Atmospheric Chemistry and Ozone

Chemical ozone loss cannot be deduced from isolated measurements of ozone alone, since ozone values at a given location are always determined not only by chemistry, but also by transport.

From the different approaches developed to overcome this problem, the Match approach developed at AWI (Rex et al., 1999) is the most straightforward and reliable solution. In the Match approach, an air mass probed by an ozonesonde is measured for a second time by predicting the trajectory of the air parcel and a coordinated sonde launch at a different station.

Since 1992, more than 15 international measurement campaigns involving a large number of ozonesonde stations have been conducted in the Arctic and Antarctic.

Low stratospheric temperatures at the altitude of the ozone layer are a prerequisite for Arctic ozone depletion. Temperatures inside the Arctic polar vortex show a large variability between different years. The polar vortex is a low-pressure system in the stratosphere at an altitude of 15 to 50 kilometres that forms over the Arctic every autumn and may persist until spring. Chlorine, which is normally present in chemically inactive reservoir gases, can be released from these gases at the surface of polar stratospheric clouds at low temperatures. Subsequently, chlorine species in combination with bromine destroy ozone when exposed to sunlight.

The relation between column ozone loss accumulated during a winter/spring season and the area of the vortex below the formation temperature of polar stratospheric clouds is linear to a good approximation (Rex et al., 2004; 2006; Tilmes et al., 2006; Pommereau et al., 2018). The Match campaigns (see preceding segment) provide the amount of ozone loss during many Arctic winter/spring seasons in the past for this study. As expected lower temperatures lead to more ozone losses.  Moreover, Rex et al. (2004, 2006) showed that there is a tendency for cold stratospheric Arctic winters to get colder. This trend was confirmed by von der Gathen et al., (2021) with the help of several state-of-the-art meteorological data sets, i.e. ERA5, MERRA-2, JRA-55, and CFSR/CFSv2.

Von der Gathen et al., (2021) investigated the future stratospheric temperature trend in the polar vortex in the output from 53 computer models of the international “Coupled Model Intercomparison Project Phases 5 and 6” (CMIP5, CMIP6).  Depending on the greenhouse gas emission scenarios used in these models the trends project into the future with different magnitude, with larger temperature trends related to stronger greenhouse gas emissions. That finding clearly shows that the stratospheric temperature trend in the polar vortex is part of climate change and therefore the product of global greenhouse-gas emissions.

Despite the production ban issued in the 1987 Montreal Protocol, substances like chlorofluorocarbons (CFCs) and halons, which contain ozone-destroying chlorine and bromine atoms, are still abundant in the atmosphere, because of their long lifetimes. The concentrations of these substances in the polar vortex continued to rise until the year 2000. Since then, they have been on decline and are currently (2021) at roughly 90 percent of the maximum. Only by the end of the century they will reach values below 50 percent of the maximum (WMO, 2018). These reductions will eventually lead to a recovery of the global ozone layer and even to a super-recovery in mid-latitudes, where gas-phase catalytic destruction cycles are prevalent. I.e., in the global mean, there will be more ozone in the stratosphere 2100 compared to the years before 1980. However, the situation inside the Arctic polar vortex can be different due to the strength of the climate change and the related temperature trend inside the vortex. In an extreme emission scenario the amount of ozone losses can even increase until the end of the century (von der Gathen et al., 2021).

Related press release

The Tropical Western Pacific and the Asian Summer Monsoon are the main entry regions for tropospheric air into the stratosphere. Hence, atmospheric processes and composition in these regions determine the composition and chemistry of the global stratosphere and of the stratospheric aerosol layer within. This can have far-reaching consequences, e.g., for polar ozone depletion or the radiative balance of the Earth.

Since 2015, AWI operates a temporary research station in Palau (7°N, 134°E), in the center of the Tropical Western Pacific warm pool. Our previous measurements of very low ozone concentrations during the TransBrom campaign in 2009 revealed a low local oxidizing capacity in this region, which controls chemical lifetimes and thus the abundance of chemical species entering the stratosphere here (Rex et al., Atmos. Chem. Phys., 2014). The Palau Atmospheric Observatory was established during the EU-project StratoClim, hosts a variety of ground-based remote sensing instruments like a cloud and aerosol lidar and facilitates regular balloon launches with ozone, water vapour and aerosol sondes. With our measurements, we contributed to international research campaigns like the NASA-led project POSIDON in 2016 and the NCAR/NASA project ACCLIP in 2022. The unprecedented, growing time series of tropospheric ozone measurements fills an observational gap in this key region (Müller et al., Atmos. Chem. Phys., preprint 2023).

We perform modelling studies on the origin and chemistry of the air entering the stratosphere and develop and operate a detailed trajectory-based transport model with an explicit simulation of convection and a model of SO₂ chemistry and microphysics. Recent areas of study include

• ammonium nitrate in the Asian monsoon (Höpfner et al., Nature Geoscience, 2019),
• the characterization and origin of air masses in the Tropical Western Pacific (Müller, 2020)
• explaining the concentrations of short-lived pollution species in the Asian monsoon (Johansson et al., Atmos. Chem. Phys., 2020), and 
• modelling the contribution of tropospheric SO₂ to the stratospheric aerosol layer.

ATLAS is a Lagrangian (trajectory-based) model for the global modelling of chemistry and transport in the stratosphere (Wohltmann et al., 2010). It has been used for a wide range of applications, including estimation of ozone loss (e.g. Manney et al., 2011; Wohltmann et al., 2020) and sensitivity of polar ozone depletion to uncertainties in stratospheric chemistry and microphysics (Wohltmann et al., 2013).

An algorithm, that was originally developed for the automatic determination of catalytic ozone destruction cycles from the output of a chemical model (Lehmann, 2004), is now used for a wider range of problems. It was also applied to determine the dominant reaction sequences in Antarctic stratospheric chlorine chemistry, mesospheric ion chemistry and in the atmospheres of Mars and potential extra-solar planets.