Upper tropospheric NO_x controls, in part, the distribution of ozone in this greenhouse sensitive region of the atmosphere. Many factors control NO_x in this region. As a result it is difficult to assess uncertainties in anthropogenic perturbations to NO from aircraft, for example, without understanding the role of the other major NO_x sources in the upper troposphere. These include in situ sources (lightning, aircraft), convection from the surface (biomass burning, fossil fuels, soils), stratospheric intrusions, and photochemical recycling from HNO₃. This work examines the separate contribution to upper tropospheric “primary” NO_x from each source category and uses two different chemical transport models (CTMs) to represent a range of possible atmospheric transport. Because aircraft emissions are tied to particular pressure altitudes, it is important to understand whether those emissions are placed in the model stratosphere or troposphere and to assess whether the models can adequately differentiate stratospheric air from tropospheric air. We examine these issues by defining a point-by-point “tracer tropopause” in order to differentiate stratosphere from troposphere in terms of NO_x perturbations. Both models predict similar zonal average peak enhancements of primary NO_x due to aircraft (≈10–20 parts per trillion by volume (pptv) in both January and July); however, the placement of this peak is primarily in a region of large stratospheric influence in one model and centered near the level evaluated as the tracer tropopause in the second. Below the tracer tropopause, both models show negligible NO_x derived directly from the stratospheric source. Also, they predict a typically low background of 1-20 pptv NO_x when tropospheric HNO₃ is constrained to be 100 pptv of HNO₃. The two models calculate large differences in the total background NO_x (defined as the source of NO_x from lightning + stratosphere + surface + HNO₃) when using identical loss frequencies for NO_x. This difference is primarily due to differing treatments of vertical transport. An improved diagnosis of this transport that is relevant to NO_x requires either measurements of a surface-based tracer with a substantially shorter lifetime than ²²²Rn or diagnosis and mapping of tracer correlations with different source signatures. Because of differences in transport by the two models we cannot constrain the source of NO_x from lightning through comparison of average model concentrations with observations of NO_x.

Simulations of ²²²Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean ²²²Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of ²²²Rn concentrations in the upper troposphere, and none reproduces the high ²²²Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-²²²Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of ²²²Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.