Background nitrogen dioxide (NO ₂) over the United States and its implications for satellite observations and trends: effects of nitrate photolysis, aircraft, and open fires

page6272Tropospheric nitrogen dioxide (NOinline-formula₂) measured from satellites has been widely used to track anthropogenic NOinline-formula_x emissions, but its retrieval and interpretation can be complicated by the free tropospheric NOinline-formula₂ background to which satellite measurements are particularly sensitive. Tropospheric NOinline-formula₂ vertical column densities (VCDs) from the spaceborne Ozone Monitoring Instrument (OMI) averaged over the contiguous US (CONUS) show no trend after 2009, despite sustained decreases in anthropogenic NOinline-formula_x emissions, implying an important and rising contribution from the free tropospheric background. Here, we use the GEOS-Chem chemical transport model applied to the simulation of OMI NOinline-formula₂ to better understand the sources and trends of background NOinline-formula₂ over CONUS. The previous model underestimate of the background is largely corrected by the consideration of aerosol nitrate photolysis, which increases the model NOinline-formula₂ VCDs by 13 % on an annual basis (25 % in spring) and also increases the air mass factor (AMF) to convert the tropospheric slant column densities (SCDs) inferred from the OMI spectra into VCDs by 7 % on an annual basis (11 % in spring). The increase in the AMF decreases the retrieved NOinline-formula₂ VCDs in the satellite observations, contributing to the improved agreement with the model. Accounting for the 2009–2017 increase in aircraft NOinline-formula_x emissions drives only a 1.4 % mean increase in NOinline-formula₂ VCDs over CONUS and a 2 % increase in the AMF, but the combination of decreasing surface NOinline-formula_x emissions and increasing aircraft emissions is expected to drive a 14 % increase in the AMF over the next decade that will be necessary to account for in the interpretation of satellite NOinline-formula₂ trends. Fire smoke identification with the National Oceanic and Atmospheric Administration (NOAA) Hazard Mapping System (HMS) indicates that wildfires contribute 1 %–8 % of OMI NOinline-formula₂ VCDs over the western US in June–September and that this contribution has been increasing since 2009, contributing to the flattening of OMI NOinline-formula₂ trends. Future analyses of NOinline-formula₂ trends from satellite data to infer trends in surface NOinline-formula_x emissions must critically consider the effects of a rising free tropospheric background due to increasing emissions from aircraft, fires, and possibly lightning.