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

. The National Emission Inventory (NEI) of the US Environmental Protection Agency (EPA) reports a steady decrease in US NOx 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 NO2 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...