Trends in OMI NO2 observations over the United States: Effects of emission control technology and the economic recession
Observations of tropospheric NO2 vertical column densities over the\nUnited States (US) for 2005-2011 are evaluated using the OMI Berkeley\nHigh Resolution (BEHR) retrieval algorithm. We assess changes in NO2 on\nday-of-week and interannual timescales to assess the impact of changes\nin emissions from mobile and non-mobile sources on the observed trends.\nWe observe consistent decreases in cities across the US, with an average\ntotal reduction of 32 +/- 7% across the 7 yr. Changes for large power\nplants have been more variable (-26 +/- 12%) due to regionally-specific\nregulation policies. An increasing trend of 10-20% in background NO2\ncolumns in the northwestern US is observed. We examine the impact of the\neconomic recession on emissions and find that decreases in NO2 column\ndensities over cities were moderate prior to the recession (-6 +/- 5%\nyr(-1)), larger during the recession (-8 +/- 5% yr(-1)), and then\nsmaller after the recession (-3 +/- 4% yr(-1)). Differences in the\ntrends observed on weekdays and weekends indicate that prior to the\neconomic recession, NO2 reductions were dominated by technological\nimprovements to the light-duty vehicle fleet but that a decrease in\ndiesel truck activity has contributed to emission reductions since the\nrecession. We use the satellite observations to estimate a 34% decrease\nin NO2 from mobile sources in cities for 2005-2011 and use that value to\ninfer changes in non-mobile sources. We find that reductions in NO2 from\nnon-mobile sources in cities have been both more modest and more\nvariable than NO2 reductions from mobile sources (-10 +/- 13%).