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 Auriol Laboratoire d'Optique Atmosphérique (LOA/CNRS), Université des Sciences et Technologies de Lille, Villeneuve d’Ascq, FRANCE. Iolanda Ialongo, Anna Maria Siani, Giuseppe Rocco Casale Physics Department, SAPIENZA University of Rome, P.le A. Moro 2, 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 O 3 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 = 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 February 2007 Collection 3 All skies … clear days Seasonal effect : SZA effect Stronger for DOAS (same as collection 2) Good agreement TOMS - like DOAS DOAS - like points 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 February 2007 Only few points nm380.1 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) nm380.1 nm SZA > 65° Weak correlation between relative differences and aerosols (AOT) (low irradiances)

Comparison of EDR at overpass time at VdA Period : October February 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 July 2006 Clear skies  Bias (OMI > spectro) Bias  15% for clear skies 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 ( nm) Erythemal Dose Rate (EDR) AOD at nm (Sellitto et al., 2006) SSA/Absorbing AOD at 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 December 2006Period : September December 2006 OMI ozone data: collection 3

Comparison of UV spectral irradiances at noon at ROME Brewer 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) nm nm nm Brewer UV data uncertainty: 5% Period : September July 2006

Comparison of EDR at noon at ROME BrewerYES UVB radiometer Langley plot derived AOD at nm AOD uncertainty: 0.04 Only at SZA>55° r>0.4 Positive bias OMI > ground Period : September July 2006

Comparison of EDD at ROME YES UVB radiometer Positive bias OMI > ground Period : September July 2006

Role of absorbing aerosols on OMI UV estimates at ROME Irradiances at nmEDR Brewer Absorbing AOD at 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 nm:

Summary of the OMI validation results at Rome Bias (%)r OMI-TOMS/Brewer O OMI-DOAS/Brewer O CSASCSAS OMI/Brewer Irradiance at nm OMI/Brewer Irradiance at nm OMI/Brewer Irradiance at nm OMI/Brewer EDR OMI/YES EDR OMI/YES EDD 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 km 2 )

Thank you! Contact: Virginie Buchard Iolanda Ialongo