Aerosol emission estimates through SPEXone/PACE data assimilation in ECHAM6.3-HAM2.3: implications for aerosol radiative forcing

Abstract. Aerosols exert an effective radiative forcing (ERFaer) on climate through directly scattering and absorbing solar radiation (ARI) and indirectly interacting with clouds (ACI). Aerosol emissions, due to limited observational constraints, remain among the largest sources of uncertainty in modeled ERFaer. Here, we estimate aerosol emissions globally by assimilating observations from SPEXone/PACE for the period 2024-08 to 2025-07, using a local ensemble transform Kalman smoother in combination with the global aerosol–climate model ECHAM6.3-HAM2.3. We assimilate aerosol optical depth at 550 nm (AOD550), Ångström exponent between 440 nm and 670 nm (AE440–670), and single scattering albedo at 440 nm (SSA440) and additionally evaluate aerosol absorption optical depth at 440 nm (AAOD440). The data assimilation experiment (DAS) reduces root mean square error (RMSE) against SPEXone for all four variables, with global mean bias improving from −0.017 to −0.001 for AOD550 and from 0.336 to 0.183 for AE440–670. Validation against independent AERONET and MODIS observations likewise shows improvements. The emission estimates based on DAS lead to relatively more emissions of coarse particles, particularly over the ocean, and more weakly absorbing emissions, particularly over biomass-burning regions. Compared to the control experiment (CTL), DAS results in a more negative ERFaer (−2.16 Wm−2 vs −1.34 Wm−2), with changes of −0.53 Wm−2 from ARI and −0.29 Wm−2 from ACI. By assimilating SPEXone observations for aerosol emission estimation, this study highlights the critical role of accurate emissions from both natural and anthropogenic sources in determining aerosol radiative forcing.