Absorption of sunlight by cloud-borne black carbon aerosol

Description

Absorption of sunlight by black carbon (BC) aerosol can be enhanced several-fold when the particles are embedded within a larger scattering medium such as water. This enhancement is thought to contribute to the large estimates of radiative warming by anthropogenic BC. However, cloud-borne BC constitutes only a small fraction of the total atmospheric burden of BC, so solar absorption by cloud-borne BC is unlikely to contribute a large fraction to the total atmospheric absorption. Yet the collocation of cloud-borne BC with clouds suggests a potentially important role through the so-called semi-direct mechanism in which warming due to solar absorption in clouds reduces the cloud liquid water content. We have implemented solar absorption by cloud-borne BC in the Community Atmosphere Model (CAM5) and have estimated the impact of anthropogenic aerosol on the planetary energy balance with and without that absorption. We find that the direct effect of the cloud-borne BC is smaller than 0.01 W/m2, and the semi-direct effect is a cooling of less than 0.1 W/m2, with contributions from cloud changes on longwave radiation larger than shortwave radiation. This suggests that absorption of solar radiation by cloud-borne BC can be neglected in climate change simulations.