Droughts and PM_2.5 air pollution in California: the roles of wildfires

Although individual relationships between droughts, wildfires, and ambient PM_2.5 air pollution have been reported, the role of wildfires in the pathways from droughts to PM_2.5 remains uncertain. This study investigated the association between droughts, defined by the Standardized Precipitation Evapotranspiration Index (SPEI), and PM_2.5 concentrations in California, U.S., from 2006 to 2020. A generalized linear mixed model showed that PM_2.5 concentrations increased by 1.47 μg/m³ (SE = 0.10; p < 0.0001), on average, as drought conditions intensified by 1 unit of SPEI (lower SPEI values indicating drier conditions and greater drought severity). In stratified analysis, drought-related PM_2.5 increases were larger during wildfire days [3.29 μg/m³ (SE = 0.36)] compared to non-wildfire days [0.97 μg/m³ (SE = 0.08)] per unit decrease in SPEI. The likelihood of wildfire-impacted days increased by 89.9 % [odds ratio (OR) = 1.899] per unit decrease in SPEI, rising with the severity of droughts. On wildfire days, wildfire-induced PM_2.5 increased by 3.11 μg/m³ (SE = 0.33) per unit decrease in SPEI, on average. Wildfires were a crucial contributor to the drought-related PM_2.5 increases, as supported by the decline in the association per unit decrease in SPEI from 3.29 μg/m³ (p < 0.0001) to -0.10 μg/m³ (p = 0.1307) after adjusting for wildfire-induced PM_2.5 during wildfire days. PM_2.5 increases due to droughts and wildfires pose substantial challenges for air quality management because the likelihood of PM_2.5 exceedance days was elevated by wildfires during droughts (OR = 2.978 per unit decrease in SPEI). Understanding the role of wildfires informs air quality management in preparation for future droughts and wildfires.