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)

- 4 -

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

- 5 -

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)

- 6 -

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?!

- 7 -

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

- 8 -

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

- 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!

- 10 -

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

- 11 -

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

- 12 -