NO₂ columns in the western United States observed from space and simulated by a regional chemistry model and their implications for NO_x emissions

International audienceThere are many isolated sources of NOx emissions across the western United States, including electrical power generation plants and urban areas. In this manuscript, two satellite instruments measuring NO2 vertical columns over these sources and an atmospheric chemical-transport model are used to evaluate bottom-up NOx emission inventories, model assumptions, and satellite retrieval algorithms. We carried out simulations with the Weather Research and Forecasting-Chemistry (WRF-Chem) model for the western U.S. domain during the summer of 2005 using measured power plant NOx emissions. Model NO2 vertical columns are compared with a retrieval of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite instrument data by the University of Bremen and retrievals of the Ozone Monitoring Instrument (OMI) data by the U.S. National Aeronautics and Space Administration (NASA) and a modified version of the NASA OMI retrieval produced by the University of Bremen. For areas dominated by power plant NOx emissions, the model NO2 columns serve as a comparison standard for satellite retrievals because emissions are continuously monitored at all large U.S. power plants. An extensive series of sensitivity tests of the assumptions in both the satellite retrievals and the model are carried out over the Four Corners and San Juan power plants, two adjacent facilities in the northwest corner of New Mexico that together represent the largest NOx point source in the United States. Overall, the SCIAMACHY and OMI NO2 columns over western U.S. power plants agree well with model NO2 columns, with differences between the two being within the variability of the model and satellite. In contrast to regions dominated by power plant emissions, model NO2 columns over large urban areas along the U.S. west coast are approximately twice as large as satellite NO2 columns from SCIAMACHY and OMI retrievals. The discrepancies in urban areas are beyond the sensitivity ranges in the model simulations and satellite observations, implying overestimates of these cities' bottom-up NOx emissions, which are dominated by motor vehicles. Taking the uncertainties in the satellite retrievals into account, our study demonstrates that the tropospheric columns of NO2 retrieved from space-based observations of backscattered solar electromagnetic radiation can be used to evaluate and improve bottom-up emission inventories