Sensitivity of Aerosol Radiative Forcing to Mixing State and Non-Sphericity

Abstract:

Mixing state and particle non-sphericity of aerosols are two major sources of uncertainties in estimation of aerosol direct radiative forcing (DRF). To examine the sensitivity of optical properties to the mixing state, aerosols were collected on filter papers and analyzed for detailed chemical composition in Delhi national capital region (NCR) during 2007-2008. Black carbon, BC was measured directly by Aethalometer. They are grouped into four major aerosol species - dust, water soluble (WS), water insoluble (WINS) and BC. Eight different mixing cases - external mixing, internal mixing, and six combinations of core-shell type which includes two modes of dust (accumulation and coarse) have been considered for the present study. Core-shell mixing cases are BC over dust, WS over dust, BC over WS and, WS over BC. These core shell mixed components are then externally mixed with rest of the aerosol species. The aerosol optical properties for each of the mixing state cases are utilized to estimate the radiative forcing using a radiative transfer model. The surface-reaching fluxes for each of the cases are compared with measured clear-sky surface radiation. MISR aerosol product were analyzed to understand the seasonal variations of the bulk aerosol properties that may help in interpreting the sensitivity results. We observed that for the winter season (DJF), core-shell mixed case; BC over dust (accumulation) (207.7±4.28 Wm^-2) and BC over WS (207.25±8.4 Wm^-2) are almost comparable with measured surface flux (206.46±70.06 Wm^-2), while for the monsoon season (JAS) external mixing is closest to observations. None of the mixing cases show compatible comparison, for the pre-monsoon (MAMJ) and post-monsoon season (ON). The remaining discrepancy may be attributed to the assumption of uniform vertical distribution (calculated from CALIPSO data) for each individual aerosol species, whereas ideally different vertical profile should be considered. Secondly, dust is assumed to be spherical whereas literature suggests it to be non-spherical. Three non-spherical shapes - spheroid, cylinder and chebyshev are considered and the changes in optical properties are estimated. Our results quantified the sensitivity of aerosol DRF to mixing state and non-sphericity in a representative site of the Indo-Gangetic basin, India.