrogen oxide levels exceeded 30 parts per billion.