1. Validation of OMI NO 2 data using ground-based spectrometric NO 2 measurements at Zvenigorod, Russia A.N. Gruzdev and A.S. Elokhov A.M. Obukhov Institute of Atmospheric Physics Moscow

2. Zvenigorod (55.7°N, 36.8°E) The station is located in a rural area 50 km west of Moscow. The station is exposed to pollution episodes most frequent and intensive in winter Zvenigorod Kislovodsk Lovozero Minsk Issyk-Kul Tomsk Petrodvorets Zhigansk Salekhard

3. The method of NO 2 measurements Measurements are done by a zenith viewing grating scanning monochromator MDR-23 in 435-450 nm wavelength spectral range (versus 415-465 nm for OMI) Spectral resolution is 0.7 nm (versus ~0.5 nm for OMI) Measurements are done in morning and evening twilight at solar zenith angles 84-96° The instrument and method of measurements of a slant column NO 2 were certified by the NDACC following the results of intercomparison carried out at Zvenigorod in 1997 Using measured slant column NO 2 abundances, vertical distribution of NO 2 is retrieved by solving inverse problem with Chahine method A NO 2 vertical profile is represented by NO 2 abundances in ten 5-km thickness layers and in the thin surface layer The method allows separating unpolluted and potentially polluted parts of the NO 2 column content

4. Examples of NO 2 vertical profiles at Zvenigorod retrieved for conditions of clean and polluted boundary layer During pollution episodes, the NO 2 abundance within the atmospheric boundary layer can be significantly larger than in the above troposphere- stratosphere layer The profiles correspond to solar zenith angle 84° The axis is broken

5. Comparison with data of SAGE II and CRISTA 2 satellite measurements CRISTA 2 август 1997 г. SAGE II Altitude (km) NO 2 concentration (10 8 mol/cm 3 ) 7 April 1993 23 March 1992 Zvenigorod SAGE II (52N, 39.2E) SAGE II (57.7N, 41.5E) August 1997 Zvenigorod CRISTA 2

6. Methodology of comparison with data of OMI measurements In deriving slant column NO 2 abundances, the NO 2 absorption cross sections by Vandaele et al. (1998) are used corresponding to temperature 220 K The distance of OMI ground pixel centers from the Zvenigorod station is chosen to be less than 45 km NO 2 contents retrieved from ground-based measurements correspond to solar zenith angle 84° Data of ground-based measurements are then interpolated to time of OMI overpass measurements with the help of photochemical modeling

7. Quantities used for comparison OMIGround-based 1 “Unpolluted” vertical column NO 2 abundance (NO2Unpol product) Vertical column NO 2 abundance above the surface layer (includes NO 2 contents in ten 5- km thickness layers) 2 Tropospheric vertical column NO 2 abundance (NO2Trop product) Tropospheric (0-10 km) vertical column NO 2 abundance (includes NO 2 contents in two tropospheric 5-km thickness layers and in the surface layer)

8. Comparison of OMI and ground-based measurements “Unpolluted” column NO 2 OMI data are generally between morning and evening ground-based data

9. “Unpolluted” column NO 2

10. Difference (OMI - ground-based) Mean difference: –(0.30  0.03)∙10 15 cm -2 or –(11.2  1.2)% Mean square root diff.: 0.6∙10 15 cm -2 or 22% Mean difference: –(0.17  0.04)∙10 15 cm -2 or –(6.9  1.6)% Mean square root diff.: 0.47∙10 15 cm -2 or 19%

11. Correlation of “unpolluted” column NO 2 values Correlation coefficient ~0.9 Linear regression equation OMI = 0.82GB + 0.2∙10 15 cm -2

12. Possible reasons of discrepancies between OMI and ground-based stratospheric NO 2 data Different spectral regions used for measurements Different spectral resolutions of the two instruments Different methods of measurements resulting, in particular, in different spatial resolution (smoothing) of results of measurements Different sensitivity to tropospheric pollution Temperature dependence of NO 2 cross sections that may differently affect derived NO 2 contents due to different spectral regions and spectral resolutions of the two instruments

13. Comparison of OMI and ground-based measurements Tropospheric column NO 2 There is a significant discrepancy between OMI and ground-based estimates of tropospheric NO 2 contents It is related to strong spatial inhomogeneity ant temporal variability of pollution part of NO 2 and is due to different spatial averaging of satellite and ground-based observations Mean OMI-GB difference: –(1.4  0.5)∙10 15 cm -2 Mean square root difference 7.9∙10 15 cm -2 (~200%)

14. Correlation of tropospheric column NO 2 values Correlation coefficient for daily data ~0.4 Linear regression equation OMI = 0.22GB + 2.2∙10 15 cm -2 Correlation coefficient for monthly mean data ~0.45 Linear regression equation OMI = 0.16GB + 2.3∙10 15 cm -2

15. Conclusions OMI “unpolluted” NO 2 columns underestimate ground-based measurements at Zvenigorod by (0.30  0.03)∙10 15 molecules/cm -2 (~11%), if all data are used, and by (0.17  0.04)∙10 15 molecules/cm -2 (~7%), if a part of data are used for comparison for which OMI tropospheric NO 2 columns is not large (< 2∙10 15 cm -2 ) OMI tropospheric NO 2 columns are on average by (1.4  0.5)∙10 15 molecules/cm -2 (~35%) less than those derived from ground-based measurements On the whole, more detail investigation of lower tropospheric pollution effects is needed for more accurate validation of OMI data