Impact of Heatwaves and Declining NO on Nocturnal Monoterpene Oxidation in the Urban Southeastern United States.

Nighttime oxidation of monoterpenes (MT) via the nitrate radical (NO) and ozone (O) contributes to the formation of secondary organic aerosol (SOA). This study uses observations in Atlanta, Georgia from 2011-2022 to quantify trends in nighttime production of NO (PNO) and O concentrations and compare to model outputs from the EPA's Air QUAlity TimE Series Project (EQUATES). We present urban-suburban gradients in nighttime NO and O concentrations and quantify their fractional importance (F) for MT oxidation. Both observations and EQUATES show a decline in PNO, with modeled PNO declining faster than observations. Despite decreasing PNO, we find that NO continues to dominate nocturnal boundary layer (NBL) MT oxidation (F = 60%) in 2017, 2021, and 2022, which is consistent with EQUATES (F = 80%) from 2013-2019. This contrasts an anticipated decline in F based on prior observations in the nighttime residual layer, where O is the dominant oxidant. Using two case studies of heatwaves in summer 2022, we show that extreme heat events can increase NO concentrations and F, leading to short MT lifetimes (<1 h) and high gas-phase organic nitrate production. Regardless of the presence of heatwaves, our findings suggest sustained organic nitrate aerosol formation in the urban SE US under declining NO emissions, and highlight the need for improved representation of extreme heat events in chemistry-transport models and additional observations along urban to rural gradients.