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 H ₂SO ₄-H ₂O binary homogeneous nucleation (BHN), the presence of ionization (i.e., IMN) halves H ₂SO ₄ 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.