Satellite (OMI) and Surface Observations Confirm Steady Decline of US NOx Emissions over the 2004-2017 Period

The National Emission Inventory (NEI) of the US EPA reports a steady decrease of US NO_x emissions over the 2004-2017 period at a rate of -0.8 Mt a^-1, reflecting sustained efforts to improve air quality. Tropospheric NO₂ columns observed by the OMI satellite instrument over the US show a steady decrease until 2009 but a flattening afterward. This has been attributed to a decreasing effectiveness of NO_x emission controls after 2009, implying that NEI trends are incorrect. We show here that the steady 2004-2017 decrease of NO_x emissions reported by the NEI is in fact consistent with observed trends in nitrate wet deposition fluxes and surface NO₂ concentrations when only sites with continuous records are considered. The post-2009 flattening of the OMI NO₂ trend can be explained by an underestimate of background free tropospheric NO₂ in the air mass factors (AMFs) used to account for increased satellite measurement sensitivity with altitude. This underestimate of background free tropospheric NO₂ is supported by GEOS-Chem model simulations, SEAC⁴RS aircraft observations, and cloud-slicing OMI retrievals isolating free tropospheric NO_2. As US NO_x emissions decrease, background free tropospheric NO₂ makes an increasing (and now dominant) relative contribution to the tropospheric columns measured from space. Steady decline of US NO_x emissions over 2004-2017 is also consistent with the steady improvement of surface ozone air quality observed during that period. Better understanding is needed of the factors controlling free tropospheric NO₂ in order to relate satellite observations of tropospheric NO₂ columns to the underlying NO_x emissions.