Abstract
Abstract. 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.
Highlights
Aerosol particles formed in the atmosphere influence climate indirectly by acting as cloud condensation nuclei (CCN) that affect cloud properties and precipitation (Twomey, 1977; Albrecht, 1989)
The high H2SO4 burdens extending from northern Chile to northern Argentina and southern Brazil are primarily due to volcanic emissions in the Andes
Because of the attractive interaction between the HSO−4 ion and the electric dipole of H2SO4, ion-mediated nucleation (IMN) occurs at [H2SO4] lower than that needed for binary homogeneous nucleation (BHN) to occur
Summary
Aerosol particles formed in the atmosphere influence climate indirectly by acting as cloud condensation nuclei (CCN) that affect cloud properties and precipitation (Twomey, 1977; Albrecht, 1989). Kazil et al (2010) investigated the impact of the individual aerosol nucleation mechanisms (neutral and charged nucleation of sulfuric acid throughout the troposphere, and cluster activation limited to the forested boundary layer) on the Earth’s energy balance, and showed that the change in the net top of atmosphere shortwave radiative flux associated with nucleation is around −2.55 W m−2. These previous studies highlight the importance of a clear understanding of atmospheric particle nucleation processes and proper representation of these processes in the climate models.
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