Multiple site ground-based evaluation of carbonaceous aerosol mass concentrations retrieved from CAMS and MERRA-2 over the Indo-Gangetic Plain

Recent progress in the availability of reanalysis data of Earth system variables with high spatial–temporal resolution provides valuable information for estimating the impacts of atmospheric aerosols. However, the aerosol module of reanalysis data has high uncertainty and must be validated with ground-based measurements for better accuracy and precision. In the present study, carbonaceous aerosol (black carbon and organic carbon) mass concentrations simulated by CAMS (Copernicus Atmosphere Monitoring Service) and MERRA-2 (Modern-Era Retrospective analysis for Research and Applications, version 2) were evaluated with ground-based measurements over different sites in the Indo-Gangetic Plain (IGP). The discrepancies in the reanalysis data for Black Carbon (BC) and Organic Carbon (OC) showed the influence of fresh and aged aerosols. Both CAMSRA and MERRA-2 reanalysis data underestimated the BC mass concentrations over the NW-IGP while overestimated over the central-IGP. These discrepancies were mainly due to inadequate knowledge about emissions and aging processes, especially over the central-IGP. Although the error in BC simulations is less than that in OC simulations, the inclusion/revision of local emissions will be helpful for accurate and precise simulations of carbonaceous aerosols over the IGP. For OC, observed deviations were complex mainly because of biases associated with complex atmospheric processes. At all four IGP study sites, CAMSRA overestimated OC, while MERRA-2 underestimated OC. The results clearly showed that the overestimation of CAMSRA simulated OC mass concentrations was due to the hygroscopic growth scheme, besides the poorly constrained escaped semi-volatile species from the primary organic aerosols. In contrast, the underestimation of MERRA-2 simulated OC was mainly due to existing ambiguities related to emissions at the NW-IGP while it was due to the aging scheme at the central-IGP. In addition, underestimation of organics could also be due to the formation of secondary organic aerosols (SOA) in the atmosphere from the regionally transported gas-phase “pool” of efficient precursors.