Latest results on the comparison between OMI and ground-based data at two European

sites (Rome and Villeneuve d’Ascq)

 

Virginie Buchard, Colette Brogniez, Frédérique AuriolLaboratoire d'Optique Atmosphérique (LOA/CNRS),

Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, FRANCE.

 Iolanda Ialongo, Anna Maria Siani, Giuseppe Rocco Casale

Physics Department, SAPIENZA University of Rome,P.le A. Moro 2, 00185 Rome, ITALY.

 

OMI - meeting25 June 2008

OUTLINE

- Presentation of the Villeneuve d’Ascq station

- Results of the comparison between OMI and ground-based Ozone and UV data at Villeneuve d’Ascq

- Presentation of the Rome station

- Results of the comparison between OMI and ground-based Ozone and UV data at Rome

Measurements of UV and O3 at VILLENEUVE D‘ASCQ (VdA) LOA - University of LILLE (50.61°N, 3.14°E, 70 m a.s.l.)

UV Spectroradiometer : Spectral irradiance measurements on a horizontal surface Double monochromator JOBIN-YVON HD10

Resolution: ~ 0.75 nm – thermally regulated

range = 280-450 nm, step = 0.5 nm Scan duration: ~ 6 min – every 15 min Alternately : global (diffuse + direct) and diffuse

Uncertainties : ~ 5% (400 nm) – ~8% (300 nm)

• Erythemal Dose Rate (EDR)

• Total ozone: Differential absorption technique (comparison between 2 ratios of irradiances at 2 wavelengths, one ratio simulated and stocked in a LUT, the other calculated from UV measurements).

Uncertainties ~ 3% (clear skies) to 7% depending on the cloudiness

• Aerosol optical thickness (AOT) is obtained from the ground-based direct irradiance (global - diffuse),

Uncertainties : ~ 0.05 at SZA about 40° ~ 0.03 at SZA about 70°

• Erythemal Dose Rate (EDR)

• Total ozone: Differential absorption technique (comparison between 2 ratios of irradiances at 2 wavelengths, one ratio simulated and stocked in a LUT, the other calculated from UV measurements).

Uncertainties ~ 3% (clear skies) to 7% depending on the cloudiness

• Aerosol optical thickness (AOT) is obtained from the ground-based direct irradiance (global - diffuse),

Uncertainties : ~ 0.05 at SZA about 40° ~ 0.03 at SZA about 70°

Comparison of Daily Mean Total Ozone at VdA Daily averages for SZA < 75° Period : October 2005 - February 2007

Collection 3

All skies

… clear days

Seasonal effect : SZA effect

Stronger for DOAS (same as collection 2)

Good agreement

TOMS - like DOASDOAS - like+ + + 670 points + + + 687 points

Comparison of UV spectral irradiances at overpass time at VdA (1)

For clear skies (flags OMI and spectro)

Deconvolution of the measurements by the instrument’s slit function (FWHM ≈ 0.75 nm) and reconvolution by the FWHM of OMI ≈ 0.55 nm.

Period : October 2005 - February 2007

Only few points

324.1 nm 380.1 nm

Excellent correlation

bias at 324 nm OMI > spectro

At 380 nm : slope 1

Larger differences for low irradiances

Comparison of UV spectral irradiances at overpass time at VdA (2)

324.1 nm 380.1 nm

+ + + SZA > 65°

Weak correlation between relative differences and aerosols (AOT)

(low irradiances)

Comparison of EDR at overpass time at VdA

Period : October 2005 - February 2007

+ + + 723 points

Bias (OMI > spectro)

Cloud Optical Depth at 360 nm : OMI UV algorithm

Dependence with COD

Weak correlation between relative differences and aerosols (AOT)

Comparison of EDD at VdA Period : October 2005 - July 2006

Clear skies

Bias (OMI > spectro) Bias 15% for clear skies

+ + + 349 points

Ozone data: good agreement for both methods (little better for TOMS method), seasonal effect for DOAS-method.

UV spectral irradiance : satisfying agreement at 324 nm (bias about 6.5%) and 380 nm (bias at small irradiances).

Weak correlation between relatives differences and AOT.

Erythemal dose rates : bias OMI>spectro.

Large relative differences for large COD.

Weak correlation between relative differences and AOT.

Erythemal daily doses : bias OMI>spectro even by clear skies (about 15 %).

Summary of the OMI validation results at VdA

Brewer spectrophotometer 067 (model MKIV)

Total ozone

Spectral irradiances (290-325 nm)

Erythemal Dose Rate (EDR)

AOD at 320.1 nm (Sellitto et al., 2006)

SSA/Absorbing AOD at 320.1 nm

Broad-band radiometer (model YES UVB-1)

Erythemal Dose Rate (EDR)

Erythemal Daily Dose (EDD)

(Webb et al., 2006)

Measurements of O3 and UV at ROME Sapienza - University of Rome (41.9°N, 12.5°E, 75 m a.s.l.)

Comparison of Daily Mean Total Ozone at ROME

Brewer Direct Sun measurements (standard deviation < 2.5 DU)

Brewer Total Ozone data uncertainty: 1%

Good agreement

Period : October 2005 - December 2006Period : September 2004 - December 2006

OMI ozone data: collection 3

Comparison of UV spectralirradiances at noon at ROMEBrewer measurements have been deconvoluted from the instrument slit function (FWHM 0.63 nm) and convoluted with the triangular OMI slit function (FWHM 0.55 nm).

324.1 nm 305.1 nm

310.1 nm

Brewer UV data uncertainty: 5%

Period : September 2004 - July 2006

Comparison of EDR at noon at ROME

Brewer YES UVB radiometer

Langley plot derived AOD at 320.1 nmAOD uncertainty: 0.04Only at SZA>55° r>0.4

Positive bias OMI > ground

Period : September 2004 - July 2006

Comparison of EDD at ROME

YES UVB radiometer

Positive bias OMI > ground

Period : September 2004 - July 2006

Role of absorbing aerosols on OMI UV estimates at ROME

Irradiances at 324.1 nm EDR

Brewer

Absorbing AOD at 320.1 nm derived according to Bais et al. (2005)AAOD uncertainty: 10%Absorbing aerosol free atmosphere: bias=4-7%

Correlation between relatives differences and AOD at 320.1 nm: 0.40-0.56

Summary of the OMI validation results at Rome

Bias (%) rOMI-TOMS/Brewer O3 -1.8 0.97OMI-DOAS/Brewer O3 -0.7 0.96

CS AS CS AS

OMI/Brewer Irradiance at 305.1 nm 22 23 0.99 0.97

OMI/Brewer Irradiance at 310.1 nm 7 13 0.98 0.95

OMI/Brewer Irradiance at 324.1 nm 19 26 0.98 0.92

OMI/Brewer EDR 28 33 0.99 0.96

OMI/YES EDR 23 30 0.98 0.91

OMI/YES EDD 21 23 0.99 0.97

- The aerosols (especially the absorbing aerosols) could affect the UV OMI-ground bias

AS: All skies

CS: Clear skies

Further causes of uncertainty in UV comparison:- atmospheric conditions at overpass time could not correspond to conditions at solar noon: estimated magnitude of changes in atmospheric transmission can reach 50%- the Rome station atmospheric conditions may be not representative of the OMI pixel area (13x24 km2)

Thank you!

Contact:

Virginie Buchard (buchard@loa.univ-lille1.fr)

Iolanda Ialongo (iolanda.ialongo@uniroma1.it)