Stratosphere-troposphere exchange and the impact of commercial aviation on the atmosphere

Abstract

The exchange of mass and chemical constituents between the stratosphere and the troposphere is examined on a variety of time scales using various techniques. Estimates are developed of the stratosphere-troposphere exchange of important chemical species, and commercial aircraft emissions. Various methods for stratosphere-troposphere exchange are discussed. The circulation of the stratosphere (expressed using the Transformed Eulerian Mean residual circulation) is used to estimate fluxes of ozone, methane, nitrous oxide and CF2Cl2. These fluxes are found to be within the range of previous estimates. Depending upon the tropopause definition, between 18% and 44% of global aircraft emissions are deposited annually into the stratosphere. The distribution of emissions is found to be most sensitive to the tropopause definition, and not sensitive to the data set used or to inter-annual variability of the tropopause. Using a transport model as well as a trajectory model, the residence time of these aircraft emissions in the stratosphere is determined to be about 50 days, shorter in January than July. Simulations of water vapor emissions in the upper troposphere and lower stratosphere are found to qualitatively reproduce the water vapor distribution in this region. The stratosphere-troposphere exchange of water vapor across the tropical tropopause is examined in detail. Large scale processes dominate the stratosphere-troposphere exchange of water vapor, but small scale processes are important in the upper tropical troposphere. A diagnostic of cross-tropopause transport is calculated at the tropopause and evaluated. Sensitivity tests and theoretical considerations indicate that the two-way cross tropopause flux may be significantly exaggerated by this method. The exaggeration is plausibly related to the type of data used to evaluate the diagnostic. The various methods for examining stratosphere troposphere exchange are summarized, along with illustrative quantitative estimates from the literature. Different methods are found to yield consistently different results. Observed and hypothesized mechanisms for stratosphere-troposphere exchange are discussed along with conclusions about the impact of aircraft emissions on the atmosphere.