Steven L. Baughcum

An evaluation of upper troposphere NO x with two models

Upper tropospheric NOx controls, in part, the distribution of ozone in this greenhouse sensitive region of the atmosphere. Many factors control NOx 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 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 “primary” NOx 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 NOx perturbations. Both models predict similar zonal average peak enhancements of primary NOx 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 NOx derived directly from the stratospheric source. Also, they predict a typically low background of 1-20 pptv NOx when tropospheric HNO3 is constrained to be 100 pptv of HNO3. The two models 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 primarily due to differing treatments of vertical transport. An improved diagnosis of this transport that is relevant to NOx 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 NOx from lightning through comparison of average model concentrations with observations of NOx.

Direct deposition of subsonic aircraft emissions into the stratosphere

The direct deposition of emissions from the 1992 aircraft fleet is studied using different aircraft emissions inventories, meteorological data from 1983–1996, and definitions of the tropopause. Globally, between 18 and 44% of current emissions of water vapor and nitrogen oxides are deposited in the stratosphere, depending on the exact definition of the tropopause and on season, in agreement with previous work. Largest emissions in the stratosphere are found to occur in January. The sensitivity of this result is tested with respect to the definition of the tropopause, the data used to analyze the tropopause, and the interseasonal and interannual variability of tropopause height. On a global basis, the stratospheric deposition is most sensitive to the definition of the tropopause. Approximately 20% of emissions are within ±1 km of the tropopause, and 35% are within ±2 km of the tropopause. Deposition into the stratosphere is not significantly affected by the choice of meteorological data set or by the interannual variability of the tropopause. These results have important implications for chemical model studies of aircraft emissions impact because they highlight the sensitivity to the exact flight altitudes, choice of tropopause height, and resolution in the tropopause region.