1 Relationship between ground-level NO 2 concentrations and OMI tropospheric NO 2 columns Martin Steinbacher, Empa; Edward Celarier, SGT Inc. Eric Bucsela, NASA GSFC; Edward Dunlea, CIRES Joseph Pinto, U.S. EPA Lok Lamsal and Randall Martin Tropospheric NO 2 workshop, Sept 2007, KNMI, De Bilt 2 Motivation Surface NO 2 Toxic pollutant Associated with mortality Surface NO 2 data to provide information on level of toxicity in air Surface measurements of NO 2 Sparse measurement sites Large gradient in NO 2 field May not reflect personal exposure to NO 2 Interference from reactive nitrogen Correlation of NO 2 and PM 2.5 with PAH and VOC [Brook et al., 2007] PAH VOC Correlation coefficient 3 Tropospheric column as a proxy for surface NO 2 Tropospheric columns related with surface NO x emissions NO 2 in the urban mixed layer can make 70-90% contribution to tropospheric column Tropospheric column a proxy to infer surface NO 2 San Francisco Los Angeles Phoenix Houston Dallas Chicago Toronto 4 GEOS-Chem and our approach to derive surface NO 2 from OMI GEOS-CHEM In situ 100% difference in GEOS-Chem surface NO 2 < 10% error in derived surface NO 2 in polluted areas Tropospheric NO 2 column (standard product) S NO 2 Surface NO 2 T NO 2 Tropospheric NO 2 column 5 Ground-based NO 2 measuring instruments Chemiluminescent NO x analyzer Molybdenum converter Photolytic converter Commonly used instrument Specific to NO Indirect measurement of NO 2 Significant interference from other reactive nitrogen DOAS, LIF, TILDAS (research grade instruments) Specific to NO 2 Some interference from HONO 6 Interference in molybdenum converter analyzer: Comparison with DOAS measurements Interference~50%, HNO 3 ~60%, Alkyl nitrates~10-30% Mexico City, Apr 2003 [ Dunlea et al. 2007] OMI overpass time (12.00 to 14:00 local time) Molybdenum Conv. DOAS 7 Interference in molybdenum converter analyzer: Comparison with photolytic converter measurements Taenikon/Switzerland Jan-Dec 2000 Steinbacher et al Interference:20-60% PAN:30-50% HNO 3, particulate nitrates 8 Interference in molybdenum converter analyzer: Experiments 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, 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 9 Correction for interference Bias = 40% Bias = 8% Taenikon, Switzerland 10 Correction for interference for US and Canadian sites 11 Comparison between corrected in situ and OMI-derived surface NO 2 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 12 Comparison between corrected in situ and OMI-derived surface NO 2 DJF MAM JJA SON uncorrected in situ corrected in situ OMI-derived 13 Comparison between corrected in situ and OMI-derived surface NO 2 J F M A M J J A S O N D 14 Comparison between OMI and in situ measurements according to land type Urban, suburban, rural Mean bias -21 to -48% Seasonal bias in OMI retrievals DJF MAM JJA SON 33% -20% -58% -15% 15 Conclusions Ground-level NO 2 from OMI using Tropospheric NO 2 columns NO 2 profile shapes from GEOS-Chem model Derived NO 2 not sensitive to model profile Interference in chemiluminescent NO x analyzer Interfering species: PAN, alkyl nitrates, and HNO 3 Developement of a correction algorithm following lab/field measurements and using GEOS-Chem Comparison between corrected in situ and OMI-derived surface NO 2 OMI-derived surface NO 2 less than corrected in situ by 21 to 48% Seasonal bias in OMI-derived surface NO 2