AN EVALUATION OF UPPER TROPOSPHERE NOX WITH 2 MODELS

Upper tropospheric NOx controls, in part, the distribution of ozone in this greenhouse-sensitive region of the atmosphere. Many factors cont rol NOx in this region. As a result it is difficult to assess uncertai nties in anthropogenic perturbations to NO from aircraft, for example, without understanding the role of the ether major NOx 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 ''primar y'' NOx from each source category and uses two different chemical tran sport models (CTMs) to represent a range of possible atmospheric trans port. Because aircraft emissions are tied to particular pressure altit udes, it is important to understand whether those emissions are placed in the model stratosphere or troposphere and to assess whether the mo dels can adequately differentiate stratospheric air from tropospheric air. We examine these issues by defining a point-by-point ''tracer tro popause'' in order to differentiate stratosphere from troposphere in t erms of NOx perturbations. Both models predict similar zonal average p eak enhancements of primary NOx due to aircraft (approximate to 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 stratos pheric influence in one model and centered near the level evaluated as the tracer tropopause in the second. Below the tracer tropopause, bot h models show negligible NOx derived directly from the stratospheric s ource. Also, they predict a typically low background of 120 pptv NOx w hen tropospheric HNO3 is constrained to be 100 pptv of HNO3. The two m odels calculate large differences in the total background NOx (defined as the source of NOx from lightning + stratosphere + surface + HNO3) when using identical loss frequencies for NOx. This difference is prim arily due to differing treatments of vertical transport. An improved d iagnosis of this transport that is relevant to NOx requires either mea surements of a surface-based tracer with a substantially shorter lifet ime than Rn-222 or diagnosis and mapping of tracer correlations with d ifferent source signatures. Because of differences in transport by the two models we cannot constrain the source of NOx from lightning throu gh comparison of average model concentrations with observations of NOx .