Approximation of black carbon light absorption property based on AERONET data and comparison with MERRA-2 data

Abstract

The contribution of black carbon in absorption aerosol optical depth (BC AAOD) can be retrieved using the optical properties of BC, including depolarization ratio (DPR, δ^p), single scattering albedo (SSA, ω), and Angstrom exponent (AE, å). AErosol RObotic NETwork (AERONET) inversion products provide ground-based long-term observations of aerosol optical properties based on the sun photometers measurements. In this study, the AERONET-derived (version 3 level 2.0 products) BC’s optical properties are utilized to approximate the BC AAOD on a regional and global scale for two decades. Also, the Modern-Era Retrospective analysis for Research and Applications, version 2.0 (MERRA-2) data, has been used to represent the long-term variation of BC aerosol optical depth (BC AOD) over the selected AERONET sites. We found the highest AERONET-derived BC AAOD over China, Nigeria, India, Brazil, and Mexico, peaked in China and Nigeria. During the past two decades, a falling trend has been found in BC AAOD over China, Brazil, Greece, Seoul city in South Korea, and Cape Verde. On a regional scale, we found the maximum values of BC AAOD in Osaka and Gwangju cities, indicating a downward trend in both sites. The MERRA-2-derived BC AOD showed the maximum values over China, India, Ilorin, and Mexico. Unlike the AERONET-based BC AAOD that represented higher values in Brazil, the MERRA-2 data showed lower BC AOD over this site. We concluded that the high AERONET-derived BC AAOD and MERRA-2-retrieved BC AOD over the highly populated areas mainly owes to the considerable BC emission from transportation, industrial, and residential sectors in response to the high population. We attributed the decreasing tendency in BC AAOD to the decrease in BC emissions, likely owing to the governmental policies to reduce BC emissions. Our results indicated that MERRA-2 underestimated the BC values compared to AERONET over the Americas, Middle East, and West Africa. This underestimation is particularly evident in Sao-Paulo. The inconsistency of MERRA-2 and AEORNET data over Sao-Paulo city in Brazil might be influenced by the low number of AERONET data over this site. Further, since Sao-Paulo is one of the most populated cities worldwide, the very low MERRA-2 BC AOD over this city might indicate the underestimation of MERRA-2 compared to AERONET. Also, our retrieval methodology, which more likely describes the aerosol absorption in anthropogenic pollution than the biomass-burning, might affect the discrepancy between the AERONET and MERRA-2 BC data over Brazil.