1. Estimating Ground-level NO2 Concentrations from OMI Observations
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
Ruud Dirksen, KNMI
Folkert Boersma, KNMI

2. Two Major Applications of OMI Tropospheric NO2 Columns
Estimates of Surface Air Quality
Top-down NOx Emissions
Based on PM2.5, O3, and NO2

3. Large Regions Have Insufficient In Situ NO2 Measurements for Air Quality NO2 Associated with Mortality (Brook et al. 2007) In Situ Monitors Contaminated with Reactive Nitrogen
Highest NO2 maximum quarterly mean by county, 2001
NO2 strongly associated with other toxic gases. Current U.S. standard 0.5 ppm annual average.
long-term cohert studies have difficulty contrasting polluted vs rural areas

4. OMI Tropospheric NO2 Column Proxy for Surface Concentration
NO/NO2  
with altitude
Satellite more representative for Exposure
OMI Standard Product: October 2004 – September 2007 Inclusive

5. Simulated Concentrations (GEOS-Chem) over North America
Strong Correlation Between Modeled NO2 Column and Surface NO2 Concentration
Simulated Concentrations (GEOS-Chem) over North America
Day (March 5, 2005)
Annual Mean (2005)
r2=0.96 n=652
r2=0.86 n=652

6. Promising Relationship Between Modeled and In-Situ NO2 Profiles
GEOS-Chem
Texas AQS
Martin et al., 2004
Eastern North America
New England
Mixed layer can comprise 60-75% of tropospheric column
In Situ
GEOS-Chem
ICARTT
Martin et al., 2006

7. General Approach to Estimate Surface NO2
Coincident Model NO2 Profile
Daily OMI Tropospheric NO2 Column
In Situ
GEOS-Chem
S → Surface NO2
Ω → Tropospheric NO2 column

8. Tropospheric NO2 Column (1015 molec cm-2)
Specific Approach to Estimate Surface NO2 Use OMI to Infer Sub-Grid Variation in NO2 Profile
OMI (2o x 2.5o)
OMI (0.1o x0.1o)
Tropospheric NO2 Column (1015 molec cm-2) O
S → Surface NO2
Ω → Tropospheric NO2 column
ΩF → Free tropospheric NO2
Specific approach changes result by 10-30%

9. Surface NO2 for 2005 Inferred from OMI
MAM
DJF
JJA
SON

10. Sample Comparison of In-situ and OMI-derived Surface NO2

11. Significant Correlation Between In Situ and OMI-derived Surface NO2
Standard Product
Near-Real-Time
Correlation of Coincident Observations over 2005

12. Comparison of In Situ and OMI-derived Surface NO2 for 2005 Indirect Validation of OMI
DJF MAM JJA SON
OMI (STD) r=0.76
OMI (NRT) r=0.78
In situ

13. Seasonal Comparison by Land Type

14. Conclusions Future Directions
Promising satellite-based surface NO2 estimate
Surface measurements provide indirect validation of NO2 columns
Need to understand seasonal differences
Future Directions
High resolution profile information (nested and regional models) Evaluate with ground-based measurements of “true NO2” More sophisticated algorithms for emission inversions (i.e. adjoint)
Supported by NASA and NSERC