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: December 2018

Abstract. Water vapour convectively injected into the mid-latitude lowermost stratosphere could affect stratospheric ozone. The associated potential ozone loss process requires low temperatures and an elevated water vapour mixing ratio. An increase in sulphate aerosol surface area due to a volcanic eruption or geoengineering could increase the likelihood of occurrence of this process. 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 is expected. Understanding the influence of low temperatures, elevated water vapour and enhanced sulphate particles on this ozone loss mechanism is a key step in estimating the impact of climate change and potential sulphate geoengineering on mid-latitude ozone. Here, we analyse the ozone loss mechanism and its sensitivity to various stratospheric conditions in detail. Conducting a box-model study with the Chemical Lagrangian Model of the Stratosphere (CLaMS), chemistry was simulated along a 7-day backward trajectory. This trajectory was calculated neglecting mixing of neighbouring air masses. Chemical simulations were initialized using measurements taken during the Study 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 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 197–203 K, a threshold in water vapour of 11.0–11.6 ppmv has to be exceeded and maintained for stratospheric ozone loss to occur. We investigated the sensitivity of the water vapour threshold to temperature, sulphate content, inorganic chlorine (Cly), inorganic nitrogen (NOy) and inorganic bromine (Bry). The water vapour threshold is mainly determined by the temperature and sulphate content. However, the amount of ozone loss depends on Cly, NOy, Bry and the duration of the time period over which chlorine activation can be maintained. Our results show that to deplete ozone, a chlorine activation time of 24 to 36 hours for conditions of the water vapour threshold with low temperatures and high water vapour mixing ratios must be maintained. A maximum ozone loss of 9 % was found for a 20 ppmv water vapour mixing ratio at North American Monsoon (NAM) tropopause standard conditions with the model run along a realistic trajectory. For the same trajectory, using observed conditions (of 10.6 ppmv), whether ozone loss occurs was simulated dependent on the sulphate 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 leads to significant ozone loss.