1. 1 Surface nitrogen dioxide concentrations inferred from Ozone Monitoring Instrument (OMI) 12.04.2007 3 rd GEOS-Chem USERS ` MEETING, Harvard University Lok Lamsal and Randall Martin Department of Physics and Atmospheric Science, Dalhousie University James Gleason Goddard Space Flight Center NASA

2. 2 Motivation  Surface NO 2 is a toxic pollutant that affects human health  Association between surface NO 2 and non-accidental mortality (Burnett et al., 2004; Brook et al., 2007)  Association stronger than for PM 2.5  Limited number of sites  Global map of surface NO 2 not possible without satellite measurements Surface measurement sites in US and Canada

3. 3 OMI tropospheric NO 2 columns as a proxy for deriving surface NO 2  Tropospheric column correlated with NO 2 at surface (Ordóñez et al., 2006)  Tropospheric column a proxy  Similar work for ground level PM 2.5 from van Donkelaar et al., 2006; Liu et al., 2004 Global distribution of tropospheric column NO 2 (Sep. 2005)

4. 4 Derivation of surface NO 2 from OMI OMI grids GEOS-CHEM grid  Need model profile & OMI tropospheric column NO 2.  GEOS-CHEM NO 2 profile shape consistent with in situ measurements (Martin et al.,2004) GEOS-CHEM In situ Houston: 29 ° N-31°N, 96.25°W-93.75°W Texas Air Quality Study aircraft campaign 16 August-13 September 2000 Averaged over OMI overpass time

5. 5 NO 2 mixing ratio inferred from OMI

6. 6 Comparison between OMI surface NO 2 and in situ measurements  About 300 US & Canadian stations  Collocation radius 12 km, overpass time  Agreement incredible at Essex r =.75 r =.48

7. 7 Comparison of ground based and OMI surface NO 2 r =.65 r =.27

8. 8 Correlation of OMI NO 2 with surface measurements  Correlation coefficient for 307 stations  Correlation coefficient higher for polluted regions Year 2005

9. 9 Comparison based on land use ratio

10. 10 Why is OMI surface NO 2 lower than in situ measurements? Ground based instruments (equipped with molybdenum converter) overestimate surface NO 2.  Equipped with molybdenum converter  Interference from HNO 3, PAN, alkyl nitrates  Instrument with photolytic converter more accurate Schaub et al., 2006; Ordóñez et al., 2006; Dunlea et al., 2007 interference large in summer

11. 11 Correction of GB measurements for interference from HNO 3, PAN, and alkyl nitrates  Determine correction factor from GEOS-CHEM  Correction factor  Seasonal means of correction factor

12. 12 Correction of GB measurements for interference Correction factor

13. 13 Correction of GB measurements for interference Seasonal means of surface NO 2 (2005) GB after correction OMI

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