Abstract
Abstract. Observations of aerosol scattering and absorption offer valuable information about aerosol composition. We apply a simulation of the Ultraviolet Aerosol Index (UVAI), a method of detecting aerosol absorption from satellite observations, to interpret UVAI values observed by the Ozone Monitoring Instrument (OMI) from 2005 to 2015 to understand global trends in aerosol composition. We conduct our simulation using the vector radiative transfer model VLIDORT with aerosol fields from the global chemical transport model GEOS-Chem. We examine the 2005–2015 trends in individual aerosol species from GEOS-Chem and apply these trends to the UVAI simulation to calculate the change in simulated UVAI due to the trends in individual aerosol species. We find that global trends in the UVAI are largely explained by trends in absorption by mineral dust, absorption by brown carbon, and scattering by secondary inorganic aerosol. Trends in absorption by mineral dust dominate the simulated UVAI trends over North Africa, the Middle East, East Asia, and Australia. The UVAI simulation resolves observed negative UVAI trends well over Australia, but underestimates positive UVAI trends over North Africa and Central Asia near the Aral Sea and underestimates negative UVAI trends over East Asia. We find evidence of an increasing dust source from the desiccating Aral Sea that may not be well represented by the current generation of models. Trends in absorption by brown carbon dominate the simulated UVAI trends over biomass burning regions. The UVAI simulation reproduces observed negative trends over central South America and West Africa, but underestimates observed UVAI trends over boreal forests. Trends in scattering by secondary inorganic aerosol dominate the simulated UVAI trends over the eastern United States and eastern India. The UVAI simulation slightly overestimates the observed positive UVAI trends over the eastern United States and underestimates the observed negative UVAI trends over India. Quantitative simulation of the OMI UVAI offers new insight into global trends in aerosol composition.
Highlights
Atmospheric aerosols have significant climate impacts due to their ability to scatter and absorb solar radiation and to their indirect effect through modification of cloud properties
The sudden suppression of observations for specific viewing geometries could cause an additional spurious trend in the Ultraviolet Aerosol Index (UVAI) trend calculation. We address this concern by considering only scan positions 3–23, which remain unaffected by the row anomaly, and by using the recently reprocessed OMAERUV UVAI that is less sensitive to scan-angle-dependent cloud artifacts due to the implementation of a Mie-scattering-based water cloud model (Torres et al, 2018)
We focus on cloud-filtered observations by excluding scenes with Ozone Monitoring Instrument (OMI) UVAI radiative cloud fraction exceeding 5 % to further reduce uncertainty due to clouds
Summary
Atmospheric aerosols have significant climate impacts due to their ability to scatter and absorb solar radiation and to their indirect effect through modification of cloud properties. Over the past decade positive trends in satellite AOD have been observed over Asia and Africa, corresponding to regions experiencing industrial growth (de Meij et al, 2012; Chin et al, 2014; Mao et al, 2014; Mehta et al, 2016), while negative trends in satellite AOD have been observed over North America and Europe, largely due to pollution controls (Hsu et al, 2012; de Meij et al, 2012; Chin et al, 2014; Mehta et al, 2016) Studies such as these demonstrate the information about the evolution of aerosol abundance offered by total column AOD retrievals, but measurements of absorption would complement the scattering information in AOD retrievals by providing independent information on aerosol composition.
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