Meteorological conditions associated with visible CCN and INP perturbations on low clouds at industrial aerosol hot spots

Abstract. Aerosol impacts on clouds remain the most uncertain driver of anthropogenic climate change. Anthropogenic aerosols can brighten liquid-water clouds by serving as cloud condensation nuclei (CCN), but some aerosols also promote glaciation of supercooled liquid-water clouds by serving as ice-nucleating particles (INPs). Conditions favouring these contrasting responses remain poorly understood. Here, we compare meteorological conditions associated with visible CCN and INP perturbations at five isolated industrial aerosol sources in Russia and Canada during 2000–2021. We detect perturbations visually in satellite imagery and characterise the associated conditions using satellite remote sensing and atmospheric reanalysis. At the four sites with both perturbation types, 46 % of liquid-cloud days showed no visible aerosol signal, 33 % showed CCN perturbations and 21 % INP perturbations, with strong seasonality in all three categories. Synoptic-scale circulation strongly influenced whether liquid-phase clouds and visible aerosol perturbations occurred, but separated CCN and INP perturbations only weakly. Relative to climatology, both perturbation types occurred under anomalously high sea level pressure and dry air. INP perturbations occurred in substantially colder environments than CCN perturbations, mainly reflecting their cold-season occurrence and the presence of susceptible supercooled clouds. Several other differences, such as higher static stability during INP perturbations, largely disappeared once seasonality was removed. The CCN-to-INP perturbation ratio differed strongly between sites, with only INP perturbations detected near the cement plant in Volsk. This is consistent with expected differences in emitted aerosol properties and suggests that the properties of emitted aerosols strongly influence whether the CCN or INP effect dominates.