Short-term changes in ozone precursors during 2018 California wildfires observed from TROPOMI

While it is widely accepted that wildfires raise ground-level particulate matter levels, the extent to which wildfires influence ozone air quality is uncertain. Biomass burning emits substantial amounts of nitrogen oxides (NO_x) and volatile organic compounds (VOCs), which react in the presence of sunlight to produce tropospheric ozone (O₃). While current satellites cannot retrieve the abundance of ground-level O₃, they have provided continuous global observations of O₃ precursors, namely tropospheric columns of NO₂ and formaldehyde (HCHO, a proxy for VOCs) for over two decades. The NO₂ and HCHO products from these satellites, such as the Ozone Monitoring Instrument (OMI), are challenging to retrieve accurately for applications seeking to quantify day-to-day variability or the detailed spatial patterns of HCHO and NO₂ within urban areas. The TROPospheric Monitoring Instrument (TROPOMI), on board the Copernicus Sentinel-5 Precursor satellite, provides daily early afternoon observations of NO₂and HCHO since late 2017 with fine spatial resolution (7 × 3.5 km²) and less affected by clouds. We analyze day-to-day variability of HCHO, NO₂and their ratio, which provides insights into the ozone formation regime and its sensitivity to NO_x versus VOC precursors, with the new TROPOMI products during the California wildfires in 2018. We assess whether TROPOMI products capture transported plumes of O₃precursors from wildfires, and how this transport of O₃ precursors affects O₃ production over downwind areas. We show that wildfires can lead to large and rapid changes of ozone chemical regimes over downwind cities. For example, during the California Camp Fire in November 2018, we find a large enhancement of HCHO but little enhancement of NO₂ over San Francisco, which led to a rapid transition of the local ozone production regime from VOC-limited to NO_x-limited. In this case, the ozone production efficiency per NO_x increased as the wildfire plume provided abundant VOC to mix with urban NO_x and form O₃ outside of the typical "O₃ season". Our study aims to demonstrate how space-based HCHO/NO₂ from TROPOMI complements in-situ ozone networks and model simulations by providing information on the spatial heterogeneity and temporal evolution of ozone chemical regimes during wildfire events.