Silvern, Rachel F.; Jacob, Daniel J.; Mickley, Loretta J.; Sulprizio, Melissa P.; Travis, Katherine R.; Marais, Eloise A.; Cohen, Ronald C.; Laughner, Joshua L.; Choi, Sungyeon; Joiner, Joanna; Lamsal, Lok N.

The National Emission Inventory (NEI) of the US Environmental Protection Agency (EPA) reports a steady decrease in US inline-formulaNOx emissions over the 2005–2017 period at a rate of 0.1 Tg N ainline-formula−1 (53 % decrease over the period), reflecting sustained efforts to improve air quality. Tropospheric inline-formulaNO2 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 inline-formulaNOx emissions, contradicting the NEI. We show here that the steady 2005–2017 decrease in inline-formulaNOx emissions reported by the NEI is in fact largely consistent with observed network trends of surface inline-formulaNO2 and ozone concentrations. The OMI inline-formulaNO2 trend is instead similar to that observed for nitrate wet deposition fluxes, which is weaker than that for anthropogenic inline-formulaNOx emissions, due to a large and increasing relative contribution of non-anthropogenic background sources of inline-formulaNOx (mainly lightning and soils). This is confirmed by contrasting OMI inline-formulaNO2 trends in urban winter, where the background is low and OMI inline-formulaNO2 shows a 2005–2017 decrease consistent with the NEI, and rural summer, where the background is high and OMI inline-formulaNO2 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 inline-formulaNO2, which we attribute to a model underestimate of free tropospheric inline-formulaNO2. Better understanding is needed of the factors controlling free tropospheric inline-formulaNO2 in order to relate satellite observations of tropospheric inline-formulaNO2 columns to the underlying inline-formulaNOx 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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Silvern, Rachel F. / Jacob, Daniel J. / Mickley, Loretta J. / et al: Using satellite observations of tropospheric NO2 columns to infer long-term trends in US NOx emissions: the importance of accounting for the free tropospheric NO2 background. 2019. Copernicus Publications.

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