Abstract
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.
Highlights
Atmospheric aerosols play an important role in Earth’s climate system through direct and indirect modification of the radiation budget.[1,2] Most anthropogenic and biomass-burning aerosol particles of radiative significance belong to the accumulation mode, from ~0.1 to ~2 μm.[3]
Atmospheric models that have been validated by precipitation observations may be reasonably able to predict the mean condensed-water production in a particular 4D domain, but it is difficult for current atmospheric models to predict the mean SSlsd value in a particular 4D domain with verified accuracy because in a non-adiabatic precipitating cloud, environmental supersaturation is highly sensitive to many physical processes at both macroscopic to microscopic scales.[9,10]
We estimated the SSlsd value experienced by the removed aerosols from the results of a detailed comparison between the black carbon (BC)-containing aerosols in the surface air before the rainfall and the BC particles in the rainwater, under the assumption that the former had been transformed to the latter through localized moist convection (BC tracer method: Fig. 1)
Summary
Atmospheric aerosols play an important role in Earth’s climate system through direct and indirect modification of the radiation budget.[1,2] Most anthropogenic and biomass-burning aerosol particles of radiative significance belong to the accumulation mode, from ~0.1 to ~2 μm.[3]. In each LSD, the aerosol’s critical supersaturation (SSc) value and the environmental supersaturation (SSlsd) value together determine the proportion of the aerosol particles susceptible to nucleation scavenging. The rainout amount of aerosols in a particular space-and-time (4D) domain comprising an ensemble of LSDs (e.g., a computational grid cell and the computational time interval in an atmospheric model) is determined primarily by two 4D-domainaveraged variables: condensed-water production and the SSlsd value maintained during droplet nucleation in each LSD. Atmospheric models that have been validated by precipitation observations may be reasonably able to predict the mean condensed-water production in a particular 4D domain, but it is difficult for current atmospheric models to predict the mean SSlsd value in a particular 4D domain with verified accuracy because in a non-adiabatic precipitating cloud, environmental supersaturation is highly sensitive to many physical processes at both macroscopic to microscopic scales.[9,10] As a result, the prognostic environmental supersaturation value provided as an output variable by the aerosol activation parameterization scheme[11,12,13] of an atmospheric model is not necessarily close to the 4D-domain-averaged
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