[paper] The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere

[Govindasamy Bala]

Hi Andrews,

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019EF001326  

This paper in Earth's Future (published online last week) quantifies the climatic effects of hygroscopic growth of sulfate aerosols in the stratosphere. Here, the cooling effect is more when aerosols are placed in the lower stratosphere. This is exactly opposite to the sedimentation related residence time effects and the effects associated with the stratospheric heating induced by aerosols (our recent study published in Earth System Dynamics, both abstracts are shown below). Looks like too many factors are involved in deciding the net effect. 

The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere

Abstract

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 which 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‐IR shortwave absorption by the aerosols which 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.

https://www.earth-syst-dynam.net/10/885/2019/ 

Climate system response to stratospheric sulfate

aerosols: sensitivity to altitude of aerosol layer

Abstract. Reduction of surface temperatures of the planet by injecting sulfate aerosols in the stratosphere has been suggested as an option to reduce the amount of human-induced climate warming. Several previous studies
have shown that for a specified amount of injection, aerosols injected at a higher altitude in the stratosphere would produce more cooling because aerosol sedimentation would take longer. In this study, we isolate and assess the
sensitivity of stratospheric aerosol radiative forcing and the resulting climate change to the altitude of the aerosol layer.We study this by prescribing a specified amount of sulfate aerosols, of a size typical of what is produced by
volcanoes, distributed uniformly at different levels in the stratosphere.We find that stratospheric sulfate aerosols are more effective in cooling climate when they reside higher in the stratosphere. We explain this sensitivity in
terms of effective radiative forcing: volcanic aerosols heat the stratospheric layers where they reside, altering stratospheric water vapor content, tropospheric stability, and clouds, and consequently the effective radiative
forcing. We show that the magnitude of the effective radiative forcing is larger when aerosols are prescribed at higher altitudes and the differences in radiative forcing due to fast adjustment processes can account for a substantial part of the dependence of the amount of cooling on aerosol altitude. These altitude effects would be additional to dependences on aerosol microphysics, transport, and sedimentation, which are outside the scope of this study. The cooling effectiveness of stratospheric sulfate aerosols likely increases with the altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have higher stratospheric residence time; these two effects are likely to be of comparable importance.

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With Best Wishes,

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G. Bala
Professor
Center for Atmospheric and Oceanic Sciences
Indian Institute of Science
Bangalore - 560 012
India

Tel: +91 80 2293 3428; +91 80 2293 2505
Fax: +91 80 2360 0865; +91 80 2293 3425
Email: gb...@iisc.ac.in; bala.gov@gmail.com
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[Alan Robock]

Dear Bala,

These studies are very interesting, but they only examine isolated processes.  For example, if you allow the aerosols to grow, they will fall out of the stratosphere more rapidly, negating their enhanced radiative effects.  Have you planned an experiment with an interactive aerosol model, such as in WACCM6, to see how this all plays out?  Or have you just looked at already conducted studies, such as GLENS, to diagnose these processes?

Alan

Alan Robock, Distinguished Professor
  Associate Editor, Reviews of Geophysics
Department of Environmental Sciences             Phone: +1-848-932-5751
Rutgers University                    E-mail: rob...@envsci.rutgers.edu
14 College Farm Road            http://people.envsci.rutgers.edu/robock
New Brunswick, NJ 08901-8551  USA      ☮ http://twitter.com/AlanRobock

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