Observational constraint of in-cloud supersaturation for simulations of aerosol rainout in atmospheric models

Quantitative simulation of an aerosol’s lifecycle by regional-scale and global-scale atmospheric models is mandatory for unbiased analysis and prediction of aerosol radiative forcing and climate change. Globally, aerosol deposition is dominated by the rainout process, which is mostly triggered by activation of aerosols to liquid droplets in supersaturated domains of precipitating clouds. However, the actual environmental supersaturation value that aerosols experience in precipitating clouds is difficult for models to predict, and it has never been constrained by observations; as a result, there is large uncertainty in atmospheric aerosol simulations. Here, by a particle-tracer analysis of 37 rainfall events in East Asia, near the largest source region of anthropogenic aerosols in the northern hemisphere, we observed that the environmental supersaturation actually experienced by the removed aerosols in precipitating clouds averaged 0.08 ± 0.03% and ranged from 0.03 to 0.2%. Simulations by a mixing-state-resolved global aerosol model showed that the simulated long-range transport efficiency and global atmospheric burden of black carbon aerosols can be changed by a factor of two or three as a result of a change in the environmental supersaturation in precipitating clouds within just 0.08 ± 0.03%. This result is attributable to the fact that the sensitivity of an aerosol’s rainout efficiency to environmental supersaturation is higher for the less-aged black carbon concentrated near source regions. Our results suggest that observational constraints of environmental supersaturation in precipitating clouds, particularly near source regions, are of fundamental importance for accurate simulation of the atmospheric burden of black carbon and other aerosols. Atmospheric science: Direct measurements of cloud conditions required for aerosol rainout A delicate balance determines whether or not precipitation will generate the rainout, or removal, of atmospheric aerosols. The process is critical for climate and human health, but is only approximately simulated in most models. A multi-institution team led by Nobuhiro Moteki from the University of Tokyo has quantified the cloud supersaturation conditions that are required to generate ‘nucleation scavenging’, the mechanism ultimately leading to aerosol rainout. By measuring black carbon aerosols in surface air before convective precipitation and comparing it to aerosols within the precipitation itself, the team established t