Indirect Radiative Forcing by Ion-Mediated Nucleation of Aerosol

A clear understanding of particle formation mechanisms is critical for assessing aerosol indirect radiative forcing and associated climate feedback processes. Recent studies reveal the importance of ion-mediated nucleation (IMN) in generating new particles and cloud condensation nuclei (CCN) in the atmosphere. Here we implement the IMN scheme into the Community Atmosphere Model version 5 (CAM5). Our simulations show that, compared to globally averaged results based on H2SO4-H2O binary homogeneous nucleation (BHN), the presence of ionization (i.e., IMN) halves H2SO4 column burden, but increases the column integrated nucleation rate by around one order of magnitude, total particle number burden by a factor of ∼ 3, CCN burden by ∼ 10 % (at 0.2 % supersaturation) to 65 % (at 1.0 % supersaturation), and cloud droplet number burden by ∼ 18 %. Compared to BHN, IMN increases cloud liquid water path by 7.5 %, decreases precipitation by 1.1 %, and increases total cloud cover by 1.9 %. This leads to an increase of total shortwave cloud radiative forcing (SWCF) by 3.67 W m−2 (more negative) and longwave cloud forcing by 1.78 W m−2 (more positive), with large spatial variations. The effect of ionization on SWCF derived from this study (3.67 W m−2) is a factor of∼ 3 higher that of a previous study (1.15 W m−2) based on a different ion nucleation scheme and climate model. Based on the present CAM5 simulation, the 5-yr mean impacts of solar cycle induced changes in ionization rates on CCN and cloud forcing are small (∼−0.02 W m−2) but have larger inter-annual (from −0.18 to 0.17 W m−2) and spatial variations.