AEROSOL-INDUCED CHEMICAL PERTURBATIONS OF STRATOSPHERIC OZONE - 3-DIMENSIONAL SIMULATIONS AND ANALYSIS OF MECHANISMS

An atmospheric general circulation model is coupled with a stratospher ic photochemical model to simulate the chemical/dynamical perturbation s associated with background and volcanically perturbed aerosols in th e lower stratosphere. The present work focuses on short-term anomalies at middle and high latitudes in the northern hemisphere, where large ozone depletions have been observed in late winter and early spring, p articularly following the eruption of Mount Pinatubo. Five fully coupl ed simulations are analyzed, corresponding to a control case with only gas phase chemistry, and cases including heterogeneous chemistry on b ackground aerosols, on El Chichon-type, and on Pinatubo-type aerosols. It is found that heterogeneous reactions occurring on sulfate aerosol s (background or postvolcanic) can strongly perturb the chemical parti tioning in the lower stratosphere, leading to significant ozone deplet ion through enhanced chlorine, bromine, and odd-hydrogen catalytic cyc les. In the Arctic lower stratosphere, the maximum zonal and March mon thly mean local ozone reductions (with respect to the control case) ca n exceed 15% for the background aerosol case, 40% for the El Chichon c ase, and 50% for the Pinatubo case. The corresponding zonal mean total column ozone decreases are roughly 5% and 15% for the background and volcanic aerosol cases, respectively. In the most extreme case tested (post-Pinatubo), a large ozone depletion below 30 mbar is offset to so me extent by an ozone increase above that level. The results of a sens itivity study (in which the aerosols are distributed closer to the tro pics, as might occur early after an eruption at low latitude) lead tp relatively small total ozone depletions at northern high latitudes, an d small ozone increases in the tropical lower stratosphere. The reduce d impact on total ozone at high latitudes is associated both with loca l ozone increases above 30 mbar and with poleward transport of enhance d ozone from the tropical lower stratosphere. The ozone increase at lo w latitudes is the net result of compensating changes in the catalytic destruction cycles involving odd-nitrogen and chlorine species activa ted by heterogeneous processes at the low temperatures and abundant su nlight found near the tropical tropopause. Our:simulations indicate th at ozone variations triggered by volcanic injections of aerosols depen d on the global distribution as well as th abundance of the particles and-their evolution over time, and on the initial dynamical-radiative- chemical state of the atmosphere, which itself exhibits large seasonal and interannual variability.