1. OMI validation workshop - 22 nd June 2006 Louisa. J. Kramer (1), Paul. S. Monks (2), Roland. J. Leigh (1) (1) Earth Observation Science, Space Research Centre, University of Leicester, Leicester, LE1 7RH (2) Department of Chemistry, University of Leicester, Leicester, LE1 7RH Validation of tropospheric NO 2 measurements from OMI, over Leicester, U.K.

2. OMI validation workshop - 22 nd June 2006 The CMAX-DOAS Instrument

3. OMI validation workshop - 22 nd June 2006 View of entrance slit and CCD image Output to CCD Marconi 48-20 UV-Coated Back illuminated Frame transfer Oriel MS257 Imaging Spectrometer 25cm Focal Length Multi-track fibre input to imaging Spectrometer

4. OMI validation workshop - 22 nd June 2006 Fitting Procedure Cross SectionTemp. (K)ConvolutionReference Ring-Wavelength dependent GaussianWinDOAS (calculated) O4O4 296Wavelength dependent GaussianGreenblatt et. al.,1990 (with Burkholder -0.2nm shift) O3O3 223Wavelength dependent Gaussian with I 0 correction (1 x 10 20 mol/cm 2 ) Bogumil et. al., 2003 H2OH2O-Wavelength dependent GaussianHitran Database (Rothman et. al., 2003) NO 2 220Wavelength dependent Gaussian with I 0 correction (5 x 10 16 mol/cm 2 ) Vandaele et al., 1998 NO 2 293Wavelength dependent Gaussian with I 0 correction (5 x 10 16 mol/cm 2 ) Vandaele et al., 1998 Fitting window 430-508 nm

5. OMI validation workshop - 22 nd June 2006 Corrections for line shape etc? Strength of NO2 signal Prim/Sec analysis? Corrections applied: Wavelength shift, pixel sensitivity.

6. OMI validation workshop - 22 nd June 2006 Comparison with in situ monitor N Chemiluminescence detector CMAX-DOAS

7. OMI validation workshop - 22 nd June 2006 Zoom in on 2 plumes  The absorption by NO 2 outside of the boundary layer is the same for all viewing geometries including zenith.  Clouds are a uniform layer above the boundary layer and increased absorption due to scattering is identical in all viewing geometries.  The 5 degree viewing angle samples a 2 km path of the polluted boundary layer, which is not sampled by the zenith view.

8. OMI validation workshop - 22 nd June 2006 In situ and CMAX

9. OMI validation workshop - 22 nd June 2006

12. Wind direction Southerly winds blow emissions away from the measurement field of the CMAX-DOAS instrument. Wind Speed Fast winds - cause NO 2 plumes to disperse rapidly or be blown under the viewing geometry of CMAX- DOAS. Slow winds - cause temporal offsets of NO 2 peaks.

13. OMI validation workshop - 22 nd June 2006

14. Comparison between OMI and CMAX-DOAS tropospheric NO 2 columns performed. Include all OMI pixels within 50 km of the CMAX-DOAS instrument for all data points between 1st December 2006 and 14th March 2006. NO 2 data from CMAX-DOAS is obtained approximately every minute. To ensure an accurate comparison the data is averaged over a 5 minute period either side of the OMI over pass time. This ensures that any short term enhancement in the CMAX- DOAS NO 2 data, due to plumes passing across the field-of-view (FOV) of the telescope, will not affect the comparison. Validation of OMI NO 2

15. OMI validation workshop - 22 nd June 2006 December 2005 January 2006 Key CMAX-DOAS In Situ OMI

16. OMI validation workshop - 22 nd June 2006 February 2006 March 2006 Key CMAX-DOAS In Situ OMI

17. OMI validation workshop - 22 nd June 2006 Weekly Cycle Weekly cycle December 05 – March 06

18. OMI validation workshop - 22 nd June 2006

21. Summary The data from the CMAX-DOAS instrument has previously been validated with a chemiluminescence detector located in the city centre, with good agreement in the right wind conditions. Have to ability to reliably monitor weekly cycles, and diurnal variability at high temporal resolution. Weekly cycle from OMI, CMAX-DOAS and in situ monitor all show the expected decrease in tropospheric NO 2 at weekends. Column densities of tropospheric NO 2 for the period December 2005- March 2006 in a polluted urban region in the UK have been produced for the comparison with OMI tropospheric columns. Correlation between OMI and CMAX-DOAS is good when cloud clearing is implemented.

22. OMI validation workshop - 22 nd June 2006 Future work A continuous NO 2 data set will be produced from CMAX-DOAS for the validation of tropospheric NO 2 from OMI. A long time series of data will enable the trends in tropospheric NO 2 to become more apparent. Additional datasets from other validation sites around Leicester and the United Kingdom will be performed to obtain measurements of polluted and clean regions. Further investigations into weekly and monthly cycles will be performed to enable us to produce an accurate validated dataset of OMI tropospheric NO 2.

23. OMI validation workshop - 22 nd June 2006 Acknowledgements Thanks to the following: Leicester City Council: Evan Davies Paul Hodges OMI validation team NERC (£) If you would like more information: Leigh, R. J., G. K. Corlett, P. S. Monks, A concurrent Multi-Axis Differential Optical Absorption Spectroscopy System for the Measurement of Nitrogen Dioxide, Applied Optics, Submitted October 2005. Thank you for your attention.