Indirect validation of tropospheric nitrogen dioxide retrieved from the OMI satellite instrument : insight into the seasonal variation of nitrogen oxides at northern midlatitudes

We examine the seasonal variation in lower tropospheric nitrogen oxides (NOx = NO + NO2) at northern midlatitudes by evaluating tropospheric NO2 columns observed from the Ozone Monitoring Instrument (OMI) satellite instrument with surface NO2 measurements (SouthEastern Aerosol Research and Characterization and Air Quality System) and current bottom-up NOx emission inventories, using a global model of tropospheric chemistry (GEOS-Chem). The standard (SP) and DOMINO (DP) tropospheric NO2 column products from OMI exhibit broadly similar spatial and seasonal variation, but differ substantially over continental source regions. A comparison of the two OMI tropospheric NO2 products with in situ surface NO2 concentrations and bottom-up NOx emissions over the southeast United States indicates that annual mean NO2 columns from the DP are biased high by 21%–33% and those from the SP are biased high by 27%–43%. The bias in SP columns is highly seasonal, 67%–74% in summer compared with -6% to -1% in winter. Similar seasonal differences exist between top-down and bottom-up NOx emission inventories over North America, Europe, and East Asia. The air mass factor largely explains the observed seasonal difference between the DP and SP, and in turn the seasonal SP bias. We develop a third product (DP_GC) using averaging kernel information from the DP and NO2 vertical profiles from GEOS-Chem. This product reduces to 5%–21% the annual mean bias over the southeast United States. We use the seasonal variation in the DP_GC to estimate the seasonal variation in the lifetime of lower tropospheric NOx against oxidation to HNO3 over the eastern United States. The effective NOx lifetime at OMI overpass time (early afternoon) ranges from 7.6 h in summer to 17.8 h in winter, consistent within 3 h of the simulated lifetime. GEOS-Chem calculations reveal that the seasonal variation in OMI NO2 columns largely reflects gas-phase oxidation of NO2 in summer with an increasing role for heterogenous chemistry in winter