The Impacts of Wildfires on Ozone Production and Boundary Layer Dynamics in California's Central Valley

Abstract

We investigate the role of wildfire smoke on ozone photochemical production (P(O 3 )) and atmospheric boundary layer (ABL) dynamics in California's Central Valley during June-September, 2016-2020. Wildfire events are identified by the Hazard Mapping System (HMS) and Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT). Air quality and meteorological data are acquired from 10 monitoring sites operated by the California Air Resources Board (CARB) across the Central Valley. During wildfire influenced periods, maximum daily 8h averaged (MDA8) O 3 was enhanced by about 5 ppb (˜10%) across the entire valley after the temperature correction. The photochemical ozone production rate calculated from a modified Leighton relationship was also found to be higher by 35% on average compared to non-fire periods despite the average diminution of by ˜7% due to the shading effect of the wildfire plumes. Furthermore, the in-situ ozone production rates are found to be elevated due to an increase of both peroxyl radicals (˜24%) and NO (˜11%). Surface heat flux measurements from two AmeriFlux sites in the Northern San Joaquin Valley show midday surface buoyancy fluxes decrease by 30% on average when influenced by wildfire smoke. Further, ABL height measured from a radio acoustic sounding system (RASS) located in Visalia in the Southern San Joaquin Valley were found to decrease 80 m and virtual potential temperatures in ABL are higher on average by ˜1.5 K when wildfire smoke is present. The increased temperature is likely the result of shortwave-radiation absorption by the additional aerosols in the wildfire smoke.