Abstract: Tropospheric nitrogen dioxide (NO₂) measured from satellites has been widely used to track anthropogenic NO_x emissions, but its retrieval and interpretation can be complicated by the free tropospheric background to which satellite measurements are particularly sensitive. Observations from the OMI satellite instrument over the contiguous US (CONUS) shows no trend after 2009, despite sustained decreases in anthropogenic NO_x emissions, implying an important and rising contribution from the free tropospheric background. Here we use the GEOS-Chem chemical transport model applied to simulation of OMI NO₂ to better understand the sources and trends of background NO_x over CONUS. Previous model underestimate of the background is largely corrected by the consideration of aerosol nitrate photolysis and by using a new aircraft emission inventory. The increase in aircraft emissions over the past decades not only increases the background NO₂ but also affects the satellite retrieval by altering the NO₂ vertical profile. Increasing wildfire emissions also contributed to the post-2009 increase in the NO₂ background over the western US.