Evidence for Changes in Arctic Cloud Phase Due to Long‐Range Pollution Transport.

Reduced precipitation rates allow pollution within air parcels from midlatitudes to reach the Arctic without being scavenged. We use satellite and tracer transport model data sets to evaluate the degree of supercooling required for 50% of a chosen ensemble of low‐level clouds to be in the ice phase for a given meteorological regime. Our results suggest that smaller cloud droplet effective radii are related to higher required amounts of supercooling but that, overall, pollution plumes from fossil fuel combustion lower the degree of supercooling that is required for freezing by approximately 4 °C. The relationship between anthropogenic plumes and the freezing transition temperature from liquid to ice remains to be explained. Plain Language Summary: Anthropogenic pollution plumes from midlatitudes can be transported long distances to the Arctic. In this study, we analyze the impact of these plumes on how easily liquid clouds over the Arctic Ocean freeze by using a novel combination of satellite measurements and a pollution transport model. We find that liquid clouds in polluted air switch phase to become ice clouds at temperatures that are 4 °C higher they would otherwise in pristine air. Because ice clouds in the Arctic precipitate more easily than liquid clouds, the potential is that distant industrial pollution sources are acting to reduce arctic cloud life time. Key Points: MODIS and POLDER‐3 can be used to provide space‐based measurements of phase transitions in liquid arctic cloudsA smaller droplet effective radius is associated with an increase in the amount of supercooling required for freezingAnthropogenic pollution plumes are associated with a decrease in the amount of supercooling required for freezing [ABSTRACT FROM AUTHOR]