Impact of Heatwaves and Declining NOx on Nocturnal Monoterpene Oxidation in the Urban Southeastern United States (Invited Presentation)

Nighttime oxidation of monoterpenes (MT) via the nitrate radical (NO₃) and ozone (O₃) contributes to the formation of secondary organic aerosol (SOA). We use observations in Atlanta, Georgia from 2011–2022 to quantify trends in nighttime production of NO₃ (PNO₃) and O₃ concentrations and compare to model outputs from the EPA’s Air QUAlity TimE Series Project (EQUATES). We present urban-suburban gradients in nighttime NO₃ and O₃ concentrations and quantify their fractional importance (F) for MT oxidation.

Both observations and EQUATES show a decline in PNO₃, with modeled PNO₃ declining faster than observations. Despite decreasing PNO₃, we find that NO₃ continues to dominate nocturnal boundary layer (NBL) MT oxidation (F_NO3 = 60%) in 2017, 2021, and 2022, which is consistent with EQUATES (F_NO3 = 80%) from 2013–2019. This contrasts an anticipated decline in F_NO3 based on prior observations in the nighttime residual layer, where O₃ is the dominant oxidant.

Using two case studies of heatwaves in summer 2022, we show that extreme heat events can increase NO₃ concentrations and F_NO3, 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_x emissions, and highlight the need for improved representation of extreme heat events in chemistry-transport models and additional observations along urban to rural gradients.