1. Validation of a Satellite Retrieval of Tropospheric Nitrogen Dioxide Randall Martin Dalhousie University Smithsonian Astrophysical Observatory Daniel Jacob Elisabeth Sturges Harvard University Kelly Chance Chris Sioris Smithsonian Astrophysical Observatory David Parrish National Oceanic and Atmospheric Administration INTRODUCTION Nitrogen oxide radicals (NO x = NO + NO 2 ) originating from combustion, lightning, and soils largely control tropospheric ozone production. Tropospheric ozone plays a key role in determining the oxidizing power of the atmosphere, is an important greenhouse gas, and is toxic to biota. Previous comparison of tropospheric nitrogen dioxide (NO 2 ) retrieved from the Global Ozone Monitoring Experiment (GOME) [Burrows et al., 1999] and simulated with a global 3-D model of tropospheric chemistry (GEOS-CHEM model [Bey et al., 2001; Martin et al., 2003a]) showed that retrieved tropospheric NO 2 columns over Houston are half of simulated columns during summer [Martin et al., 2003b]. Here we examine the consistency of tropospheric NO 2 retrieved from GOME with in situ measurements over Houston. Texas Air Quality Study The Texas Air Quality Study (TexAQS) measured tropospheric NO 2 [Ryerson et al., 2000] over Houston on 14 separate flights over August 16 - September 13, 2000. More than 15% of the in situ measurements were taken in one grid cell directly over Houston. The spatial coverage of aircraft flights during the campaign is comparable to the scale of a GOME footprint. The measured profile shows that tropospheric NO 2 is largely confined within the lowest kilometer. Retrieval of Tropospheric NO 2 Comparison Number of in situ measurements taken within a tropospheric column of 0.5 o x 0.5 o resolution during TexAQS. White areas indicate regions without measurements. The white lines show an example GOME footprint. SCIAMACHY Tropospheric NO 2 determined from SCIAMACHY by limb- nadir matching. Validation of measurements from higher resolution instruments will be facilitated by smaller pixel size, but sub-grid scale heterogeneity may remain an issue. References Measured NO 2 as part of TexAQS. The yellow dots show each 1-second measurement. The green line is the mean. The median is represented by the red line, with error bars showing the 17 th and 83 rd percentiles respectively. We retrieve tropospheric NO 2 from GOME using the algorithms described in Martin et al. [2002; 2003b]. This includes the air mass factor (AMF) formulation of Palmer et al. [2001] which computes the AMF as the integral over the tropospheric column of the relative vertical distribution of NO 2 (shape factor) weighted by the local sensitivity to NO 2 of the solar radiation backscattered to space (scattering weights). The shape factor is specified from a global 3-D model simulation (GEOS-CHEM model). Tropospheric NO 2 retrieved from GOME observations during the TexAQS campaign. The relative vertical distribution of NO 2 (shape factors [Palmer et al., 2001]) over Houston (29 o to 31 o lat by -96.25 o to -93.75 o lon). The blue line shows the shape factors calculated with the GEOS-CHEM model as used in the AMF calculation. The red line shows the shape factors determined from in situ measurements as part of TexAQS. Tropospheric NO 2 column determined from in situ measurements. The column is integrated in 500 m increments from the surface to 6 km. The 17 th percentile of all in situ measurements within a 500 m slab is assumed for slabs without any in situ measurements for each column calculation. The 17 th percentile is chosen to reduce sampling biases from the mission focus on plume studies. The GEOS-CHEM model value of 5x10 14 molecules cm -2 was added to all columns to represent tropospheric NO 2 above 6 km. Comparison of GOME and in situ measurements. The blue lines show the average GOME measurement during TexAQS that falls within a 4.5 o range (-97.25 o to -92.75 o lon), plotted as a function of the latitude of the center of the GOME footprint. The red line is the average over -97.25 o to -92.75 o lon of the in situ column as a function of latitude. Blue error bars are the GOME retrieval error [Martin et al., 2003b]. Red lower bounds use the 5 th percentile instead of the 17 th percentile in the in situ column calculation. Red upper bounds average over 3.5 o (-96.75 o to -93.25 o lon). Relating GOME NO 2 with in situ measurements from TexAQS requires accounting for the spatial sampling characteristics of both measurements. We achieve more uniform spatial weighting of the in situ measurements by calculating tropospheric NO 2 columns at 0.5 o by 0.5 o resolution and subsequently averaging the cells. The resolution of 0.5 o by 0.5 o is chosen as a spatial scale over which the in situ measurement is representative given a NO x lifetime of a few hours and nominal wind speeds of 10 m s -1. Bey, I., D.J. Jacob, J.A. Logan, and R.M. Yantosca, Asian chemical outflow to the Pacific: origins, pathways and budgets, J. Geophys. Res., 106, 23097-23114, 2001. Burrows, J.P., et al., The Global Ozone Monitoring Experiment (GOME): Mission concept and first scientific results, J. Atmos. Sci., 56, 151-175, 1999. Leue, C., M. Wenig, T. Wagner, O. Klimm, U. Platt, and B. Jahne, Quantitative analysis of NOx emissions from GOME satellite image sequences, J. Geophys. Res., 106, 5493-5505, 2001. Martin, R.V., et al., An improved retrieval of tropospheric nitrogen dioxide from GOME, J. Geophys. Res., 107, 4437, doi:10.1029/2001JD001027, 2002. Martin, R.V., D.J. Jacob, R.M. Yantosca, M. Chin, and P. Ginoux, Global and regional decreases in tropospheric oxidants from photochemical effects of aerosols, J. Geophys. Res., 108, 4097, doi:10.1029/2002JD002622, 2003a. Martin, R.V., D.J. Jacob, K. Chance, T.P. Kurosu, P.I. Palmer, and M.J. Evans, Global inventory of nitrogen oxide emissions constrained by space-based observations of NO2 columns, J. Geophys. Res., in press, 2003b. Palmer, P.I., et al., Air mass factor formulation for spectroscopic measurements from satellites: Application to formaldehyde retrievals from the Global Ozone Monitoring Experiment, J. Geophys. Res., 106, 14539-14550, 2001. Ryerson, T.B., E.J. Williams, and F.C. Fehsenfeld, An efficient photolysis system for fast-response NO2 measurements, J. Geophys. Res., 105, 26447-26461, 2000.