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Possible influence of anthropogenic aerosols on cirrus clouds and anthropogenic forcing

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Cirrus clouds have a net warming effect on the atmosphere and cover about 30% of the Earth's area. Aerosol particles initiate ice formation in the upper troposphere through modes of action that include homogeneous freezing of solution droplets, heterogeneous nucleation on solid particles immersed in a solution, and deposition nucleation of vapor onto solid particles. Here, we examine the possible change in ice number concentration from anthropogenic soot originating from surface sources of fossil fuel and biomass burning, from anthropogenic sulfate aerosols, and from aircraft that deposit their aerosols directly in the upper troposphere. We find that fossil fuel and biomass burning soot aerosols exert a radiative forcing of 0.68 to 0.01 Wm 2 while anthropogenic sulfate aerosols exert a forcing of 0.01 to 0.18 Wm 2. Our calculations show that the sign of the forcing by aircraft soot depends on the model configuration and can be both positive or negative, ranging from 0.16 to 0.02 Wm 2. The magnitude of the forcing in cirrus clouds can be comparable to the forcing exerted by anthropogenic aerosols on warm clouds, but this forcing has not been included in past assessments of the total anthropogenic radiative forcing of climate.


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