Ice nucleation by anthropogenic aerosols downwind of industrial point sources of air pollution

Satellite observations reveal the glaciation of supercooled stratiform liquid phase clouds by anthropogenic aerosols acting as ice-nucleating particles downwind of industrial aerosol point sources. Glaciation events are observed downwind of oil refineries, coal-fired power plants, cement, metal smelting and processing, chemical plants, and other anthropogenic air pollution sources in Europe, Asia, North America and Australia. Glaciation leads to snowfall, reduced cloud cover and reduced back-scattering of solar radiation to space. The polluted areas with decreased cloud cover are plume-shaped, with a distinctive head pointing towards the pollution source, similar to aerosol-polluted cloud tracks in liquid-water clouds (Toll et al 2019 Nature https://doi.org/10.1038/s41586-019-1423-9). Moreover, heavily polluted areas derived by simulating aerosol dispersion from strong anthropogenic aerosol point sources overlap with the areas of glaciation, snowfall, and decreased cloud cover strikingly well. These lines of evidence suggest that anthropogenic aerosols are the cause of the observed glaciation events.

Glaciation-induced snowfall downwind of aerosol sources is observed using ground-based precipitation radars, and tracks of snow are also seen on the ground in satellite imagery. Thus, at least locally, anthropogenic ice nucleating particles could significantly affect precipitation amounts. At multiple aerosol sources, glaciation events are more frequent than polluted tracks in liquid-phase clouds. Thus, at least locally at some aerosol sources in the middle and high latitudes, the warming effect induced by aerosols acting as ice-nucleating particles likely exceeds the cooling effect induced by aerosols acting as cloud condensation nuclei. Further research is needed to quantify the global radiative forcing by anthropogenic ice nucleating particles.