Emissions of nitrogen oxides from US urban areas: estimation from Ozone Monitoring Instrument retrievals for 2005–2014

Satellite remote sensing of tropospheric nitrogen dioxide (NO₂) can provide valuable information for estimating surface nitrogen oxides (NO_<i>x</i>) emissions. Using an exponentially modified Gaussian (EMG) method and taking into account the effect of wind on observed NO₂ distributions, we estimate 3-year moving-average emissions of summertime NO_<i>x</i> from 35 US (United States) urban areas directly from NO₂ retrievals of the Ozone Monitoring Instrument (OMI) during 2005–2014. Following conclusions of previous studies that the EMG method provides robust and accurate emission estimates under strong-wind conditions, we derive top-down NO_<i>x</i> emissions from each urban area by applying the EMG method to OMI data with wind speeds greater than 3–5 m s⁻¹. Meanwhile, we find that OMI NO₂ observations under weak-wind conditions (i.e., < 3 m s^&minus;1) are qualitatively better correlated to the surface NO_<i>x</i> source strength in comparison to all-wind OMI maps; therefore, we use them to calculate the satellite-observed NO₂ burdens of urban areas and compare with NO_<i>x</i> emission estimates. The EMG results show that OMI-derived NO_<i>x</i> emissions are highly correlated (<i>R</i> > 0.93) with weak-wind OMI NO₂ burdens as well as with bottom-up NO_<i>x</i> emission estimates over 35 urban areas, implying a linear response of the OMI observations to surface emissions under weak-wind conditions. The simultaneous EMG-obtained effective NO₂ lifetimes (~ 3.5 ± 1.3 h), however, are biased low in comparison to the summertime NO₂ chemical lifetimes. In general, isolated urban areas with NO_<i>x</i> emission intensities greater than ~ 2 Mg h⁻¹ produce statistically significant weak-wind signals in 3-year average OMI data. From 2005 to 2014, we estimate that total OMI-derived NO_<i>x</i> emissions over all selected US urban areas decreased by 49 %, consistent with reductions of 43, 47, 49, and 44 % in the total bottom-up NO_<i>x</i> emissions, the sum of weak-wind OMI NO₂ columns, the total weak-wind OMI NO₂ burdens, and the averaged NO₂ concentrations, respectively, reflecting the success of NO_<i>x</i> control programs for both mobile sources and power plants. The decrease rates of these NO_<i>x</i>-related quantities are found to be faster (i.e., −6.8 to −9.3 % yr^&minus;1) before 2010 and slower (i.e., −3.4 to −4.9 % yr^&minus;1) after 2010. For individual urban areas, we calculate the <i>R</i> values of pair-wise trends among the OMI-derived and bottom-up NO_<i>x</i> emissions, the weak-wind OMI NO₂ burdens, and ground-based NO₂ measurements, and high correlations are found for all urban areas (median <i>R</i>= 0.8), particularly large ones (<i>R</i> up to 0.97). The results of the current work indicate that using the EMG method and considering the wind effect, the OMI data allow for the estimation of NO_<i>x</i> emissions from urban areas and the direct constraint of emission trends with reasonable accuracy