Interpreting the ultraviolet aerosol index observed with the OMI satellite instrument to understand absorption by organic aerosols: implications for atmospheric oxidation and direct radiative effects

摘要

Satellite observations of the ultraviolet aerosol index (UVAI) are sensitiveto absorption of solar radiation by aerosols; this absorption affectsphotolysis frequencies and radiative forcing. We develop a global simulationof the UVAI using the 3-D chemical transport model GEOS-Chem coupled withthe Vector Linearized Discrete Ordinate Radiative Transfer model (VLIDORT).The simulation is applied to interpret UVAI observations from the OzoneMonitoring Instrument (OMI) for the year 2007. Simulated and observed valuesare highly consistent in regions where mineral dust dominates the UVAI, buta large negative bias (−0.32 to −0.97) exists between simulated and observedvalues in biomass burning regions. We determine effective optical propertiesfor absorbing organic aerosol, known as brown carbon (BrC), and implementthem into GEOS-Chem to better represent observed UVAI values over biomassburning regions. The inclusion of absorbing BrC decreases the mean biasbetween simulated and OMI UVAI values from −0.57 to −0.09 over West Africain January, from −0.32 to +0.0002 over South Asia in April, from −0.97 to−0.22 over southern Africa in July, and from −0.50 to +0.33 over SouthAmerica in September. The spectral dependence of absorption after includingBrC in the model is broadly consistent with reported observations forbiomass burning aerosol, with absorbing Ångström exponent (AAE) valuesranging from 2.9 in the ultraviolet (UV) to 1.3 across the UV–Near IRspectrum. We assess the effect of the additional UV absorption by BrC onatmospheric photochemistry by examining tropospheric hydroxyl radical (OH)concentrations in GEOS-Chem. The inclusion of BrC decreases OH by up to30 % over South America in September, up to 20 % over southern Africa inJuly, and up to 15 % over other biomass burning regions. Global annualmean OH concentrations in GEOS-Chem decrease due to the presence ofabsorbing BrC, increasing the methyl chloroform lifetime from 5.62 to5.68 years, thus reducing the bias against observed values. We calculate thedirect radiative effect (DRE) of BrC using GEOS-Chem coupled with theradiative transfer model RRTMG (GC-RT). Treating organic aerosol ascontaining more strongly absorbing BrC changes the global annual meanall-sky top of atmosphere (TOA) DRE by +0.03 W m and all-skysurface DRE by −0.08 W m. Regional changes of up to +0.3 W m at TOA and down to −1.5 W m at the surface are found over majorbiomass burning regions.