1. Improvement and validation of OMI NO 2 observations over complex terrain A contribution to ACCENT-TROPOSAT-2, Task Group 3 Yipin Zhou, Dominik Brunner, and Brigitte Buchmann Empa, Swiss Federal Laboratories for Materials Testing and Research Dübendorf, Switzerland Folkert Boersma and Ruud Dirksen Royal Netherlands Meteorological Institute De Bilt, The Netherlands

2. 2 Goal: Study air quality and pollution sources over domain of a small country with complex topography (focusing on NO 2 ) GOME SCIA OMI Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

3. 3 Evolution of sensor resolution PhD D. SchaubGOME (launch 1995) 320  40 km 2 SCIAMACHY (launch 2002) 60  40 km 2 Schaub, D. et al., Atmos. Chem. Phys. 5, 23-37, 2005 Schaub, D. et al., Atmos. Chem. Phys. 6, 3211-3229, 2006 Schaub, D. et al., Atmos. Chem. Phys. 7, 5971-5987, 2007 PhD candidate Y. ZhouOMI (launch 2004) up to 24  13 km 2 up to 24  13 km 2 Jan 2004 – Dec 2007 mean Jan 2004 – Dec 2007 mean Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

4. 4 x a = a priori tropospheric NO 2 profile b = forward model parameters - cloud fraction - cloud pressure - surface pressure - surface albedo - aerosols - viewing geometry x a = a priori tropospheric NO 2 profile b = forward model parameters - cloud fraction - cloud pressure - surface pressure - surface albedo - aerosols - viewing geometry Tropospheric NO 2 from satellites Tropospheric air mass factor AMF trop computed with radiative transfer model: AMF trop = f(x a,b) Retrieval method Sources of uncertainty in AMF trop calculation Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

5. 5 Representation of surface topography Topography as used in DOMINO retrieval Topography at full resolution of OMI pixels TM4/ECMWF topography, consistent with a priori NO 2 profiles GTOPO-30 topography reduced to OMI pixel resolution Too loo low over Alps Too high over adjacent plains m m Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

6. 6 Possible implications of inaccurate pixel surface height Schaub et al., ACP (2007) „old picture“ Averaging kernel: sensitivity decreasing towards surface Strong sensitivity to altitude change „new picture“ Averaging kernel: sensitivity decreasing towards surface Modest sensitivity to altitude change AK profile changes when surface height changes Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

7. 7 Box air mass factor profile (m l ) A priori NO 2 subcolumn profile (x a,l ) Possible implications of inaccurate pixel surface height p surf = 788 hPa p surf = 928 hPa (ref.) p surf = 1008 hPa Note: NO 2 volume mixing ratio profile preserved by vertical scaling Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

8. 8 Effect of clouds Possible implications of inaccurate pixel surface height NO 2 profile AK profile cloudy part cloud z z TM4/ECMWF topography GTOPO topography Illustration of effect of inaccurate topography for cloudy pixels AMF cloud significantly overestimated over planes when using TM4/ECMWF topo z c : cloud top height z 0 : surface terrain height w: cloud radiance fraction Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

9. 9 Results: Effect on retrieved NO 2 Difference in terrain height (GTOPO – TM4 topography) +40 +15 0 -15 Difference (%) in retrieved NO 2 (GTOPO – TM4)/TM4 December 2006 mean difference, cloud radiance fraction < 50% Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

10. 10 Results: Effect on retrieved NO 2 Relative difference (%) in retrieved NO 2 VTC, December 2006 (GTOPO – TM4)/TM4 Cloud radiance fraction < 50% Cloud radiance fraction < 10% +40 +15 0 -15 +40 +15 0 -15 Differences of about 30% over parts of Swiss plateau and over Po basin Differences of about 10% Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

11. 11 Monthly mean difference in retrieved NO 2 for selected point over Po basin (cloud rad. fraction < 50%) Difference in retrieved NO 2 column (%) absolute difference NO 2 with TM4 topo NO 2 with GTOPO relative difference (%) Results: Effect on retrieved NO 2 Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

12. 12 Validation with ground-based in situ NO 2 Approach: -Data obtained from about 60 stations in Lombardy region (Po basin) -Select OMI pixels with centers close to station -Construct „ground-based vertical tropospheric columns“ using TM4 a priori NO 2 profile for each individual OMI pixel Example: OMI pixels for station Motta (rural station south of Milano) Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

13. 13 Validation with ground-based in situ NO 2 Motta: rural station, moderately polluted (crfrac < 50%) 211 pixels Ground based column OMI TM4 topo column OMI GTOPO column Ground based versus OMI columns Comparison between mean annual cycles Improved agreement in winter with GTOPO Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

14. 14 Validation with ground-based in situ NO 2 Agrate: moderately polluted (crfrac < 50%) 164 pixels Ground based versus OMI columns Comparison between mean annual cycles Improved agreement in winter with GTOPO Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions Ground based column OMI TM4 topo column OMI GTOPO column

15. 15 Validation with ground-based in situ NO 2 Ground based versus OMI columns Comparison between mean annual cycles Abbiategrasso: urban, heavily polluted Bergamo: rural, least polluted Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

16. 16  Using correct surface pressure is important, but less important than previously thought  Using correct surface pressure more important for cloudy part of pixel than for clear part  NO 2 over Po basin underestimated by current retrieval by about 5% in summer and up to 20% in winter assuming a cloud radiance fraction threshold of 50% (equivalent to a cloud fraction of about 20%)  Agreement with in situ surface observations improves with new retrieval using accurate surface pressure (seasonal cycle is more pronounced in better agreement with observations) Next steps after improving surface pressure:  Use a high resolution albedo data set (e.g. snow cover)  Use high resolution NO 2 a priori profiles (e.g. from a regional air quality model)  Reprocess OMI data over Europe Summary and outlook Goal Retrieval basics Potential topography effects cloud free cloudy Results NO 2 change cloud effect seasonal Validation Conclusions

17. 17 Overview Idea Algorithms ART R-L Ideal world 1-D ex. 2-D ex. Real world Effect of noise OMI data Outlook Effect of clouds Possible implications of inaccurate pixel surface height f 1-f f = cloud fraction cloud NO 2 profile AK profile cloudy part cloud z z TM4/ECMWF topography GTOPO topography Illustration of effect of inaccurate topography for cloud pixels AMF cloud significantly overestimated over planes when using TM4/ECMWF topo z c : cloud top height z 0 : surface terrain height w: cloud radiance fraction