Impact of Heatwaves and Declining NOxon Nocturnal Monoterpene Oxidation in the Urban Southeastern United States

Nighttime oxidation of monoterpenes (MT) via the nitrate radical (NO3) and ozone (O3) contributes to the formation of secondary organic aerosol (SOA). This study uses observations in Atlanta, Georgia from 2011 to 2022 to quantify trends in nighttime production of NO3(PNO3) and O3concentrations and compare to model outputs from the EPA's Air QUAlity TimE Series Project (EQUATES). We present urban‐suburban gradients in nighttime NO3and O3concentrations and quantify their fractional importance (F) for MT oxidation. Both observations and EQUATES show a decline in PNO3, with modeled PNO3declining faster than observations. Despite decreasing PNO3, we find that NO3continues to dominate nocturnal boundary layer (NBL) MT oxidation (FNO3= 60%) in 2017, 2021, and 2022, which is consistent with EQUATES (FNO3= 80%) from 2013 to 2019. This contrasts an anticipated decline in FNO3based on prior observations in the nighttime residual layer, where O3is the dominant oxidant. Using two case studies of heatwaves in summer 2022, we show that extreme heat events can increase NO3concentrations and FNO3, leading to short MT lifetimes (<1 hr) 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 NOxemissions, and highlight the need for improved representation of extreme heat events in chemistry‐transport models and additional observations along urban to rural gradients. Monoterpenes are important precursors of secondary organic aerosol (SOA), which influence air quality and climate. At night, they react with the nitrate radical (NO3) and ozone (O3). Trends in these two oxidants and their role on air quality in the southeastern United Sates (SE US) is partially dictated by changes in nitrogen oxide (NOx= NO + NO2) emissions which have declined in recent years. We find that NO3dominates present‐day monoterpene loss in the summer, and may continue to do so even as NOxconcentrations decrease in the future. We show that heatwaves in the SE US further elevate both O3and NO3concentrations at night, with a larger relative importance for NO3. We compare observations to a regional air quality model, and find the model overpredicts both oxidant concentrations. This overprediction may impact model‐based studies of future nighttime chemistry. Nitrate radical (NO3) continues to control nocturnal boundary layer monoterpene oxidation in the urban SE USRegional model overpredicts oxidant concentrations and fractional NO3contribution to monoterpene lossExtreme heat events increase nighttime NO3concentrations, monoterpene oxidation rates, and gas‐phase organic nitrate production Nitrate radical (NO3) continues to control nocturnal boundary layer monoterpene oxidation in the urban SE US Regional model overpredicts oxidant concentrations and fractional NO3contribution to monoterpene loss Extreme heat events increase nighttime NO3concentrations, monoterpene oxidation rates, and gas‐phase organic nitrate production