UV Aerosol Indices from (TROP)OMI
An investigation of viewing angle dependence
28.11.2013, 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
- 2 - 1Torres et al., JGR 1998; 2de Graaf et al., JGR 2005; 3Penning de Vries et al., ACP, 2009
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
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
320 330 340 350 360 370 380 390
refl
ecta
nce
wavelength (nm)
absorbing aerosol
scattering aerosol
Rayleigh_0.0945
Rayleigh_0.0272
λ0
λSCI = 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)
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8 Aug
9 Aug10 Aug
GOME-2UVAI
JJA 2007
OMIUVAI
June 13, 2011
GOME-2
GOME-2PMD UVAI
SCIAMACHY
0
1.5
10
01 Aug
31 July30July
• Absorbing aerosols
– Desert dust (2004-2007)
– 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
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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)
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SO2 VCD (K.Yang)
OMI pixels affected by row anomaly removed
June 13
UVAI (NASA)
SO2 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 SO2 VCD>1 DU to pick out volcanic plume
• First overpass: negative UVAI; second overpass: positive UVAI?!
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SO2 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
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JJA 2007-2010
Surface albedo
0
1
Layer top altitude:
19 km15 km11 km7 km3 km
CloudsCOT 50
AerosolsAOT 1.2SSA 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
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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!
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Application to Nabro plume
• 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
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orbit 36772 orbit 36773
Mod
eled
UV
AI
OM
I U
VA
I
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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