Space-Based Observations of Ozone Precursors within California Wildfire Plumes and the Impacts on Ozone-NO_x-VOC Chemistry

The frequency of wildfires in the western United States has escalated in recent decades. Here we examine the impacts of wildfires on ground-level ozone (O₃) precursors and the O₃-NO_x-VOC chemistry from the source to downwind urban areas. We use satellite retrievals of nitrogen dioxide (NO₂) and formaldehyde (HCHO, an indicator of VOC) from the Tropospheric Monitoring Instrument (TROPOMI) to track the evolution of O₃ precursors from wildfires over California from 2018 to 2020. We improved these satellite retrievals by updating the a priori profiles and explicitly accounting for the effects of smoke aerosols. TROPOMI observations reveal that the extensive and intense fire smoke in 2020 led to an overall increase in statewide annual average HCHO and NO₂ columns by 16% and 9%. The increase in the level of NO₂ offsets the anthropogenic NO_x emission reduction from the COVID-19 lockdown. The enhancement of NO₂ within fire plumes is concentrated near the regions actively burning, whereas the enhancement of HCHO is far-reaching, extending from the source regions to urban areas downwind due to the secondary production of HCHO from longer-lived VOCs such as ethene. Consequently, a larger increase in NO_x occurs in NO_x-limited source regions, while a greater increase in HCHO occurs in VOC-limited urban areas, both contributing to more efficient O₃ production.