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EXploring Chemistry, Composition and Circulation in the stratosphere with InnovativeTEchnologies

Periodic Reporting for period 3 - EXC3ITE (EXploring Chemistry, Composition and Circulation in the stratosphere with InnovativeTEchnologies)

Reporting period: 2018-12-01 to 2020-05-31

It is ozone that primarily heats and therefore creates the stratosphere. Human emissions of ozone-depleting substances (ODSs) have however led to dramatic stratospheric ozone losses for decades. This global problem is ongoing and of renewed concern due to recent unexpected changes. It is also likely affecting the nature of the stratosphere itself, with implications for global health and economy. In addition, emissions of greenhouse gases have been proposed to lead to a long-term acceleration of the stratospheric overturning circulation. In summary, significant stratospheric changes are to be expected from both, ozone losses and global warming.
Indications for such changes have been reported, but there are substantial uncertainties and limitations connected with these studies. In addition, current technologies to explore stratospheric composition and chemistry are very expensive and often offer only infrequent data. There is clearly a need for new and improved tools to correctly detect and quantify changes from observations.
The EXC3ITE project is investigating 3 novel avenues to explore stratospheric chemistry, composition and circulation:

1) A newly developed low-cost technology to retrieve and analyse air from the stratosphere. This will be a new way to derive budgets of all important and newly emerging ODSs directly in the stratosphere; while at the same time providing observations of many strong greenhouse gases.

2) New evidence for substantial past changes in stratospheric chemistry and circulation. An unprecedented investigation of stratospheric air archives spanning 40 years and >50 trace gases will allow new insights into these changes and

3) New diagnosis tools and a detailed comparison with state-of-the-art models will identify the implications for future climate.

The EXC3ITE project will result in a breakthrough in the understanding of stratospheric changes which are of high importance for society through their impact on climate prediction and ozone recovery.
During the first 30 months of the project, the focus was predominantly on a) building a research team and b) overcoming technical challenges mainly related to avenue no. 1, i.e. the development of the “AirCore” technology to retrieve air from the stratosphere. AirCores were invented by researchers at NOAA in the U.S. and mostly consist of a very long stainless-steel tube (up to 100 m) that is open at one end. When launched on a small balloon, the AirCore equilibrates to the low ambient atmospheric pressure while ascending to an altitude of more than 30 km, with atmospheric pressure decreasing along the way. Upon descent, the AirCore gradually fills with air as ambient pressure rises, thus collecting a vertical profile of the atmosphere. This profile can be subsequently reconstructed from simultaneous measurements of pressure, temperature and GPS positions. The air itself can be analysed for its content of important ODSs and greenhouse gases. Currently only a handful of atmospherically important gases such as carbon dioxide and methane have been measured using AirCores. The aim of the EXC3ITE project is to increase that number to >30.

Within these first 30 months, we managed to successfully establish measurements of 14 gases including most of the important ODSs such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) as well as some emerging greenhouse gases such as sulphur hexafluoride (SF6) and hydrofluorocarbons (HFCs). Our activities were boosted by collaborations with researchers in Finland, the Netherlands and Germany. These research teams carried out several AirCore launches from northern Finland from Germany and we managed to analyse the air they retrieved after it had already been analysed for its content of carbon dioxide and methane. This collaboration even resulted in three joint field campaigns in June and September 2017 as well as June 2018. While many gases are not influenced by the initial analysis from the other team, other gases are either destroyed by or emitted from internal parts of their measurement system.

We therefore developed our own launch capability in parallel and could launch 9 AirCores by the end of September 2018, the first ever to be launched from the UK. This is a major achievement as the combination of the (often strong) westerly winds with a launch site situated on an island create substantial challenges - especially when considering that the payload needs to be retrieved and brought back to the university campus for analysis within a few hours. These UK activities also entailed the construction and testing of a whole tailored launch system, including radiosonde, temperature and pressure sensors, miniature computers, and parts that do not affect any of the target trace gases. The trace gas results from the 2017 launches were still limited but we have since improved our methods and are currently able to measure a range of over 20 gases, including some that pose an emerging threat to the ozone layer. We have also been able to identify the causes for the continued problems encountered with some gases and have - again - improved our methods accordingly. A publication is currently in preparation.

In addition, some work has been carried out to explore avenue no. 2. The reanalysis of the air archives has been delayed due to technical challenges during the construction of an improved analytical system. However, by month 18 a new, automated inlet system to optimise sample throughput and precisions had been designed and built and undergone preliminary tests on the physical design and implementation. By 2018 two new team members had joined EXC3ITE. Their main task was to carry out final testing on that system. This had been deliberately postponed to January 2018 due to the intensive campaign activities. After an initial training period further progress was however hindered by repeated software and hardware issues and insufficient reliability of the system in general. Several and repeated updates and improvements were made, and the system is currently functioning and being optimised for ultimate precisions of trace gas measurements.

Moreover, we have utilised existing data to investigate novel ways to detect changes in stratospheric circulation. An established tool for this is the age-of-air. It is defined as the average time that an air parcel has spent in the stratosphere and increases as the air is being transported within the stratosphere. It can be inferred from suitable trace gases that do not break down in the stratosphere. Carbon dioxide and sulphur hexafluoride have been used previously, but we have been able to identify 5 more gases that provide additional constraints on stratospheric transport times. A manuscript that assesses the suitability of these new as well as existing age-of-air gases was published in early 2018. It also includes an analysis of the knock-on consequences for policy-relevant parameters that affect predictions of the recovery of the ozone layer (Leedham Elvidge et al., 2018). Contributions were made to a second publication on the atmospheric distributions of CFC-113a, an emerging threat to the ozone layer. CFC-113a currently remains the only CFC with substantially increasing concentrations in the atmosphere (40 % in the ~4 most recent years of the record) and strong emission sources have been identified in the East Asian region (Adcock et al., 2018).
These achievements have been largely outlined within the previous summary section. It is however worth pointing out, that

a) We have carried out the first ever ODS measurements on air retrieved from the stratosphere via an AirCore, which confirms the suitability of this technique as a cost-effective way to investigate the composition, chemistry and circulation of the stratosphere. We have carried out 8 flights from the UK and analysed 15 further AirCores that were retrieved through international collaborations.

b) We have expanded the number of trace gases suitable for tracking the stratospheric circulation from two to seven, all of which will be valuable tools for the investigation of long-term changes in the stratosphere.

c) We have identified several new threats to the ozone layer as well as alternative possibilities to improve the diagnosis of stratospheric circulation and chemistry changes.

We will be building on these results when attempting to

a) Expand the trace gas range accessible through AirCores even further and to use the results to investigate new threats to ozone layer as well as the seasonality of the circulation in the stratosphere,

b) Analyse the stratospheric air archives as well as complementing tropospheric air archives to develop new methods to investigate changes in the stratosphere over the last four decades and

c) Compare the results from a) and b) with global models to assess their performance and improve climate predictions, with a particular focus on implications arising from stratospheric changes.