Aerosol emissions estimation with POLDER

Abstract. We apply a Local Ensemble Transform Kalman Smoother (LETKS) in combination with the global aerosol climate model ECHAM-HAM to estimate aerosol emissions from POLDER-3/PARASOL observations for the year 2006. We assimilate Aerosol Optical Depth at 550 mnm (AOD₅₅₀), Ångström Exponent for 550 nm and 865 nm (AE_550-865) and Single Scattering Albedo at 550 nm (SSA₅₅₀) in order to improve modeled aerosol mass, size and absorption simultaneously. The new global aerosol emissions increase to 1419 Tg·yr^-1 (+28 %) for dust, 1850 Tg·yr^-1 (+75 %) for sea salt, 215 Tg·yr^-1 (+143 %) for organic aerosol and 13.3 Tg·yr^-1 (+75 %) for black carbon, while the sulfur dioxide emissions increase to 198 Tg·yr^-1 (+42 %) and total deposition of sulfates to 293 Tg·yr^-1 (+39 %). Organic and black carbon emissions are much higher than their prior values from bottom up inventories with a stronger increase in biomass burning sources (+193 % and +90 %) than in anthropogenic sources (115 % and 70 %). The evaluation of the experiments with POLDER (assimilated) and AERONET as well as MODIS Dark Target (independent) observations shows a clear improvement compared to the ECHAM-HAM control run. Specifically based on AERONET the global mean error of AOD₅₅₀ improves from -0.094 to -0.006 while AAOD₅₅₀ improves from -0.009 to -0.004 after the assimilation. A smaller improvement is observed also in AE_550-865 mean absolute error (from 0.428 to 0.393), with a considerably higher improvement over isolated island sites over the ocean. The new dust emissions are closer to the ensemble median of AEROCOM I, AEROCOM III and CMIP5 as well as some of the previous assimilation studies. The new sea salt emissions get closer to the reported emissions from previous studies. Indications of a missing fraction of coarse dust and sea salt particles are discussed. The biomass burning changes (based on POLDER) can be used as alternative biomass burning scaling factors for the GFAS inventory distinctively estimated for organic carbon (2.93) and black carbon (1.90), instead of the recommended scaling of 3.4 (Kaiser et al. 2012). The estimated emissions are highly sensitive to the relative humidity due to aerosol water uptake, especially in the case of the sulfates. We found that ECHAM-HAM, like most of the GCMs that participated in AEROCOM and CMIP6, overestimated the relative humidity compared to ERA-5 and as a result the water uptake by aerosols, assuming the kappa values are not underestimated. If we use the ERA-5 relative humidity, sulfate emissions must be further increased, as modeled sulfate AOD is lowered. Specifically, over East Asia, the lower AOD can be attributed to the underestimated precipitation and the lack of simulated nitrates in the model.