C.: Trends in OMI NO2 observations over the United States: effects of emission control technology and the economic recession, Atmos

Abstract. Observations of tropospheric NO2 vertical column densities over the United States (US) for 2005–2011 are eval-uated using the OMI Berkeley High Resolution (BEHR) re-trieval algorithm. We assess changes in NO2 on day-of-week and interannual timescales to assess the impact of changes in emissions from mobile and non-mobile sources on the observed trends. We observe consistent decreases in cities across the US, with an average total reduction of 32 ± 7 % across the 7 yr. Changes for large power plants have been more variable (−26 ± 12 %) due to regionally-specific reg-ulation policies. An increasing trend of 10–20 % in back-ground NO2 columns in the northwestern US is observed. We examine the impact of the economic recession on emissions and find that decreases in NO2 column densities over cities were moderate prior to the recession (−6 ± 5 % yr−1), larger during the recession (−8 ± 5 % yr−1), and then smaller af-ter the recession (−3 ± 4 % yr−1). Differences in the trends observed on weekdays and weekends indicate that prior to the economic recession, NO2 reductions were dominated by technological improvements to the light-duty vehicle fleet but that a decrease in diesel truck activity has contributed to emission reductions since the recession. We use the satellite observations to estimate a 34 % decrease in NO2 from mo-bile sources in cities for 2005–2011 and use that value to in-fer changes in non-mobile sources. We find that reductions in NO2 from non-mobile sources in cities have been both more modest and more variable than NO2 reductions from mobile sources (−10 ± 13 %).