INFLUENCE OF NONLOCAL CHEMISTRY ON TRACER DISTRIBUTIONS - INFERRING THE MEAN AGE OF AIR FROM SF6

Sulfur hexafluoride (SF6) is nearly inert in the troposphere and strat osphere and has a documented, steady increase in the troposphere due t o industrial sources, making it a useful tracer of atmospheric circula tion. Studies using SF6 to estimate the mean age of stratospheric air have assumed the influence of mesospheric photochemical destruction is negligible. However, the mean age of an air parcel may be sensitive t p small fractions, of the air that have resided for long times in the upper atmosphere. Here we use two three-dimensional chemical transport models to estimate the influence of mesospheric SF6 loss on mean age inferences in the stratosphere. Because the mechanisms of SF6 loss are uncertain, we perform a number of simulations employing a range of ma gnitudes of a simple constant mesospheric loss frequency, as well as a range of scenarios for SF6 time variation in the troposphere. Using l oss rates producing plausible global Lifetimes of SF6 (1000 to 3000 ye ars), and tropospheric time-variation matching: observations, we find that age; estimates inferred from SF6 mixing ratios may be significant ly biased and that the bias is increasing in time. For example, the pr esent-day time lag of SF6 mixing ratio from the troposphere overestima tes the mean age by up to 18% at 68 degrees S and 20 km, and up to 65% at 68 degrees S and 30 km, depending on the loss rate and model. Corr ecting for this bias would bring recent comparisons of modeled and mea sured mean age closer in line.