A global modeling study on carbonaceous aerosol microphysical characteristics and radiative forcing

Description

Recently, attention has been drawn towards black carbon aerosols as a short-term climate warming mitigation candidate. However, the global and regional impacts of the direct, cloud-indirect, and semi-direct forcing effects are highly uncertain, due to the complex nature of aerosol evolution and the way that mixed, aged aerosols interact with clouds and radiation. A detailed aerosol microphysical scheme, MATRIX, embedded within the global GISS climate model is used in this study to present a quantitative assessment of the impact of microphysical processes involving black carbon, such as emission size distributions and optical properties on aerosol cloud activation and radiative forcing. Our best estimate for net direct and indirect aerosol radiative forcing between 1750 and 2000 is -0.56 W/m2. However, the direct and indirect aerosol effects are quite sensitive to the black and organic carbon size distribution and consequential mixing state. The net carbonaceous aerosol radiative forcing can vary between -0.32 to -0.75 W/m2, depending on these carbonaceous particle properties at emission. Assuming that sulfates, nitrates, and secondary organics form a coating around a black carbon core, rather than forming a uniformly mixed particle, changes the overall net aerosol radiative forcing from negative to positive. Taking into account internally mixed black carbon particles lets us simulate correct aerosol absorption. Black carbon absorption is amplified by sulfate and nitrate coatings, but even more strongly by organic coatings. Black carbon mitigation scenarios generally showed reduced radiative forcing when sources with a large proportion of black carbon, such as diesel, are reduced; however, reducing sources with a larger organic carbon component as well, such as bio-fuels, does not necessarily lead to negative radiative forcing change. This poster will give examples on how we plan to evaluate aerosol size and mixing state properties that are crucial to better understand their climate impacts.