UV Aerosol Indices from (TROP)OMI An investigation of viewing angle dependence , Marloes Penning de Vries and Thomas Wagner Max Planck Institute for Chemistry, Mainz, Germany

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 Reminder – UV Aerosol Indices Torres et al., JGR 1998; 2 de Graaf et al., JGR 2005; 3 Penning de Vries et al., ACP, 2009

Calculation of UVAI Determine the measured reflectance at reference wavelength λ 0 : R meas (λ 0 ) Model R Rayl (λ) for Rayleigh atmosphere with R meas (λ 0 ) = R Rayl (λ 0 ) Calculate UVAI using: UVAI = -100* 10 log(R meas /R Rayl ) λ λ0λ0 λ SCI = 1.12 (UVAI = -1.12) AAI = 3.13 (UVAI = 3.13)

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

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 0 Rel. azimuth angle 180 SCIAMACHY GOME-2 (TROP)OMI

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) SO 2 VCD (K.Yang) OMI pixels affected by row anomaly removed June 13 UVAI (NASA) SO 2 VCD (K.Yang) UVAI (NASA) June 14

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

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 JJA Surface albedo 0 1 Layer top altitude: 19 km 15 km 11 km 7 km 3 km Clouds COT 50 Aerosols AOT 1.2 SSA 1.0, g 0.6

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 - 9 -

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!

Application to Nabro plume Radiative transfer modeling of UVAI of elevated sulfate plume – Plume at km – Non-absorbing aerosols with AOT (depending on SO 2 ) 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 orbit 36772orbit Modeled UVAIOMI UVAI

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