Abstract
Abstract. We investigated chemical and microphysical processes in the late winter in the Antarctic lower stratosphere, after the first chlorine activation and initial ozone depletion. We focused on a time interval when both further chlorine activation and ozone loss, but also chlorine deactivation, occur. We performed a comprehensive Lagrangian analysis to simulate the evolution of an air mass along a 10-day trajectory, coupling a detailed microphysical box model to a chemistry model. Model results have been compared with in situ and remote sensing measurements of particles and ozone at the start and end points of the trajectory, and satellite measurements of key chemical species and clouds along it. Different model runs have been performed to understand the relative role of solid and liquid polar stratospheric cloud (PSC) particles for the heterogeneous chemistry, and for the denitrification caused by particle sedimentation. According to model results, under the conditions investigated, ozone depletion is not affected significantly by the presence of nitric acid trihydrate (NAT) particles, as the observed depletion rate can equally well be reproduced by heterogeneous chemistry on cold liquid aerosol, with a surface area density close to background values. Under the conditions investigated, the impact of denitrification is important for the abundances of chlorine reservoirs after PSC evaporation, thus stressing the need to use appropriate microphysical models in the simulation of chlorine deactivation. We found that the effect of particle sedimentation and denitrification on the amount of ozone depletion is rather small in the case investigated. In the first part of the analyzed period, when a PSC was present in the air mass, sedimentation led to a smaller available particle surface area and less chlorine activation, and thus less ozone depletion. After the PSC evaporation, in the last 3 days of the simulation, denitrification increases ozone loss by hampering chlorine deactivation.
Highlights
The depletion of ozone occurring in the polar stratosphere during winter and spring is linked to processes involving clouds in the polar stratosphere (Solomon et al, 1986)
An in situ observation of an air mass when a polar stratospheric clouds (PSC) was present, by an optical particle counter and ozonometer on a balloon launched from the Antarctic station of McMurdo, where a polarization diversity lidar was operating, provided information on PSC characteristics and ozone abundance
A trajectory analysis revealed that the air mass at around the 400 K level was close to McMurdo Station 10 days later, when lidar and ozone sounding were accomplished, showing a marked ozone depletion and no sign of PSC
Summary
The depletion of ozone occurring in the polar stratosphere during winter and spring is linked to processes involving clouds in the polar stratosphere (Solomon et al, 1986). Satellite measurements of key chemical species and particles along the airmass trajectories documented its microphysical and chemical evolution This data set has been compared with simulations from chemical and microphysical box models reproducing the evolution of the cloud and evaluating its impact on the chemistry in the air mass. This well-documented case took place in early September, soon after the onset of ozone depletion from chlorine activation but before complete destruction of ozone, in a region close to the vortex edge. By modeling the microphysical and chemical processes along the trajectory and comparing simulations with observations, an assessment of the modeled denitrification, ozone chemistry, and the impact of PSC occurrence on ozone depletion can be made. The scope of this study is to provide a contribution to the most recent discussion of the relative role of PSC and liquid (background) aerosol in the ozone depletion (Drdla and Müller, 2012; Wegner et al, 2012; Wohltmann et al, 2013)
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