Global hotspots and mechanisms of extreme humid heat and air pollution co-occurrence

Abstract. Exposure to extreme humid heat and air pollution each represent significant, well-characterized environmental hazards to human health. But the questions of where, when, and why they may co-occur, and whether humid heat may exacerbate pollution relative to high temperatures alone, remain largely unexplored. Here, we identify regions worldwide where ozone (O₃) or particulate matter (PM_2.5) pollution tend to be higher during humid versus non-humid extreme heat – i.e., where increased moist heat stress tends to co-occur with increased pollution, revealing a compound hazard tendency – and characterize the meteorological and chemical drivers of this co-occurrence. We analyze 19 years of near-surface concentrations of ozone, PM_2.5, and related species (NO₂ and HCHO) in the Copernicus Atmosphere Monitoring Service global chemical reanalysis (CAMSRA), along with meteorological conditions from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5). We find that the global hotspots of worsened pollution during humid heat overlap with several global hotspots of extreme humid heat itself, and include multiple densely-populated areas. Altogether, more of the global population experiences worsened air quality during humid heat (versus dry heat) than experiences cleaner air quality. Overall, we find that humid heat and pollution co-occurrence hotspots typically occur where (1) the near-surface background chemical makeup is more urban (higher NO₂, lower HCHO), and (2) humid heat is associated with stagnation and suppressed boundary layer heights (as is common in areas that experience severe humid heat), such that the local meteorological drivers of extreme humid heat are also conducive to pollutant accumulation.