Abstract
The impact of Brown Carbon (BrC) to aerosol light absorption has been paid more attention recently and there are a large number of studies showing that the influence of BrC on radiative forcing should not be ignored. BrC also acts as an important component of haze pollution which is occurring frequently in Wuhan, China. Therefore, it is essential to estimate their optical properties, composition, and mass concentration. Considering most haze pollution happens during the coldest time, we retrieved BrC columnar content during winter in Wuhan for the first time. Our method bases on the fact that BrC showed the strong spectral dependence on UV-light absorption. Using this method, we found that BrC makes up the small proportions of total aerosol volume (less than 10%). In the winter of 2011, we retrieved the daily-averaged columnar-integrated mass concentration of BrC on clear day is 4.353 mg/m2 while that of haze day is 12.750 mg/m2. According to the sensitivity study, we found that the results highly relied on the assumed aerosol refractive index. To reduce the uncertainty of this approach, we need to gain a better understanding of the temporal variability of the radiation absorbing components of these aerosols in the future.
Highlights
Aerosols play an important role in global climate through direct and indirect radiative forcing
In order to focus on Brown Carbon (BrC), we simplified the aerosol to a mixture of Black Carbon (BC), BrC, and ammonium sulfate (AS) embedded in a water host
The method made full use of the fact that BrC showed the strong spectral dependence on UV-light absorption which led to higher imaginary index values at 440 nm
Summary
Aerosols play an important role in global climate through direct and indirect radiative forcing. Combustionproduced carbonaceous particles are the predominant radiation absorbing components of aerosols. Most direct radiative forcing models classify carbonaceous particles into two main types, strong radiation absorbing Black Carbon (BC) or negligibly radiation absorbing Brown Carbon (BrC). Bahadur found that the BrC absorption at 440 nm was about 40% of that of BC and demonstrated that current climate models treating BrC as non-absorbing were underestimating the total warming effect of carbonaceous aerosols (Bahadur et al 2012). Using a global chemical transport model and a radiative transfer model, Feng, Ramanathan, and Kotamarthi (2013) estimated the enhanced absorption of solar radiation due to BrC in a global model, and global simulations suggested that strongly absorbing BrC contributed up to 0.25 W/m2 (19%) of all absorption by anthropogenic aerosols. The retrieval of BrC based on its optical properties obtained from remote sensing has been proposed
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