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UV Aerosol Indices from (TROP)OMI

UV Aerosol Indices from (TROP)OMI. An investigation of viewing angle dependence. Reminder – UV Aerosol Indices. Indices determined at two wavelengths in the UV [1,2] Available from TOMS, GOME(-2), SCIAMACHY, OMI, OMPS, ... Most-used wavelength pair: 340/380 nm

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UV Aerosol Indices from (TROP)OMI

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  1. UV Aerosol Indices from (TROP)OMI An investigation of viewing angle dependence

  2. Reminder – UV Aerosol Indices • Indices determined at two wavelengths in the UV [1,2] • Available from TOMS, GOME(-2), SCIAMACHY, OMI, OMPS, ... • Most-used wavelength pair: 340/380 nm • UVAI≥ 0: Absorbing Aerosol Index (AAI) • UVAI≤ 0: SCattering Index (SCI) [3] • Advantages • UVAI are determined even for cloudy pixels and over highly reflective surfaces • No a priori input required (aside from surface pressure) • UVAI are very sensitive to elevated UV-absorbing particles • Absorbing (UVAI≥ 0) and non-absorbing (UVAI≤ 0) particles can be easily distinguished • Disadvantages • Quantitative interpretation difficult • Sensitive to calibration errors 1Torres et al., JGR 1998; 2de Graaf et al., JGR 2005; 3Penning de Vries et al., ACP, 2009

  3. Calculation of UVAI • Determine the measured reflectance at reference wavelength λ0:Rmeas(λ0) • Model RRayl(λ) for Rayleigh atmosphere with Rmeas(λ0) = RRayl(λ0) • Calculate UVAI using: UVAI = -100*10log(Rmeas/RRayl)λ λ SCI = 1.12 (UVAI = -1.12) λ0 AAI = 3.13 (UVAI = 3.13)

  4. UVAI Examples • Non-absorbing aerosols (new colorscale!) • Sec. Organic Aerosols over S.E. USA • Volcanic sulfate aerosols (Nabro, 2011) • Absorbing aerosols • Desert dust (2004-2007) • Biomass burning smoke (Russia, 2010) • Volcanic ash (Kasatochi, 2008) 1.5 SCIAMACHY GOME-2 UVAI JJA 2007 9 Aug 10 Aug 0 8 Aug 10 30 July 31 July GOME-2 PMD UVAI 1 Aug 0 GOME-2 OMI UVAI June 13, 2011

  5. Angle dependence of UVAI • Angle dependence was studied theoretically in de Graaf et al., JGR 2005: • Model calculations using DAK • Aerosol layer (SSA = 0.9, AOT = 1, g = 0.7) at 3-4 km, surface albedo 0.05 • Viewing angle dependence is moderate for GOME(-2) and SCIAMACHY viewing geometries, but is substantial for (TROP)OMI Rel. azimuth angle 180 Rel. azimuth angle 0 (TROP)OMI GOME-2 SCIAMACHY

  6. OMI UVAI measurements of Nabro eruption • Explosive eruption with high-altitude sulfate plume on June 12, 2011 • OMI detected the aerosol plume on June 13 (one overpass) and 14 (two overpasses) June 13 June 14 UVAI (NASA) SO2 VCD (K.Yang) OMI pixels affected by row anomaly removed UVAI (NASA) SO2 VCD (K.Yang)

  7. OMI UVAI measurements of Nabro eruption (2) OMI • Same section of plume measured twice within 100 minutes • Pixels selected with SO2 VCD>1 DU to pick out volcanic plume • First overpass: negative UVAI; second overpass: positive UVAI?! SO2 VCD (K.Yang) UVAI (NASA)

  8. RTM study – reflectances • Calculations by Steffen Dörner using McArtim3 (SZA 20) • Rayleigh phase function causes viewing angle dependence of reflectance • Aerosols and clouds have different phase functions Surface albedo Clouds COT 50 1 19 km 15 km 11 km 7 km 3 km Layer top altitude: JJA 2007-2010 0 Aerosols AOT 1.2 SSA 1.0, g 0.6

  9. RTM study – UVAI from aerosols • Viewing angle dependence most pronounced for highest AOT and highest altitude • RTM settings: • SZA 20, albedo 0.1 • Angs. coeff. = 1.5, g = 0.6 • Homog. layer, 1 km thick

  10. RTM study – UVAI from clouds • Viewing angle effect much less pronounced for clouds • Possibly not present at all; g was set to 0.6 by mistake!

  11. Application to Nabro plume orbit 36772 orbit 36773 • Radiative transfer modeling of UVAI of elevated sulfate plume • Plume at 18-19 km • Non-absorbing aerosols with AOT 0.1-0.4 (depending on SO2) • Viewing angle effect reproduced by model • This is direct evidence for high-altitude aerosol layer (>11 km) with high single-scattering albedo (>0.97) • Note: shown calculations were performed with a version of SCIATRAN that has issues with large viewing angles Modeled UVAI OMI UVAI

  12. Final words • Viewing-angle dependence of UVAI for high-altitude plumes very strong • For Nabro’s sulfate plume, change of UVAI sign was observed and modeled • From UVAI alone, we can say that the plume was at high altitude (>11 km) and was nearly non-absorbing (SSA>0.97) • Exploit this for other plumes stretching over the complete OMI/TROPOMI swath, or for plumes caught twice by the instrument (like in the presented case) • These findings imply that RT becomes complicated for large viewing angles, which may also affect trace gas retrievals

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