Interpretation of Aerosol Mass Scattering Efficiency over North America Using Ground Based Measurements from the IMPROVE Network
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Aerosol mass scattering efficiency affects climate forcing calculations, atmospheric visibility, and the interpretation of satellite observations of aerosol optical depth. We evaluated the representation of aerosol mass scattering efficiency in the GEOS-Chem chemical transport model over North America using collocated measurements of aerosol scatter and mass from IMPROVE network sites between 2000-2015. We found a positive bias in mass scattering efficiency given current assumptions of aerosol size distributions and particle hygroscopicity in the model. We found that overestimation of mass scattering efficiency was most significant in dry (RH<35%) and midrange humidity (35%<RH<65%) conditions, with biases of 87% and 38%, respectively. To address these biases, we investigated assumptions surrounding the two largest contributors to fine aerosol mass, organic and secondary inorganic aerosols. Inhibiting hygroscopic growth of SIA below 35% RH and decreasing the dry geometric mean radius (rg), from 0.069 um for SIA and 0.073 um for OA to 0.058 um for both aerosol types, significantly decreased the bias observed in dry conditions from 87% to 13%. Implementation of a widely used alternative representation of hygroscopic growth following k-Kohler theory for secondary inorganic (k=0.58) and organic aerosols (k=0.10) eliminated the overall bias in mass scattering efficiency. Simulated average sp over North America increased by 12%, with larger increases of 20-40% in Northern regions with high RH and hygroscopic aerosol fractions, and decreases in mass scattering efficiency up to 15% in southwestern U.S. where dust fractions are high and RH is low.