Insight into Global Trends in Aerosol Composition over 2005-2015 Inferred from the OMI 1 Ultraviolet Aerosol Index 2

12 Observations of aerosol scattering and absorption offer valuable information about aerosol 13 composition. We apply a simulation of the Ultraviolet Aerosol Index (UVAI), a method of 14 detecting aerosol absorption from satellite observations, to interpret UVAI values observed by the 15 Ozone Monitoring Instrument (OMI) over 2005-2015 to understand global trends in aerosol 16 composition. We conduct our simulation using the vector radiative transfer model VLIDORT with 17 aerosol fields from the global chemical transport model GEOS-Chem. We examine the 2005-2015 18 trends in individual aerosol species from GEOS-Chem, and apply these trends to the UVAI 19 simulation to calculate the change in simulated UVAI due to the trends in individual aerosol 20 species. We find that global trends in the UVAI are largely explained by trends in absorption by 21 mineral dust, absorption by brown carbon, and scattering by secondary inorganic aerosol. Trends 22 in absorption by mineral dust dominate the simulated UVAI trends over North Africa, the Middle23 East, East Asia, and Australia. The UVAI simulation well resolves observed negative UVAI trends 24 over Australia, but underestimates positive UVAI trends over North Africa and Central Asia near 25 the Aral Sea, and underestimates negative UVAI trends over East Asia. We find evidence of an 26 increasing dust source from the desiccating Aral Sea, that may not be well represented by the 27 current generation of models. Trends in absorption by brown carbon dominate the simulated UVAI 28 trends over biomass burning regions. The UVAI simulation reproduces observed negative trends 29 over central South America and West Africa, but underestimates observed UVAI trends over 30 boreal forests. Trends in scattering by secondary inorganic aerosol dominate the simulated UVAI 31