UV Aerosol Indices from (TROP)OMI An investigation of viewing angle dependence 28.11.2013, Marloes...

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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