Shekhar Chandra and Yuhang Wang
Black carbon, radiative forcing, data modeling, climate change
We analyze the evidence for the change of mixing state of aerosols from winter to summer inferred from the clear-sky surface radiative fluxes measured over the Arabian Sea. Compared to the solar absorption calculated by assuming externally mixed aerosols, we find that the estimates are in agreement with the observations in January-March but become increasingly too low from spring to summer (April-August). The discrepancy can be corrected by assuming that black carbon is coated with sulfate oxidized from SO2 or sea salt. The photochemical processing time of black carbon aerosols and SO2 increases as photochemical activity and the transport time for aerosols to reach the observation location increase substantially from winter to summer. Coating of sea salt on black carbon may also contribute significantly to solar absorption, increasing the efficacy of black carbon radiative forcing. While the assumption of external mixing is adequate for winter, it significantly underestimates the BC absorption in summer. Internal mixing assumption leads to significant overestimates in winter and summer. Seasonal change of mixing state such as the partial core-shell structure suggested in this study is needed in global and regional models for more reliable BC radiative forcing estimate
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