Abstract
Abstract. A better understanding of the chemistry of nitrogen oxides (NOx) is crucial to effectively reducing air pollution and predicting future air quality. The response of NOx lifetime to perturbations in emissions or in the climate system is set in large part by whether NOx 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 NOx and VOCs have led to a significant increase in the overall importance of RONO2 chemistry to NOx loss. We find that this shift is associated with a decreased effectiveness of NOx emissions reductions on ozone production in polluted areas and increased transport of NOx from source to receptor regions. This change in chemistry, combined with changes in the spatial pattern of NOx 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.
Highlights
Nitrogen oxides (NOx ≡ NO+NO2) play a central role in the formation of toxic air pollutants including O3 and secondary aerosols
Due to its harmful effects to the environment and human health, nitrogen oxides (NOx) has been the target of emissions control strategies since the 1970s, causing anthropogenic NOx emissions in the United States to have decreased by a factor of 2 or more over the past 30 years (United States Environmental Protection Agency, 2018)
In a WRF-Chem simulation identical to those described in Zare et al (2018), RONO2 chemistry is found to be 60 % or more of the total NOx loss across broad swathes of the southeast United States (Fig. 1), while Fisher et al (2016) found RONO2 production to be concentrated in rather small sections of the southeast
Summary
Nitrogen oxides (NOx ≡ NO+NO2) play a central role in the formation of toxic air pollutants including O3 and secondary aerosols. Direct HNO3 production was thought to be the only important NOx loss pathway, with RONO2 chemistry playing at most a minor role. Browne and Cohen (2012) modeled NOx loss over the Canadian boreal forest using WRF-Chem and Fisher et al (2016) and Zare et al (2018) studied NOx loss in the southeast United States using GEOS-Chem and WRF-Chem, respectively. Can be well described by the relative OH reactivity of NO2 and of the combined VOC mixture As both anthropogenic NOx and anthropogenic VOC emissions have decreased substantially in the United States over the past 20 years, the relative role of these two pathways has shifted as well. Combined with changing emission patterns of NOx, the shift in NOx chemistry is leading to a flatter distribution of NOx across the continental United States
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