Mechanism of ozone loss under enhanced water vapour conditions in the mid-latitude lower stratosphere in summer

Abstract

<p><strong>Abstract.</strong> 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.</p><p> 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 (SEAC<sup>4</sup>RS) 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 (NO<sub>x</sub>). 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 (Cl<sub>y</sub>), inorganic nitrogen (NO<sub>y</sub>) and inorganic bromine (Br<sub>y</sub>). The water vapour threshold is mainly determined by the temperature and sulphate content. However, the amount of ozone loss depends on Cl<sub>y</sub>, NO<sub>y</sub>, Br<sub>y</sub> 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.</p>

A 13‐year Trajectory‐Based Analysis of Convection‐Driven Changes in Upper Troposphere Lower Stratosphere Composition Over the United States

“…The final scenario, that other processes are contributing to O 3 reduction in tropical environments, seems most likely. Previous studies have identified the possibility of chemical destruction of O 3 through chlorine activation when an airmass is cold and wet enough and contains a sufficient amount of inorganic chlorine, though the extent of possible destruction is still a subject of debate (Anderson et al., 2017, 2012; M. J. Schwartz et al., 2013; Robrecht et al., 2019; Schoeberl et al., 2020). At the level of maximum O 3 reduction observed in convectively influenced MLS layers within tropical environments, temperatures are less than 205 K ∼50% of the time and less than 200 K ∼10% of the time while H 2 O concentrations are typically ∼5–7 ppmv (Figures 4c and 6b).…”

“…However, some proportion of the decreased O 3 concentrations can likely be attributed to dilution by tropospheric air. The extent to which chemical destruction of O 3 occurs is still largely unknown and is a topic of debate within the community (e.g., Robrecht et al., 2019).…”

Extreme Outliers in Lower Stratospheric Water Vapor Over North America Observed by MLS: Relation to Overshooting Convection Diagnosed From Colocated Aqua‐MODIS Data

“…Using global Version 3 observations from the Aura Microwave Limb Sounder (MLS) over the years 2005–2012, Schwartz et al (2013) (“S2013” hereinafter) revealed that OCEs over the North American monsoon area (“NA”) yield elevated H 2 O in the LMS in up to 1% of profiles. While numerous studies discuss localized negative correlations between OCEs and ozone concentrations (e.g., Anderson et al, 2012; Héron et al, 2020; Solomon et al, 2005), the overall impact of OCEs on stratospheric ozone is currently believed to be low (e.g., Homeyer et al, 2014; Robrecht et al, 2019; Schwartz et al, 2013; World Meteorological Organisation, 2018).…”

Erythemal Radiation, Column Ozone, and the North American Monsoon

“…Recently, Robrecht et al (2019) has revisited the SEAC 4 RS data and used the CLAMS model to explore the possibility of chemical ozone loss following the convective injection of water vapor into the lower stratosphere. With a series of model experiments, they found that the injected water vapor amount had to exceed 10 ppmv, and there had to be sufficient aerosol nuclei and high amounts of reservoir chlorine (~1 ppbv of Cl y ) to produce~60% ozone loss.…”

“…With a series of model experiments, they found that the injected water vapor amount had to exceed 10 ppmv, and there had to be sufficient aerosol nuclei and high amounts of reservoir chlorine (~1 ppbv of Cl y ) to produce~60% ozone loss. They found that under normal summer midlatitude conditions, Cl y is believed to be~0.15 ppbv (based on observed correlation with CH 4 ) and with a 20 ppmv injection of water, Robrecht et al (2019) computed an ozone loss of only~9%. According to MLS observations compiled by Schwartz et al (2013) convectively injected water levels exceeding 20 ppmv are rarely observed at 16 km (~100 hPa) and higher altitudes.…”