Using satellite observations of tropospheric NO_2 columns to infer long-term trends in US NO_x emissions: the importance of accounting for the free tropospheric NO_2 background

Abstract

The National Emission Inventory (NEI) of the US Environmental Protection Agency (EPA) reports a steady decrease in US NO_x emissions over the 2005–2017 period at a rate of 0.1 Tg N a^(−1) (53 % decrease over the period), reflecting sustained efforts to improve air quality. Tropospheric NO_2 columns observed by the satellite-based Ozone Monitoring Instrument (OMI) over the US show a steady decrease until 2009 but a flattening afterward, which has been attributed to a flattening of NO_x emissions, contradicting the NEI. We show here that the steady 2005–2017 decrease in NO_x emissions reported by the NEI is in fact largely consistent with observed network trends of surface NO_2 and ozone concentrations. The OMI NO_2 trend is instead similar to that observed for nitrate wet deposition fluxes, which is weaker than that for anthropogenic NO_x emissions, due to a large and increasing relative contribution of non-anthropogenic background sources of NO_x (mainly lightning and soils). This is confirmed by contrasting OMI NO_2 trends in urban winter, where the background is low and OMI NO_2 shows a 2005–2017 decrease consistent with the NEI, and rural summer, where the background is high and OMI NO_2 shows no significant 2005–2017 trend. A GEOS-Chem model simulation driven by NEI emission trends for the 2005–2017 period reproduces these different trends, except for the post-2009 flattening of OMI NO_2, which we attribute to a model underestimate of free tropospheric NO_2. Better understanding is needed of the factors controlling free tropospheric NO_2 in order to relate satellite observations of tropospheric NO_2 columns to the underlying NO_x emissions and their trends. Focusing on urban winter conditions in the satellite data minimizes the effect of this free tropospheric background.

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