Abstract
Abstract. Satellite remote sensing of tropospheric nitrogen dioxide (NO2) can provide valuable information for estimating surface nitrogen oxides (NOx) 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 NOx 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 NOx 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−1) are qualitatively better correlated to the surface NOx 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 NOx emission estimates. The EMG results show that OMI-derived NOx emissions are highly correlated (R > 0.93) with weak-wind OMI NO2 burdens as well as with bottom-up NOx 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 NOx 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 NOx emissions over all selected US urban areas decreased by 49 %, consistent with reductions of 43, 47, 49, and 44 % in the total bottom-up NOx 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 NOx control programs for both mobile sources and power plants. The decrease rates of these NOx-related quantities are found to be faster (i.e., −6.8 to −9.3 % yr−1) before 2010 and slower (i.e., −3.4 to −4.9 % yr−1) after 2010. For individual urban areas, we calculate the R values of pair-wise trends among the OMI-derived and bottom-up NOx emissions, the weak-wind OMI NO2 burdens, and ground-based NO2 measurements, and high correlations are found for all urban areas (median R= 0.8), particularly large ones (R 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 NOx emissions from urban areas and the direct constraint of emission trends with reasonable accuracy.
Highlights
Nitrogen oxides (NOx), the sum of nitrogen dioxide (NO2) and nitric oxide (NO), is one of the six criteria pollutants identified by the US EPA
In terms of the Ozone Monitoring Instrument (OMI)-observed NO2 trend during 2005–2014, a significant reduction in tropospheric vertical column densities (TVCDs) of up to 50 % is observed in visible hotspots and an increase of up to ∼ 0.3 × 1015 molecules cm−2 is observed in rural areas, in the central Us (Fig. 3c)
Significant differences are found between the OMI NO2 maps averaged from all valid data and those from data with slow wind speeds only, and such differences are not uniform across all urban areas but depend on local meteorological conditions
Summary
Nitrogen oxides (NOx), the sum of nitrogen dioxide (NO2) and nitric oxide (NO), is one of the six criteria pollutants identified by the US EPA Kunhikrishnan et al (2004) conducted a similar analysis over the Arabian Sea outflow region to estimate the regional NOx lifetime for the Indian subcontinent This method was revised by Beirle et al (2004), who fitted the GOME-observed NO2 columns across the shipping lane between Sri Lanka and Indonesia with an exponentially modified Gaussian (EMG) function and derived the mean NOx lifetime and the corresponding ship emissions for 1996–2001. de Foy et al (2014) evaluated the performance of the EMG method using simulated column densities over a point source with known emissions under three chemical lifetime cases They found that the EMG method generally provided reliable emission estimates at fast-wind-speed conditions (> 3 m s−1); the lifetime estimates were biased low and quite sensitive to the selection of the wind-speed cutoff and the accuracy of the plume rotation. The rest of the paper is organized as follows: Sect. 2 documents the methodology and data sets; Sect. 3 highlights the effect of wind on the OMI NO2 observations (Sect. 3.1), presents the relationship between the EMG-derived NOx emissions and the OMI NO2 observations (Sect. 3.2), and compares the trends of various NOx-related quantities (Sect. 3.3); and Sect. 4 summarizes the major findings of this work
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