1. 1 Ground-level nitrogen dioxide concentrations inferred from the satellite-borne Ozone Monitoring Instrument Lok Lamsal and Randall Martin with contributions from Martin Steinbacher, Empa Edward Celarier, SGT Inc. Eric Bucsela, NASA GSFC Edward Dunlea, CIRES Joseph Pinto, U.S. EPA 13.08.2007

2. 2 Nitrogen dioxide  Is a reddish-brown gas and has a sharp biting odor  Is a prominant toxic air pollutant  Current WHO guideline: 200 µg/m 3 (hourly average), 40 µg/m 3 (annual average)

3. 3 NO x sources Surface sources: 85%

4. 4 NO x : Impacts on public health and the environment Acid deposition Toxic products Visibility impairment Eutrophication Health effects

5. 5 Ambient NO 2 as indicator of toxic air pollution  Strong association between between NO 2 and mortality.  NO 2 strongly correlated with other air pollutants  NO 2 can serve as an indicator of level of toxicity in air VOC PAH Brook et al., 2007 Correlation of NO 2 and PM 2.5 with certain PAH and VOC

6. 6 Ground-based NO 2 measurements  NO 2 measuring instruments:CLNO x (molybdenum, photolytic), DOAS, LIF, TILDAS  Sparse fixed sites  Surface NO 2 shows a large gradient  Don‘t reflect personal exposure to NO 2  Satellite-derived NO 2 would be very useful  Not possible to retrieve surface NO 2 from satellite

7. 7 Tropospheric column as a proxy for surface NO2  Tropospheric column retrieval from satellite  Strong relation between tropospheric columns and surface NO x emissions  NO 2 in the lower mixed layer makes 70-90% contribution to tropospheric column  Aim: derive ground-level NO 2 from OMI San Francisco Los AngelesPhoenix Houston Dallas Chicago Toronto

8. 8 NO 2 from OMI  Launch 15 th July 2004 onboard EOS-Aura  OMI instrument is from the contribution from NIVR and FMI  Spectral coverage: 270-500 nm  Ozone, NO 2, SO 2, HCHO, BrO, OClO, cloud and aerosol properties  Horizontal resolution ≥ 13 x 24 km 2  Daily global coverage

9. 9 GEOS-Chem GEOS-CHEM In situ  Need of information on NO 2 profile from a 3D model  GEOS-Chem at 2x2.5, version 7-03-06, 12:00-14:00 local time  GEOS-Chem NO 2 profile shape consistent with in situ measurements

10. 10 Approach to derive surface NO 2 GEOS-CHEM In situ GEOS-Chem NO 2 [ppbv] SAT

11. 11 Sensitivity studies: Effect of model profile OMI grids GEOS-Chem grids Error < 10% in polluted areas polluted unpolluted

12. 12 In situ NO 2 measurements: Chemiluminescent NO x analyzer Chemiluminescent NO x analyzer does not provide true NO 2 NO + O 3 → NO 2 * + O 2 NO 2 * → NO 2 + h NO mode NO x mode NO 2 → NO NO 2 = NO x - NO

13. 13 Comparison with DOAS measurements OMI over pass time (12.00 to 14:00 local time) Interference~50% HNO 3 ~60% ∑AN~10-30% Mexico City Apr 2003 Dunlea et al. 2007

14. 14 Comparison with photolytic analyzers Taenikon/Switzerland Jan-Dec 2000 Steinbacher et al. 2007 Interference:20-60% PAN:30-50% HNO 3, particulate nitrates

15. 15 Interference in chemiluminescent analyzers with molybdenum converter CompoundsConversion efficiencyExperiments NO, NO 2, ethyl nitrate~ 100%Winer et al., 1974 PAN92%Winer et al., 1974 HNO 3, PAN, n-propyl nitrate, n-butyl nitrate ≥98% Grosjean and Harrison, 1985 Ammonia, gas phase olefins, particulate nitrate No significant interference Dunlea et al., 2007 Difficult issue: Loss of HNO 3 on stainless steel of inlet Difficult to quantify the conversion efficiency

16. 16 Correction of interference Bias = 40% Bias = 8% Taenikon, Switzerland

17. 17 Correction for interference for USA and Canada

18. 18 Comparison between OMI and in situ measurements  Selected 214 stations  201 US and 13 Canada  Collocation criteria  Radius 10 km, time = 12:00 to 14:00 local time ► Mean correlation 0.51, maximum 0.86 ► Stronger correlation in polluted areas Correlation map

19. 19 Comparison between OMI and in situ measurements ► Uncorrected > corrected by up to a factor of 3 ►OMI and corrected in situ measurements consistent ►summer minima ►OMI<corrected in situ measurements DJF MAM JJA SON

20. 20 Comparison between OMI and in situ measurements J F M A M J J A S O N D

21. 21 Comparison between OMI and in situ measurements according to land type  Urban, suburban, rural  Mean bias -21 to -48%  Underestimation of tropospheric column by OMI  Seasonal bias in OMI retrievals

22. 22 Seasonal bias in OMI retrievals 33%-20%-58%-15% DJF MAM JJA SON

23. 23 Conclusions  We derived ground-level NO 2 from OMI using information from the GEOS-Chem model.  OMI-derived surface NO 2 is robust to model profile in polluted areas.  In situ measurements have significant interference from PAN, alkyl nitrates, and HNO 3  Following laboratory studies and field measurements, we developed a correction algorithm to the in situ measurements.  We compared OMI-derived surface NO 2 with the corrected in situ measurements from US and Canadian stations to validate our approach  OMI derived surface NO 2 agrees to 21 to 48% with the corrected in situ measurements. The remaining discrepancy is associated with the OMI retrievals. Submitted to JGR atmospheres