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标题：The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere
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作者：K.S. Krishnamohan ; Govindasamy Bala ; Long Cao 等
期刊名称：Earth's Future
电子版ISSN：2328-4277
出版年度：2020
卷号：8
期号：2
页码：1-14
DOI：10.1029/2019EF001326
出版社：John Wiley & Sons, Ltd.
摘要：Solar geoengineering by deliberate injection of sulfate aerosols in the stratosphere is one of the proposed options to counter anthropogenic climate warming. In this study, we focus on the effect of a specific microphysical property of sulfate aerosols in the stratosphere: hygroscopic growth—the tendency of particles to grow by accumulating water. We show that stratospheric sulfate aerosols, for a given mass of sulfates, cause more cooling when prescribed at the lower levels of the stratosphere because of hygroscopic growth. The larger relative humidity in the lower stratosphere causes an increase in the aerosol size through hygroscopic growth that leads to a larger scattering efficiency. In our study, hygroscopic growth provides an additional cooling of 23% (0.7 K) when 20 Mt‐SO4 of sulfate aerosols, an amount that approximately offsets the warming due to a doubling of CO2 , are prescribed at 100 hPa. The hygroscopic effect becomes weaker at higher levels as relative humidity decreases with height. Hygroscopic growth also leads to secondary effects such as an increase in near‐infrared shortwave absorption by the aerosols that causes a decrease in high clouds and an increase in stratospheric water vapor. The altitude dependence of the effects of hygroscopic growth is opposite to that of sedimentation effects or the fast adjustment effects due to aerosol‐induced warming identified in a recent study. Plain Language Abstract The injection of sulfate aerosols into the stratosphere is one of the proposed solar geoengineering methods to reduce anthropogenic warming. There are several uncertainties associated with the effects of stratospheric aerosol injection such as aerosol microphysics, transport, removal, and other physical and chemical changes of the aerosols. In this study, we have investigated the effects of a single source of uncertainty—the hygroscopic growth which depends on the ambient relative humidity in the stratosphere. Hygroscopic growth could lead to an increase in the size of the aerosols and consequently an increase in scattering efficiency. As the relative humidity is larger in the lower stratosphere, we find that the cooling efficiency of a fixed mass of sulfates is larger when they are prescribed in the lower levels of the stratosphere. This effect of hygroscopic growth can be substantial: for 20 Mt‐SO4 that offsets the warming from a doubling of CO2 , we find an additional cooling of about 20%. Our results indicate the need for an accurate parametrization of aerosol microphysical and radiative processes in climate models for an improved understanding of the effects of stratospheric aerosol injection.
关键词：stratospheric sulfate geoengineering;hygroscopic growth;scattering efficiency;sulfate aerosols;radiative forcing