Atmospheric chlorine and stratospheric ozone nonlinearities and trend detection

Abstract

The percent decrease of total stratospheric ozone column content due to the injection of fluorocarbons (F-11 and F-12) has been found to be a nearly linear function of the atmospheric mixing ratio of ClX between 1.2 and 10 parts per billion by volume (ppbv). In contrast, Cicerone et al. (1983) found a broad region of slightly positive change in ozone column content between 10 and 80 km for small ClX perturbations in addition to a nonlinear negative decrease for larger perturbations. The presence of a nonlinear response in column ozone to moderate ClX changes appears to depend strongly on the method of diurnal averaging employed in the calculation. The immediate decrease in ozone column content for our small ClX perturbation results leads to the prediction of an earlier date for possible experimental detection of total ozone trends related to chlorine. For larger ClX perturbations the response in column ozone is nonlinear when the amount of ClX in the stratosphere becomes comparable to or larger than the amount of NOₓ. On the basis of our time-dependent calculations using a specified rate of fluorocarbon injection obtained from Logan et al. (1978), and similar previously obtained results from other models, the first evidence of an ozone decrease induced by ClX may be observable in the vicinity of 40 km by 1987–1990 with present-day satellite instrumentation (Nimbus 7). The actual detection of an ozone decrease by 1990 might be obscured by seasonal and longer-term atmospheric variations. Because of increasing fluxes of several minor constituents (e.g., CH4), possible long-term temperature changes in the stratosphere, and possible secular solar flux changes, observation of an ozone decrease is not necessarily an indicator of damage to ozone caused by atmospheric chlorine. Comparing the percent difference curves near 40 km for the diurnal variation of ozone corresponding to different amounts of ClX may provide a means of distinguishing ClX effects from other atmospheric changes. The change should be detectable when ClX finally reaches 5 ppbv in the upper stratosphere (about 2020). By the year 2020 the total ozone decrease due to ClX should be about 3% (2.5% with increasing methane) and may already pose environmental problems.