Mechanism of ozone loss under enhanced water vapour conditions in the mid-latitude lower stratosphere in summer
Authors: Robrecht Sabine , Vogel Bärbel , Grooß Jens-Uwe , Rosenlof Karen , Thornberry Troy , Rollins Andrew , Krämer Martina , Christensen Lance , Müller Rolf 
Published: May 2019

Abstract. Water vapour convectively injected into the mid-latitude lowermost
stratosphere could affect stratospheric ozone. The associated potential ozone
loss process requires low temperatures together with elevated water vapour
mixing ratios. Since this ozone loss is initiated by heterogeneous chlorine
activation on liquid aerosols, an increase in sulfate aerosol surface area
due to a volcanic eruption or geoengineering could increase the likelihood of
its occurrence. However, the chemical mechanism of this ozone loss process
has not yet been analysed in sufficient detail and its sensitivity to various
conditions is not yet clear. Under conditions of climate change associated
with an increase in greenhouse gases, both a stratospheric cooling and an
increase in water vapour convectively injected into the stratosphere are
expected. Understanding the influence of low temperatures, elevated water
vapour and enhanced sulfate particles on this ozone loss mechanism is a key
step in estimating the impact of climate change and potential sulfate
geoengineering on mid-latitude ozone. Here, we analyse the ozone loss mechanism and its sensitivity to various
stratospheric conditions in detail. By conducting a box-model study with the
Chemical Lagrangian Model of the Stratosphere (CLaMS), chemistry was
simulated along a 7 d backward trajectory. This trajectory was calculated
neglecting mixing of neighbouring air masses. Chemical simulations were
initialized using measurements taken during the Studies of Emissions and
Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys
(SEAC4RS) aircraft campaign (2013, Texas), which encountered an elevated
water vapour mixing ratio of 10.6 ppmv at a pressure level around 100 hPa.
We present a detailed analysis of the ozone loss mechanism, including the
chlorine activation, chlorine-catalysed ozone loss cycles, maintenance of
activated chlorine and the role of active nitrogen oxide radicals
(NOx). Focussing on a realistic trajectory in a temperature
range from 197 to 202 K, a threshold in water vapour of 10.6 ppmv has to be
exceeded and maintained for stratospheric ozone loss to occur. We
investigated the sensitivity of the water vapour threshold to temperature,
sulfate content, inorganic chlorine (Cly), inorganic
nitrogen (NOy) and inorganic bromine (Bry).
The water vapour threshold is mainly determined by the temperature and
sulfate content. However, the amount of ozone loss depends on
Cly, Bry and the duration of the time
period over which chlorine activation can be maintained. NOy
affects both the potential of ozone formation and the balance between
reactions yielding chlorine activation and deactivation, which determines the
water vapour threshold. Our results show that in order to deplete ozone, a
chlorine activation time of 24 to 36 h for conditions of the water vapour
threshold with low temperatures must be maintained. A maximum ozone loss of
9 % was found for a 20 ppmv water vapour mixing ratio using North
American Monsoon (NAM) tropopause standard conditions with a chemical
box-model simulation along a realistic trajectory. For the same trajectory,
using observed conditions (of 10.6 ppmv H2O), the occurrence of
simulated ozone loss was dependent on the sulfate amount assumed. Detailed
analysis of current and future possibilities is needed to assess whether
enhanced water vapour conditions in the summertime mid-latitude lower
stratosphere lead to significant ozone loss.