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. The National Emission Inventory (NEI) of the US EnvironmentalProtection Agency (EPA) reports a steady decrease in US NOx emissionsover the 2005–2017 period at a rate of 0.1 Tg N a −1 (53 % decreaseover the period), reflecting sustained efforts to improve air quality.Tropospheric NO2 columns observed by the satellite-based OzoneMonitoring Instrument (OMI) over the US show a steady decrease until 2009but a flattening afterward, which has been attributed to a flattening of NOx emissions, contradicting the NEI. We show here that thesteady 2005–2017 decrease in NOx emissions reported by the NEI is infact largely consistent with observed network trends of surface NO2 andozone concentrations. The OMI NO2 trend is instead similar to thatobserved for nitrate wet deposition fluxes, which is weaker than that foranthropogenic NOx emissions, due to a large and increasing relativecontribution of non-anthropogenic background sources of NOx (mainlylightning and soils). This is confirmed by contrasting OMI NO2 trendsin urban winter, where the background is low and OMI NO2 shows a2005–2017 decrease consistent with the NEI, and rural summer, where thebackground is high and OMI NO2 shows no significant 2005–2017 trend. AGEOS-Chem model simulation driven by NEI emission trends for the 2005–2017period reproduces these different trends, except for the post-2009 flatteningof OMI NO2 , which we attribute to a model underestimate of freetropospheric NO2 . Better understanding is needed of the factorscontrolling free tropospheric NO2 in order to relate satelliteobservations of tropospheric NO2 columns to the underlying NOx emissions and their trends. Focusing on urban winter conditions in thesatellite data minimizes the effect of this free tropospheric background.

atmospheric chemistry and physics

Using satellite observations of tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; columns to infer long-term trends in US NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; emissions: the importance of accounting for the free tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; background