Comparison of the measured and simulated isoprene nitrate distributions above a forest canopy

Abstract

It is known that isoprene nitrate production can represent a significant sink for atmospheric NO x and free radicals, and therefore this chemistry is important to understanding tropospheric O 3 and the fate of NO x in forest‐impacted environments. Although six structural isoprene nitrate isomers can be produced from OH reaction with isoprene in the presence of NO, these have not been separately quantified in the atmosphere. A zero‐dimensional isoprene photochemistry model was developed based on the known gas‐phase isoprene oxidation chemistry for comparison with isoprene nitrate ambient concentration data obtained from field and laboratory measurements. The model incorporates calculated individual branching ratios for each isomer as well as rate constants for the reaction of each isomeric nitrate with the OH radical and O 3 and losses from the boundary layer by dry deposition and vertical mixing. The model indicates that under atmospheric conditions, there should be three nitrate isomers that represent 86% of the total, while our ambient measurements indicate only two dominant nitrate isomers. This contrasts significantly with what is observed in the laboratory because of atmospheric conversion to other nitrogen‐containing products. These secondary nitrates are likely to be a significant fraction of NO y in forest environments.