Arctic ozone depletion
It has been known since the early 1990s that even the Arctic can be affected by considerable ozone depletion in cold stratospheric winters. Even though the reduction and control of ozone-depleting substances agreed in the Montreal Protocol (1987) and its successors has been a success story for environmental research, the severest ozone depletion above the Arctic so far was observed in the winter of 2010/2011. The problem has therefore lost none of its urgency. Selected findings of research at Jülich on this field are presented below.
A study published in April 2007 questioned previous knowledge on polar ozone chemistry, and particularly the causes of ozone depletion. It suggested that the CIO dimer absorbs considerably less sunlight than previously assumed. According to the study, the photolysis in the stratosphere is much slower and known catalytic cycles cannot therefore be the main cause of ozone depletion. In an article in Science, IEK-7 demonstrated by means of
atmospheric measurement data that it is by no means advisable to dismiss the role of CFCs and the Montreal Protocol . This publication was supported by measurements of the absorption spectrum of dichlorine peroxide taken by IEK-7 in cooperation with the University of Wuppertal .
These publications were the starting point for the European RECONCILE project coordinated by IEK-7. From January to March 2010, an international measurement campaign was carried out as part of RECONCILE in the northern Swedish town of Kiruna to clarify unanswered questions related to polar ozone depletion and the associated interaction between ozone and climate. At the heart of the campaign were measurement flights with the Russian high-altitude research aircraft Geophysica (suitable for flights up to an altitude of 21 km), which accommodates three complex instruments belonging to IEK-7. All in all, the measurement campaign supplied a large number of data, which will constitute the basis for predicting the future development of the ozone layer and its impact on the climate more precisely than ever before. This work is currently in progress.
One important finding is that the photolysis rate of the ClO dimer, which largely determines the rate of catalytic ozone depletion after heterogeneous chlorine activation, is in the same range as most current laboratory measurements.
RECONCILE has also shown that at low temperatures chlorine cannot only be activated on polar stratospheric clouds, but also on stratospheric background aerosol (droplets of sulphuric acid). This has far-reaching implications not only for ozone depletion in the polar winter, but also in other regions and seasons, particularly against the background of possible “geoengineering” measures, such as the deliberate introduction of sulphur particles into the stratosphere to mitigate global warming.
Regarding polar stratospheric clouds (PSCs), many issues remain open and investigations on the nucleation of PSC particles have been performed. It has been shown that heterogeneous nucleation both for NAT and ice particles is an important process and is necessary to explain observations [3,4]. Solid particles of meteoritic origin immersed into sulfate aerosol may serve as heterogeneous nuclei. However, a variety of possible nuclei of different origins have been observed . Moreover, the simulation of small-scale temperature fluctuations is necessary to obtain sufficiently large cooling rates for predominantly homogeneous ice nucleation when using temperature fields from meteorological analyses such as ERA-Interim.
Furthermore, a classification of the different PSC types from spectroscopic signals from MIPAS observations (both from Envisat and from the Geophysica aircraft) was developed [6,7]. Based on the new classification approach for MIPAS, a climatology of 9 southern and 10 northern polar winter seasons has been prepared (2002-2012) . An initial comparison with the decadal satellite record of gravity wave activity in the lower stratosphere from the AIRS satellite highlights the strong link between PSC formation and mountain-wave-induced temperature fluctuations .
Findings on the nucleation of PSC particles fed into the development of the Lagrangian sedimentation module of CLaMS. Through sedimentation, PSC particles transport reactive nitrogen (NOy) downward within the stratosphere yielding denitrification at an altitude of around 17-25 km and nitrification due to evaporation below this altitude. This process was successfully implemented in CLaMS  showing good agreement with satellite and in situ observations. Simulation results based on this study were used in a variety of studies evaluating measurements obtained during the RECONCILE campaign.
The chemical impact of different types of PSCs on chlorine activation was investigated in several studies. While in the Arctic, on short time scales, the rather different rates of chlorine activation on different types of PSC are a matter of importance , this is usually not the case when ozone loss over the entire winter/spring season is considered [12,13]. In particular, heterogeneous chemistry on ice particles in the Antarctic causes only up to 5 DU of additional simulated ozone depletion. These studies are complemented by further investigations of PSCs and denitrification [14,15,16,17] and ozone trends , as well as CFCs and their lifetimes [19,20,21]
Simulation of vertical redistribution of reactive nitrogen species for winter 2015/2016 by CLaMS. Negative and positive values correspond to denitrification and nitrification, respectively. This winter was characterized by a record vertical extent of PSCs and denitrification.
The ability of the CLaMS model to successfully reproduce small-scale structures and large gradients was the reason that CLaMS was used for preparation of and flight planning in the HALO campaign POLSTRACC, in which especially the structure of the lower stratosphere was investigated. This campaign took place in winter 2015/2016 where PSCs and nitrification layers and later denitrification and chemical ozone loss were observed at flight altitudes of up to 14.5 km. The further data evaluation of POLSTRACC will benefit from the inclusion of ice into the Lagrangian sedimentation module of CLaMS, allowing the investigation of ice PSCs and vertical water vapor transport in more detail.
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