Aerosol lifetime and climate change

Abstract

The dominant removal mechanism for atmospheric aerosol is activation of particles to cloud droplets and subsequent wet deposition in precipitation. The atmospheric life- time of aerosol is thus closely coupled to the atmospheric cycling time of water vapor. 5 Changes of hydrological cycle characteristics resulting from climate change therefore directly affect aerosol lifetime, and thus the radiative forcing exerted by aerosol. This study expresses the coupling between water vapor and aerosol lifetimes and their tem- perature sensitivities in fundamental equations and in terms of the efficiency of pro- cessing of air by precipitating clouds. Based on climate model simulations these tem- 10 perature sensitivities are estimated to be on the order of +5.3%K−1, but this may be an overestimation. Generally, shifting spatial and temporal patterns of aerosol (precur- sor) emissions and precipitation, and changes in aerosol activation efficiency probably influence aerosol lifetimes more than climate change itself, resulting in a wide range of simulated aerosol lifetime sensitivities between aerosol-climate models. It is possible 15 that the climate sensitivity of models plays a role. It can be argued that climate sensi- tivity is intrinsically coupled with the simulated (temperature sensitivity of the) aerosol lifetime through the distribution of water vapor and aerosol between the lower and upper troposphere. This implies a fundamental relation between various feedback forc- ings (water vapor, lapse rate, cloud) and the aerosol forcing, illustrating the key role of 20 the hydrological cycle in different aspects of the climate system.