Cold-Air Pools and PM2.5 Exceedance Risk Across California

Wildfire smoke has become an important driver of fine particulate matter (PM2.5) exposure across the western United States, but the role of cold-air-pool conditions in shaping high-pollution episodes remains difficult to characterize across large regions. Cold-air pools can limit vertical mixing and trap pollution near the surface, creating conditions that may increase both the likelihood and severity of PM2.5 events. This study examines the relationship between cold-air-pool occurrence, wildfire smoke, and surface PM2.5 across California during 2018 using daily observations from 135 monitoring stations.

Cold-air-pool occurrence was identified using valley heat deficit, and mixed-effects models were used to evaluate its relationship with PM2.5 exceedance and high-pollution intensity while accounting for smoke exposure and station-level differences. CAP days were associated with substantially higher odds of PM2.5 exceedance above 35 µg/m³. Across CAP conditions, exceedance odds were approximately 2.6 to 4.0 times higher than on non-CAP days, indicating that stable cold-pool conditions increased the likelihood of high-PM2.5 events.

CAP conditions were also associated with stronger pollution episodes. On days already exceeding 35 µg/m³, PM2.5 concentrations were higher under CAP conditions, indicating that cold-air pools are linked not only to whether high-pollution events occur, but also to how severe those events become. This suggests that valley heat deficit can help identify periods when stable atmospheric conditions are most likely to worsen surface air quality during wildfire-smoke and stagnation episodes.