Nighttime air quality under desert conditions

Abstract Nighttime concentrations of the gas phase nitrate radical (NO 3 ) were successfully measured during a four week field campaign in an arid urban location, Reno Nevada, using long-path Differential Optical Absorbance Spectrometry (DOAS). While typical concentrations of NO 3 ranged from 5 to 20 ppt, elevated concentrations were observed during a wildfire event. Horizontal mixing in the free troposphere was considerable because the sampling site was above the stable nocturnal boundary layer every night and this justified a box modeling approach. Process analysis of box model simulations showed NO 3 accounted for approximately half of the loss of internal olefins, 60% of the isoprene loss, and 85% of the α-pinene loss during the nighttime hours during a typical night of the field study. The NO 3  + aldehyde reactions were not as important as anticipated. On a polluted night impacted by wildfires upwind of the sampling location, NO 3 reactions were more important. Model simulations overpredicted NO 2 concentrations for both case studies and inorganic chemistry was the biggest influence on NO 3 concentrations and on nitric acid formation. The overprediction may be due to additional NO 2 loss processes that were not included in the box model, as deposition and N 2 O 5 uptake had no significant effect on NO 2 levels.