Are Antarctic ozone variations a manifestation of dynamics or chemistry?

Abstract

Observations^1–3 reveal a large seasonal decrease in the column density of ozone during Antarctic early spring, followed by a rapid increase after October beyond its pre-spring value. Given the unique circumstances that exist in the Antarctic environment— relatively stable circumpolar vortex, cold air temperature achieved during polar night, and the increase in absorption of solar radiation by ozone as the Sun returns—we surmise the existence of a reverse circulation cell with rising motion in the polar lower stratosphere. The upwelling brings ozone-poor air from below 100 mbar to the stratosphere, possibly contributing to the observed ozone decline in early spring. At the same time, the Antarctic stratosphere might contain a very low concentration (<0.1 p.p.b.v. (parts per 10⁹ by volume) of NO_x(NO + NO₂), a condition that could favour a greatly enhanced catalytic removal of O₃ by halogen species^2,4,5. We argue that heterogeneous processes and formation of OC1O by the reaction BrO + ClO → OClO + Br before and after the polar night might help to suppress the NO_X levels during the early spring period. However, the dilution of the concentrations of the chlorine species by the upwelling may reduce the effectiveness of the photochemical removal of O₃.