Wildfire aerosols lofted by North American pyrocumulonimbus clouds: long-range transport and aerosol-cloud-radiative effects

Abstract. Extreme wildfires threaten health, air quality, and ecosystems. Despite extensive study of meteorological links, the feedback mechanisms by which fire weather influences the long-range smoke transport remain poorly understood. This study examines the transcontinental transport of smoke aerosols emitted by intense North American wildfires in August 2024. Our analysis reveals that pyrocumulonimbus clouds (PyroCbs) formed in extreme fires exhibit strong vertical convection, injecting large amounts of smoke aerosols into upper troposphere and lower stratosphere. These lofted aerosols exhibit enhanced hydrophilicity at high altitudes, increasing the cloud condensation nuclei by a factor of 2–3. Therefore, the water cloud droplet effective radius decreases by 1/3, and the cloud fraction increases from 0.01 to 0.64, promoting the development of optically thick, high-level clouds. PyroCb efficiently lifts aerosols, prolonging their residence time and enabling long-range transport through high-altitude winds. This process significantly affects regional and global radiation, with aerosols heating downwind Europe. Smoke aerosols produced consistent effects on radiative fluxes: they reduced longwave fluxes, while increased shortwave fluxes, resulting in net anomalies of +2.84 W m⁻² over fire sources and +3.16 W m⁻² in smoke-transported areas. Conversely, aerosols over fire-source regions caused heterogeneous radiative responses, with net cooling anomalies of -2.5 W m⁻² along the US West Coast and +5.62 W m⁻² across North America. Our findings underscore the complex interplay between wildfires, smoke aerosols, and meteorology, forming a positive feedback loop that amplifies air pollution transport and radiative perturbations across continental scales.