Planetary Boundary Layer Height and Air Quality during Heatwaves in contrasting climate regions from CALIPSO lidar retrievals.

The Height of the Planetary Boundary Layer (PBLH) plays a key role in controlling how air pollutants accumulate and disperse during heatwaves, yet its large-scale behaviour across different climate regimes remains poorly understood. In this study, we use a 10-year PBLH dataset derived from CALIPSO CALIOP Level-1 backscatter data, retrieved with a Random Forest model trained on radiosonde-based PBLH observations, to investigate boundary-layer dynamics during heatwaves across several regions of the world. The resulting product provides PBLH estimates at approximately 20 × 20 km resolution and shows good performance in mid-latitude regions under a wide range of aerosol and cloud conditions.Heatwaves are identified using ERA5 daily maximum temperature anomalies, applying region-specific percentile and persistence criteria over the Mediterranean and central Europe, the United States, eastern China megacities, and selected arid–subtropical areas. For each region, we construct composites of the diurnal evolution of PBLH during heatwave and non-heatwave summers and relate them to co-located surface PM2.5 and ozone observations from air-quality monitoring networks. This approach allows us to quantify regional differences in PBLH anomalies and in the sensitivity of PM2.5 and ozone to PBLH variations during heatwaves. We also examine how different stages of the heatwave life cycle are reflected in PBL evolution and the persistence of residual layers, highlighting implications for compound heatwave–air-pollution risks in a warming climate.