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 of US NO x emissions over the 2005–2017 period at a rate of 0.1 Mt 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 in contradiction with the NEI. We show here that the steady 2005–2017 decrease of NO x emissions reported by the NEI is in fact 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, where post-2009 flattening is due to an increasing relative contribution of non-anthropogenic background (mainly lightning and soils) and not to a flattening of anthropogenic emissions. This is confirmed by contrasting OMI NO 2 trends in urban winter, where the background is low and OMI NO 2 shows a steady 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 NO2 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.