The GCAS versus CAMx intercomparison suggests that the point source plumes are captured by the model with decent accuracy, but that NO2 is severely underestimated in the downtown area of Houston, as well as in the outer portions of the model domain. This suggests two issues with the NO2 in CAMx: an underestimate of NOx emissions near downtown Houston, and an underestimate of NO2 advected into the model domain from the boundary conditions. In following sections, we explore these model biases quantitatively. NOx emissions using Flux divergence applied to GCAS. The flux divergence (FD) method applied to the GCAS aircraft data quantified NO2 fluxes (μg/m 2 ). To our knowledge, this paper is the first to quantify NO2 fluxes by applying the FD method to aircraft remote sensing data. We identified many of the major NOx sources individually in the Houston CAMx domain: power plants and refineries as well as the IAH international airport (Figure left panel). In addition, the method identified the area of the ship channel as well as the route of the ships sailing through the Galveston Bay. Finally, the method clearly identified the major highways in the region. The center panel of Figure shows the flux divergence method applied to the CAMx simulation. In this case, the sources are known and so these simulations serve to evaluate the