Direct and Indirect Shortwave Radiative Effects of Sea Salt Aerosols

Abstract

Sea salt aerosols play a dual role in affecting the atmospheric radiative balance. Directly, sea salt particles scatter the incoming solar radiation and absorb the outgoing terrestrial radiation. By acting as cloud condensation nuclei, sea salt aerosols indirectly modulate the atmospheric radiative budget through their effective contribution to cloud formation. Using the Canadian Aerosol Module (CAM)-Canadian Centre for Climate Modelling and Analysis (CCCma) GCM, version 3 (GCM3) framework, the direct as well as the indirect shortwave (SW) radiative effects of sea salt aerosols are simulated. The model results herein suggest that sea salt aerosols exert a significant direct radiative effect over oceanic regions, with seasonal means in the range from Ϫ2 to Ϫ3 W m Ϫ2 over the Southern Ocean. Globally, sea salt's SW indirect effect (annual mean Ϫ0.38 W m Ϫ2 ) is found to be less than its direct effect (annual mean Ϫ0.65 W m Ϫ2 ). However, sea salt's indirect effect is found to be far stronger over the Southern Hemisphere than over the Northern Hemisphere, especially over the Southern Ocean with seasonal means around Ϫ4 W m Ϫ2 , which exceed its direct effect. The model results herein suggest that sea salt aerosols significantly modulate the atmospheric radiation budget over oceanic regions and need to be accounted for in global climate models.

Numerical Investigation of Sea Salt Aerosol Size Bin Partitioning in Global Transport Models: Implications for Mass Budget and Optical Depth

“…Some recent model studies by Ma et al (2008) indicate the first indirect SSA radiative forcing to be -1.34 (Wm −2 ) and the total indirect forcing to be -2.9 (Wm −2 ) if climatic feedbacks are taken into account. A much smaller assessment of the indirect effect equal to -0.38 (Wm −2 ) was reported by Ayash et al (2008). Such a large spread of direct and indirect forcing estimates is mainly related to the uncertainties in sea salt emission modeling (Ayash et al 2008).…”

“…A much smaller assessment of the indirect effect equal to -0.38 (Wm −2 ) was reported by Ayash et al (2008). Such a large spread of direct and indirect forcing estimates is mainly related to the uncertainties in sea salt emission modeling (Ayash et al 2008). The uncertainties of emission function formulation and inadequate simulation of wind velocity and its variability in global models are considered to be the main culprits (Lewis and Schwartz 2004;Pierce and Adams 2006;Ma et al 2008).…”

“…It affects atmospheric chemistry, cloud physics, atmospheric radiation, remote sensing, and air quality. The importance of SSA on Earth's radiation budget has recently been recognized (Winter and Chylek 1997;Haywood et al 1999;Jacobson 2001;Dobbie et al 2003;Satheesh and Lubin 2003;Ma et al 2008;Ayash et al 2008). SSA particles scatter solar radiation and affect the microphysics of marine clouds by serving as condensation nuclei for cloud drops (Feingold et al 1999;O'Dowd et al 1999a,b;Mason 2001;Rosenfeld et al 2002;Lohmann and Feichter 2005).…”

“…SSA particles scatter solar radiation and affect the microphysics of marine clouds by serving as condensation nuclei for cloud drops (Feingold et al 1999;O'Dowd et al 1999a,b;Mason 2001;Rosenfeld et al 2002;Lohmann and Feichter 2005). Estimates of the global mean clear-sky direct SSA radiative forcing range between −0.15 and −2.2 (Wm −2 ) (Grini et al 2002;Ma et al 2008;Ayash et al 2008;Dobbie et al 2003) and are based on model simulations. A more extreme value based on simplified emission assumptions is reported by Haywood et al (1999), who estimate SSA forcing for "high" concentration case to be as high as −5.03 Wm −2 (ocean averaged).…”

GEM-AQ/EC, an on-line global multi-scale chemical weather modelling system: model development and evaluation of global aerosol climatology

“…Dynamically coupled to the size segregated aerosol concentrations, the aerosol optical depth (AOD) of the five types of aerosols in GEM-AQ/EC is computed to enable the computation of the spatiotemporal distribution of aerosol optical characteristics, assessing model performance against observations, and interactively modeling the aerosol direct radiative effects (Ayash et al, 2008).…”