Abstract
Abstract. In this paper, we investigate the coagulation of interstitial aerosol particles (particles too small to activate to cloud droplets) with cloud drops, a process often ignored in aerosol-climate models. We use the GEOS-Chem-TOMAS (Goddard Earth Observing System-Chemistry TwO-Moment Aerosol Sectional) global chemical transport model with aerosol microphysics to calculate the changes in the aerosol size distribution, cloud-albedo aerosol indirect effect, and direct aerosol effect due to the interstitial coagulation process. We find that inclusion of interstitial coagulation in clouds lowers total particle number concentrations by 15–21% globally, where the range is due to varying assumptions regarding activation diameter, cloud droplet size, and ice cloud physics. The interstitial coagulation process lowers the concentration of particles with dry diameters larger than 80 nm (a proxy for larger CCN) by 10–12%. These 80 nm particles are not directly removed by the interstitial coagulation but are reduced in concentration because fewer smaller particles grow to diameters larger than 80 nm. The global aerosol indirect effect of adding interstitial coagulation varies from +0.4 to +1.3 W m−2 where again the range depends on our cloud assumptions. Thus, the aerosol indirect effect of this process is significant, but the magnitude depends greatly on assumptions regarding activation diameter, cloud droplet size, and ice cloud physics. The aerosol direct effect of the interstitial coagulation process is minor (< 0.01 W m−2) due to the shift in the aerosol size distribution at sizes where scattering is most effective being small. We recommend that this interstitial scavenging process be considered in aerosol models when the size distribution and aerosol indirect effects are important.
Highlights
Atmospheric aerosol particles of both anthropogenic and natural origin have important effects on human health (Dockery et al, 1993), visibility (Malm et al, 2000) and climate (Boucher et al, 2013)
We describe the GEOS-Chem-TOMAS global chemical transport model with online aerosol microphysics used in this study, the modifications we made to the model, and the various model simulations
We simulate global aerosol size distributions using the GEOS-Chem-TOMAS model, which is a coupling of the GEOS-Chem global chemical transport model with the TOMAS microphysics scheme (Adams and Seinfeld, 2002; Lee and Adams, 2012)
Summary
Atmospheric aerosol particles of both anthropogenic and natural origin have important effects on human health (Dockery et al, 1993), visibility (Malm et al, 2000) and climate (Boucher et al, 2013) The magnitude of these aerosol effects depend on the concentration, composition and size of the particles. The particles may affect climate directly by scattering and absorbing solar radiation (the aerosol direct effect; Charlson et al, 1992) and indirectly by acting as cloud condensation nuclei (CCN, the seeds for cloud-droplet formation) and affecting cloud radiative properties (the aerosol indirect effect; Twomey, 1974; Albrecht, 1989).
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