1. Ground-level NO 2 Concentrations Inferred from OMI Lok Lamsal, Dalhousie University Randall Martin, Dalhousie and Harvard-Smithsonian Aaron van Donkelaar, Dalhousie University Martin Steinbacher, Empa Edward Celarier, SGT Inc. Eric Bucsela, NASA GSFC Edward Dunlea, CIRES Joseph Pinto, EPA

2. Large Regions Have Insufficient In Situ NO 2 Measurements for Air Quality NO 2 Toxic Pollutant Associated with Mortality In Situ Monitors Contaminated with Reactive Nitrogen Highest NO 2 maximum quarterly mean by county, 2001

3. OMI Tropospheric NO 2 Column Proxy for Surface Concentration NO / NO2   with altitude

4. Tropospheric NO 2 Column Strongly Related to Ground-level Concentration over Land Martin et al., 2004 Texas AQS ICARTT Martin et al., 2006 In Situ GEOS-Chem In Situ GEOS-Chem Eastern North America New England

5. Approach to Infer Surface NO 2 from OMI 100% error in GEOS-Chem surface NO 2  < 10% error in derived surface NO 2 in polluted areas OMI Tropospheric NO 2 Column S → Surface NO 2 Ω → Tropospheric NO 2 column In Situ GEOS-Chem i GEOS-Chem NO 2 Profile OMI Local Information GEOS-Chem Grid

6. Surface NO 2 for 2005 Inferred from OMI (standard product) DJF MAM JJA SON

7. CompoundsConversion efficiencyExperiments 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, NO 3 - InsignificantDunlea et al., 2007 Difficult issue: Loss of HNO 3 on stainless steel of inlet Infer 35% measured from comparison with photolytic converter Correction for Interference in “NO 2 ” in Air Quality Networks DJF MAM JJA SON Correction Calculated with GEOS-Chem

8. Significant Correlation Between Corrected In Situ and OMI-derived Surface NO 2 J F M A M J J A S O N D Correlation of Concident Observations over 2005 OMI-derived (standard product) In Situ (corrected) In Situ (orig)

9. Comparison of In Situ and OMI-derived Surface NO 2 for 2005 Indirect Validation of OMI DJF MAM JJA SON Corrected in situ OMI-derived (standard)

10. Investigation of Seasonal Bias Investigation of Seasonal Bias OMI (standard) Top-down and Bottom-up Emissions over USA OMI (NRT) Bottom-up (NEI2002) Mar 2006 – Feb 2007 J F M A M J J A S O N D Emissions 2005 J F M A M J J A S O N D OMI (standard) In Situ NO 2 rural sites - Cannot eliminate by varying HNO 3 loss on inlet - Would need factor of 3-6 seasonal bias in GEOS-Chem NO 2 / NO y to eliminate - Preliminary: consistent seasonal bias vs NO 2 from PSS

11. Supported by NASA ROSES Promising satellite-based surface NO 2 estimate Need for additional validation of surface NO 2 with “true” NO 2 Surface measurements provide indirect validation of NO 2 columns Conclusions Acknowledgement