The Impact of Nitrate Radical Chemistry on Air Quality Modeling for Arid Conditions

The nitrate radical (NO3) is an important oxidant found in the troposphere during the nighttime. Studies show that nocturnal chemical processes involving NO3 can remove reactive nitrogen oxides (NOx) from the atmosphere subsequently affecting ozone production on the next day. A modeling analysis has been performed of nitrate radical measurements made during a four-week field campaign in an arid urban area located over Reno, Nevada. NO3 measurements were made with a long path Differential Optical Absorbance Spectrometry (DOAS) instrument. Measurements of ozone (O3), nitric oxide (NO), total nitrogen (NOy), sulfur dioxide (SO 2) and carbon monoxide (CO) were available and also included in the analysis.;During the field study, typical concentrations of nitrate radical ranged from 5 to 20 parts per trillion (ppt) with elevated concentrations that were observed during a wildfire event. A chemical box model that incorporated the Regional Atmospheric Chemistry Mechanism, version 2 (RACM2) (Goliff et al., 2012) was used for the study. Process analysis of the model simulations showed that NO3 chemistry was a significant loss process for olefins, isoprene, a-pinene, and dienes. Nitrate radical also reacted very strongly with aldehydes (acetaldehyde and formaldehyde) to form nitric acid (HNO 3). In some cases, NO3 effectively removed NOx from the atmosphere; however it did not significantly impact ozone formation on subsequent days. For the cases presented here, inorganic chemistry had the biggest influence on NO3 mixing ratios. In contrast to previous studies, these findings suggest that ozone production is not controlled or limited by NOx under urban arid desert-like conditions. The results of this study are crucial for advancing our understanding of urban nighttime chemistry, providing better air quality control strategies and ultimately improving multi-day air quality forecast models.