The changing role of organic nitrates in the removal and transport of NO x

Abstract

A better understanding of the chemistry of nitrogen oxides ( NO x ) is crucial to effectively reducing air pollution and predicting future air quality. The response of NO x lifetime to perturbations in emissions or in the climate system is set in large part by whether NO x loss occurs primarily by the direct formation of HNO3 or through the formation of alkyl and multifunctional nitrates ( RONO2). Using 15 years of detailed in situ observations, we show that in the summer daytime continental boundary layer the relative importance of these two pathways can be well approximated by the relative likelihood that OH will react with NO2 or instead with a volatile organic compound (VOC). Over the past decades, changes in anthropogenic emissions of both NO x and VOCs have led to a significant increase in the overall importance of RONO2 chemistry to NO x loss. We find that this shift is associated with a decreased effectiveness of NO x emissions reductions on ozone production in polluted areas and increased transport of NO x from source to receptor regions. This change in chemistry, combined with changes in the spatial pattern of NO x emissions, is observed to be leading to a flatter distribution of NO2 across the United States, potentially transforming ozone air pollution from a local issue into a regional one.