Satellite Retrieval of Tropospheric NO2 under Fire Conditions

Tropospheric nitrogen oxides (NO_x = NO + NO₂) are key atmospheric pollutants with adverse impacts on human health and environmental quality. In the atmosphere, nitric oxide (NO) is rapidly oxidized to nitrogen dioxide (NO₂), making satellite observations of NO₂ an effective proxy for monitoring tropospheric NO_x distributions. Open fires, such as wildfires, emit large amounts of NO_x into the atmosphere, and their impacts are becoming increasingly severe under climate change. Satellite-based NO₂ observations provide broad spatial coverage and continuous monitoring capabilities for assessing NO₂ under fire conditions. However, due to the lack of explicit consideration of fire-related priori information in current satellite NO₂ retrieval algorithms, the resulting data products exhibit large uncertainties under fire conditions. Therefore, we use the Peking University OMI NO₂ (POMINO) retrieval algorithm to investigate the impact of including fire-related priori information on the retrieval of tropospheric NO₂ vertical column densities (VCDs). We conduct sensitivity experiments by including and excluding fire-related priori information in the retrieval of tropospheric NO₂ VCDs from TROPOMI observations. These experiments focus on the western United States during September 2020, a period of intense wildfire activity. To provide priori information for these retrievals, we use GEOS-Chem simulations with and without fire emissions, as well as with different fire emission injection heights. In addition, GEOS-CF is employed for a comprehensive comparative analysis. Our results show that including fire-related priori information in the retrieval significantly increases tropospheric NO₂ VCDs. Tropospheric NO₂ VCDs increase by up to 100% in regions heavily impacted by fires and by about 80% in surrounding areas. Differences in fire emission injection height lead to approximately 30% variations in the retrieved VCDs, indicating a secondary but non-negligible effect. Validation against EPA surface NO₂ measurements shows improved agreement when fire-related priori information is included, particularly in fire-affected regions. These results highlight the importance of incorporating fire-related priori information in satellite NO₂ retrievals to obtain more accurate NO₂ data and to better support air quality assessments under fire conditions.