Observations of elemental carbon and absorption during ACE-Asia and implications for aerosol radiative properties and climate forcing

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Abstract

[1] Measurements of elemental carbon (EC) during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) show that significant amounts of EC were found in the coarse particle phase during yellow sand events. Coagulation during long-range transport is consistent with this observation. The daily averaged specific mass absorption efficiencies of EC were calculated, yielding values of 12.6 ± 2.6 and 14.8 ± 2.3 m2/g for PM10 (particulate matter <10 μm diameter) and PM1, respectively. On a limited number of days, the absorption efficiency for the coarse particles only (PM10 − PM1) was determined to be 5.7 ± 1.6 m2/g during dust days and 2.0 ± 1.0 m2/g for nondust days. These measurements suggest that fine particulate EC was internally mixed and that the dust was possibly somewhat absorbing. Specific mass absorption efficiency was observed to be inversely related to EC mass concentration, a result that does not appear to reflect only air mass aging effects. We speculate that if this observation holds on a global scale, it would reduce the effectiveness of a strategy for mitigating climate change by reducing EC emissions. Model simulations of idealized nonspherical dust radiative properties predict that scattering is strongly (by nearly a factor of 3) dependent on geometry, while absorption is a very weak function of geometry. The net change in shortwave absorption by polluted dust layers due to coagulation of EC with dust is predicted by model calculations to range between −42% and +58%, depending on the assumption about the initial mixing state of the EC and the dust optical properties, with the observations supporting values in the range of −10 to −40%.

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