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

Abstract. Observations of tropospheric NO₂ vertical column densities over the United States (US) for 2005–2011 are evaluated using the OMI Berkeley High Resolution (BEHR) retrieval 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 regulation policies. An increasing trend of 10–20% in background 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 after 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 mobile sources in cities for 2005–2011 and use that value to infer 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%).