Initial Vortex Composition

Observations of NO by Podolske et al. [1993] show descent of air into the polar regions at high northern latitudes in autumn. Using daily analyzed fields of geopotential height and temperature and calculated winds, Manney and Zurek [1993] have shown that the polar vortex intensifies into the winter season, and as it does, gradients in potential vorticity (PV) and potential temperature intensify near the polar jet, isolating air poleward of the jet from mid-latitudes. As discussed by Schoeberl et al. [1992], this air continues to cool and descend throughout the winter, and extraordinarily large horizontal chemical gradients develop across the boundary between the vortex and mid-latitudes. Measurements of long-lived tracers by G. Toon et al. [1992a,b], J. Russell et al. [1992], and Traub et al. [1994] have shown that these gradients extend throughout the column and can be used to distinguish readily between vortex and mid-latitude air. Thus, during the winter and spring, air within the vortex that originated from higher altitudes is rich in ozone, inorganic chlorine (Cl = Cl + ClO + OClO + 2xCl + 2xClO + HCl + ClONO + HOCl), water vapor, and reactive nitrogen (NO = NO + NO + NO + 2xNO + HONO + HNO + HONO + ClONO) relative to air at mid-latitudes. As shown by Strahan and Mahlman [1994a,b], Garcia and coworkers [1992,1994], and Randel et al. [1994] models also reproduce this behavior and can simulate the distributions of long-lived tracers such as NO, even accounting for differences between the arctic and antarctic polar vortices due to wave activity.

Early in the winter, cold air within the vortex is isolated from intense solar illumination, so photolysis rates are relatively slow and heterogeneous reactions on liquid sulfate aerosols can efficiently convert nitrogen oxides into nitric acid by hydrolysis of NO. The resulting build-up of nitric acid observed by UARS was reported by Roche et al. [1993] and Santee et al. [1995], and occurs by December in the Arctic and by May over Antarctica. Ground-based observations of Solomon et al. [1993] and Perner et al. [1994] found enhancements of OClO, indicative of low NO (NO + NO) and high abundances of ClO, over both polar regions after the eruption of Mount Pinatubo, apparently before extensive occurence of polar stratospheric clouds (PSCs). This suggests that reactions on sulfate aerosols can convert chlorine from long-lived reservoir forms (primarily HCl and ClONO) into reactive forms (Cl and ClO) in cold, low-sunlight conditions [ Hanson et al., 1994]. Since the observations were made after the eruption of Mount Pinatubo, it is unclear whether these processes are important under background aerosol conditions.