Quantifying the concentration of absorbing aerosol is essential for pollution tracking and calculation of atmospheric radiative forcing. To quickly obtain absorbing aerosol optical depth (AAOD) with high-resolution and high-accuracy, the gradient boosted regression trees (GBRT) method based on the joint data from Ozone Monitoring Instrument (OMI), Moderate Resolution Imaging Spectro-Radiometer (MODIS), and AErosol RObotic NETwork (AERONET) is used for TROPOspheric Monitoring Instrument (TROPOMI). Compared with the ground-based data, the correlation coefficient of the results is greater than 0.6 and the difference is generally within +/- 0.04. Compared with OMI data, the underestimation has been greatly improved. By further restricting the impact factors, three valid conclusions can be drawn: 1) the model with more spatial difference information achieves better results than the model with more temporal difference information; 2) the training dataset with a high cloud fraction (0.1-0.4) can partly improve the performance of GBRT results; and 3) when aerosol optical depth (AOD) is less than 0.3, the perform of retrieved AAODs is still good by comparing with ground-based measurements. The novel finding is expected to contribute to regional and even urban anthropogenic pollution research.

The physical treatment of internal mixing and aging of black carbon (BC) aerosols that allow for enhanced solar absorption of the BC is an important parameterization in climate models. Many climate models predict a factor of 2-3 lower aerosol absorption optical depth (AAOD) than the atmospheric columnar absorption observed from ground-based networks such as AERONET, likely because these models do not parameterize properly the BC absorption enhancement (E-MAC). Models that are configured with an internal mixing have predicted large variations of E-MAC, which are poorly constrained from ambient measurements. We determined the BC E-MAC from aerosol coatings with a two-step solvent experiment to remove both organic and inorganic coatings in ambient fine particulate matter (PM2.5). Observations in a rural North China site showed that the E-MAC varied from 1.4 to 3. The E-MAC increases simultaneously with SO42-/EC ratios, suggesting the photochemical production of sulfate coatings enhanced BC absorption. A global climate model, parameterized to account for these observational constraints, verifies that sulfates are primary drivers of the BC absorption enhancement in severely polluted area in China. This magnification of the radiative forcing of coated BC is stronger by a factor of similar to 2 than predicted by the standard parameterization (external mixing) in the climate model and is in better agreement with AERONET observations of AAOD. This result would be useful for testing the representation of solar absorption by BC-containing particles in the newer generation of climate models. Plain Language Summary Atmospheric black carbon (BC) or soot in fine particulate matter (PM2.5) is emitted from incomplete combustion of fossil fuel or biomass/biofuel. The BC is an important pollutant for both air quality and Earth's energy balance, and the BC radiative forcing maybe second only to that of CO2. The photochemical production of nonabsorbing secondary aerosols may create a coating on BC and may thereby act as a lens which may enhance the light absorption. However, this absorption enhancement is poorly constrained by ambient measurements, and thus the estimates of BC climate forcing remain highly uncertain. To this end, an aerosol filter dissolution-filtration (AFD) with two-step solvent dissolution protocol was employed to remove both organic and inorganic coatings and then investigate their effects on BC light absorption. The observations and model simulation showed that the BC warming effect likely doubled due to lens effect from secondary aerosols.

The fine-mode aerosol absorption optical depth (AAOD) retrieved from the Aerosol Robotic Network (AERONET) has been used in previous studies to calculate the radiative forcing of black carbon (BC) aerosol, assuming that the absorption by fine-mode aerosols (diameter >= 1 mu m) is primarily from BC while the absorption by larger particles (diameter > 1 mu m) is principally from dust. In the present study, the Community Earth System Model was used to simulate and quantify the contribution of fine-mode dust to fine-mode AAOD in eastern China (29-41 degrees N, 104-122 degrees E)-an area where concentrations of BC are high. The simulated fine-mode dust concentrations were constrained by observations from nine sites belonging to the Chinese Meteorological Administration Atmosphere Watch Network. Averaged over eastern China, the simulated annual mean fine-mode dust AAOD was 3.6 x 10(-3), with the maximum AAOD in spring and the minimum value in winter. The contribution of fine-mode dust to the total fine-mode AAOD (sum of fine-mode dust, BC, and organic carbon) in winter, spring, summer, and autumn was 3.4%, 25.2%, 12.5%, and 14.9%, respectively, with an annual mean value of 15.1%. The results indicate the importance of removing fine dust AAOD when the AERONET fine-mode AAOD is used for calculating the radiative forcing of BC in eastern China.