Effects of the 2020 Gigafire on Tropospheric Ozone Chemistry in the United States

Julie Christopoulos

During 2020, the United States endured unseasonably higher fire activities in the western region compared to previous years. Throughout the period August 1st to October 31st, the western United States experienced its first known “Gigafire,” which burned more than 7.8 million acres of land. Wildfires are known to generate a substantial amount of O3 precursors, however, capturing the degree of O3 production from wildfires becomes complicated due to the wildfire emissions complexity, nonlinearity in plume photochemistry, aerosol radiative effects, mixing with external air masses, and additional plume dynamics. This work aims to identify the sources of O3 forecast uncertainty by the Community Multiscale Air Quality (CMAQ) Model and understand impacts on air quality during the 2020 Gigafire. We focus on the peak of the fire season, August 1st to October 31st, 2020, and utilize CMAQ V5.3.1 to examine two scenarios, one incorporating fires, CMAQFire, and one without, CMAQNoFire . A model evaluation of O3 and its precursors (NOx, CO, VOCS) is performed with in-situ measurements obtained from the AirNow and AQS networks. An analysis of O3 concentrations surrounding the fire region illustrates the capability of the model to successfully simulate downwind O3 increases; however, O3 remains underestimated close to the fire source. Evaluation of precursors in proximity to the fire source reveals model overestimates of NO2 and further underestimates of CO. Finally, we quantify the contribution of wildfire emissions to surface O3 production using the difference of simulated O3 fields between CMAQFire and CMAQNoFire runs.