Weak impact of mixing on chlorine deactivation during SOLVE/THESEO 2000: Lagrangian modeling (CLaMS) versus ER‐2 in situ observations

Abstract

During the second and third segment of the SOLVE campaign in February and March 2000, the Arctic polar vortex began to be disturbed by planetary waves and upper stratospheric warming events. The perturbations of the vortex were associated with transport of air from low and middle latitudes into the polar region. Filaments with a lifetime exceeding two weeks were generated in regions of strong baroclinicity and peeled off the vortex edge.The Chemical Lagrangian Model of the Stratosphere (CLaMS) is used for the interpretation of filamentary structures in chemical tracer fields measured on board the ER‐2 during the March flights across the edge of the polar vortex. Both the mixing and the impact of mixing on the chemistry are considered. The isentropic version of CLaMS is initialized on 10 February at four isentropic levels: θ = 400, 425, 450, and 475 K. A comparison of the measured CH4/Halon‐1211 correlation curves and time series with corresponding CLaMS results obtained for spatial resolution of about 45 km indicates weak mixing between vortex and midlatitude air without pronounced anomalous mixing events. Thus the Arctic vortex in the altitude range 400–475 K was well isolated during the considered period without significant mass exchange across the vortex edge. The mixing intensity in CLaMS is controlled by the finite time Lyapunov exponent λ measuring the deformation rate of the horizontal wind and switching on mixing in the flow regions where λ exceeds a critical value λc. The CLaMS simulations suggest a temporally and spatially inhomogeneous mixing in the lower stratosphere with a lateral (across the wind) effective diffusion coefficient of the order 103 m2 s−1. The amount of ClONO2 formed due to chemistry induced by mixing of the activated vortex air with NOx‐rich midlatitude air does not exceed 3%. The impact of mixing on the accumulated ozone loss is less than 1%. The ClONO2 collar observed during the flight on 11 March can be understood as a result of deactivation of ClOx through the NOx produced owing to the chemical decomposition of HNO3 without a significant contribution of mixing with NOx‐rich midlatitude air.