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Accurate modeling of the scattering and absorption of ultraviolet and\udvisible radiation by aerosols is essential for accurate simulations of\udatmospheric chemistry and climate. Closure studies using in situ\udmeasurements of aerosol scattering and absorption can be used to evaluate\udand improve models of aerosol optical properties without interference from\udmodel errors in aerosol emissions, transport, chemistry, or deposition\udrates. Here we evaluate the ability of four externally mixed, fixed size\uddistribution parameterizations used in global models to simulate submicron\udaerosol scattering and absorption at three wavelengths using in situ data\udgathered during the 2008 Arctic Research of the Composition of the\udTroposphere from Aircraft and Satellites (ARCTAS) campaign. The four models\udare the NASA Global Modeling Initiative (GMI) Combo model, GEOS-Chem v9-02,\udthe baseline configuration of a version of GEOS-Chem with online radiative\udtransfer calculations (called GC-RT), and the Optical Properties of Aerosol\udand Clouds (OPAC v3.1) package. We also use the ARCTAS data to perform the\udfirst evaluation of the ability of the Aerosol Simulation Program (ASP v2.1)\udto simulate submicron aerosol scattering and absorption when in situ data on\udthe aerosol size distribution are used, and examine the impact of different\udmixing rules for black carbon (BC) on the results. We find that the GMI\udmodel tends to overestimate submicron scattering and absorption at shorter\udwavelengths by 10–23 %, and that GMI has smaller absolute mean biases for\udsubmicron absorption than OPAC v3.1, GEOS-Chem v9-02, or GC-RT. However, the\udchanges to the density and refractive index of BC in GC-RT improve the\udsimulation of submicron aerosol absorption at all wavelengths relative to\udGEOS-Chem v9-02. Adding a variable size distribution, as in ASP v2.1,\udimproves model performance for scattering but not for absorption, likely due\udto the assumption in ASP v2.1 that BC is present at a constant mass fraction\udthroughout the aerosol size distribution. Using a core-shell mixing rule in\udASP overestimates aerosol absorption, especially for the fresh biomass\udburning aerosol measured in ARCTAS-B, suggesting the need for modeling the\udtime-varying mixing states of aerosols in future versions of ASP

Evaluating model parameterizations of submicron aerosol scattering and absorption with in situ data from ARCTAS 2008