Anthropogenic aerosols, tiny solid and liquid air pollution particles, have masked a fraction of global warming caused by anthropogenic greenhouse gases. Climate researchers have known for decades that anthropogenic aerosols perturb liquid clouds by enabling the formation of a larger number of cloud droplets, making clouds brighter. A new landmark study led by the University of Tartu suggests that anthropogenic aerosols may also influence clouds by converting cloud droplets to ice at temperatures below zero degrees Celsius.
Powerplant Snow
Using satellite observations, climate researchers discovered unique plumes of ice clouds and reduced cloud cover downwind of industrial hot spots in North America, Europe and Asia. Moreover, ground-based precipitation radar data revealed plumes of snowfall in the same areas where reduced cloud cover was observed in satellite images. Combining satellite and ground-based radar observations, researchers traced the physical processes from the formation of ice to snowfall to reduced cloud cover downwind of industrial hot spots. The lead author of the study, Assoc Prof V. Toll from the University of Tartu, highlighted that collaboration among researchers with diverse expertise was essential for developing the physical understanding of the identified anthropogenic snowfall events.
Supercooled Water
Water freezes at zero degrees Celsius, right? In fact, cloud droplets can stay liquid down to temperatures as low as about -40 degrees Celsius, known as the supercooling of water. This is because suitable aerosol particles are needed to convert cloud droplets to ice at temperatures between zero and -40 degrees Celsius. The study suggests that industries such as metallurgical and cement factories, coal-fired power plants, and oil refineries emit aerosol particles that cause freezing of supercooled liquid clouds, leading to snowfall. However, it is important to note that heat and water vapour emitted by industries may also play a role in the freezing of supercooled liquid clouds.
The discovered plumes of reduced cloud cover are local phenomena, and it remains unclear if anthropogenic aerosols induce ice formation in clouds at larger spatial scales. Further research is needed to understand the ability of various types of anthropogenic aerosols to initiate the formation of ice.
