ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-11451-2012 Indirect radiative forcing by ion-mediated nucleation of aerosol Yu F. 1 Luo G. 1 Liu X. 2 Easter R. C. 2 Ma X. 1 Ghan S. J. 2 Atmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Road, Albany, NY 12203, USA Atmospheric Science & Global Change Division, Pacific Northwest National Laboratory, 3200 Q Avenue, MSIN K9-24 Richland, WA 99352, USA 03 12 2012 12 23 11451 11463 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/11451/2012/acp-12-11451-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/11451/2012/acp-12-11451-2012.pdf

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<sub>2</sub>SO<sub>4</sub>-H<sub>2</sub>O binary homogeneous nucleation (BHN), the presence of ionization (i.e., IMN) halves H<sub>2</sub>SO<sub>4</sub> 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<sup>−2</sup> (more negative) and longwave cloud forcing by 1.78 W m<sup>−2</sup> (more positive), with large spatial variations. The effect of ionization on SWCF derived from this study (3.67 W m<sup>−2</sup>) is a factor of ~3 higher that of a previous study (1.15 W m<sup>−2</sup>) 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<sup>−2</sup>) but have larger inter-annual (from −0.18 to 0.17 W m<sup>−2</sup>) and spatial variations.

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