The global effect of nitric acid on cloud activation and the associated radiative forcings

The condensation of gas-phase nitric acid onto a population of aerosol particles undergoing cloud-drop activation causes an enhancement of the activated fraction. This is due to the hygroscopicity of nitric acid, and its uneven condensation to the different sized aerosol particles: smaller particles with larger surface-to-volume ratio will absorb relatively more nitric acid within a given time, increasing their chances to become activated. However, the enhancement of the activated fraction depends on a number of parameters (nitric acid concentration, the number, size distribution, and chemical composition of the aerosol, updraft velocity, temperature), and it is therefore a complex process. We have simulated the the global effects of nitric acid on cloud droplet concentrations and to the radiative balance of the Earth using the ECHAM5.5-HAM2 climate model. The nitric acid effect was described using the parametrization of Romakkaniemi et al. (ACP 5, 879–885, 2005), which is based on a large number of adiabatic parcel model simulations. The nitric acid contribution to the present-day cloud albedo effect was found to be -0.32Wm-2 and the total indirect effect -0.46Wm-2. Differences to the study of Xu and Penner (ACPD 12, 10115, 2012) are discussed.