A key role of wildfires in the association between droughts and PM2.5 air pollution

This study investigates the role of wildfires in elevating ambient PM_2.5 concentrations during droughts in California, U.S., from 2006 to 2020. While previous research has explored individual relationships between droughts, wildfires, and PM_2.5 concentrations, a comprehensive analysis integrating all these components is lacking. This study employs a multi-tiered statistical approach to estimate each relationship among droughts, wildfires, and PM_2.5 concentrations and to quantify the contribution of wildfires to the association between droughts and PM_2.5 air pollution. During the study period, PM_2.5 concentrations increased by 1.47 µg/m³ [standard error (SE)= 0.10] on average as drought conditions worsened by 1 unit of the Standardized Precipitation Evapotranspiration Index (SPEI). Drought-related PM_2.5 increases were greater during wildfire days [3.29 µg/m³ (SE= 0.36)] than during non-wildfire days [0.97 µg/m³ (SE= 0.08)] per unit decrease in SPEI. During wildfire days, the drought-related PM_2.5 increase substantially diminished from 3.29 µg/m³ (p< 0.0001) to -0.10 μg/m³ (p= 0.1307) after adjusting for wildfire-induced PM_2.5 concentrations. Furthermore, the likelihood of PM_2.5 exceedance days increased by 198% per unit decrease in SPEI due to wildfires during droughts. These findings demonstrate that the increase in drought-related PM_2.5 concentrations is largely attributable to wildfire-induced PM_2.5. Understanding the role of wildfires is crucial for air quality management and preparedness for future extreme events in the era of climate change.