Greenhouse Gases and Ozone Depleting Substances

The main objective of these higher frequency measurements are better estimations of regional emissions of halogenated hydrocarbons. In addition, changes in atmospheric mixing ratios of new replacement substances, such as the new hydrofluoroolefines used in mobile air conditioning, are monitored.

The tropopause seperates troposphere and stratosphere. The tropopause region is a region of very high climate sensitivity, due to the strong gradients in radiatively important trace gases and temperature. Relativly small changes in the chemical composition can have large impact on radiative forcing and thus on climate, also at the ground.

Images by DLR

Through our observations of a wide range of halocarbons and related trace gases using the GhOST-MS instrument onboard the research aircraft HALO we contribute to a better understanding of chemical budgets and transport processes in the tropopause region. Transport is investigated through mean age, while the chemistry aspects are related to the bromine and chlorine budget, with an emphasis on short lived sources of chlorine and bromine top the stratosphere. These chemical active species also give information on the time scales of transport through the tropopause as a function of distance to the tropopause, as the chemical breakdown interacts with transport.

Images by GUF

In addition to observational data of stratospheric trace gases, simulations of chemistry-transport- and chemistry-climate-models are used for further analyses. This allows globally resolved studies of dynamical and chemical processes in the stratosphere in past, present and future.

One application of these model results is the validation of newly developed theoretical concepts in various regions of the atmosphere to conclude if and how similar studies could be realized using observations.  For instance, it is investigated how the stratospheric halogen budget und chemical depletion are influenced by tropospheric mixing ratios of ozone depleting substances (e.g. CFCs and halons). For this to happen, so-called age spectra are being used amongst others, which depict statistical distributions of transit times of stratospheric air. Unfortunately, those spectra can only be calculated in models and not be measured directly in reality. To solve this problem, methods are developed to derive stratospheric age spectra from trace gas measurements.

The second channel of the GhOST-MS is based on the GC/MS measurement, where an enrichment is necessary before the measurement. For this purpose, external air passed through a cooled sample loop to concentrate the trace gases. Subsequently, the trace gases are desorbed, separated chromatography and quantified with the mass spectrometer. For a particularly high sensitivity, the MS can be operated in the chemical ionization (CI). In the standard configuration, the MS can measure all brominated source gases as well as some chlorinated and iodinated with a time resolution of four minutes. Furthermore, the MS can be operated in the electron ionization (EI). This ionization leads to more fragmentation and therefore more measurable substances. In addition to the brominated source gases, a variety of chlorinated source gases can thus be measured.

AirCore is an innovative high-altitude air sampling system that was first developed at NOAA in the USA. In principle, the AirCore consists of a long tubing in the shape of a coil, which is sealed at one end and open at the other. Deployed to a balloon, it rises up into the stratosphere to approximately 30 km. During ascent, the AirCore evacuates due to the decreasing ambient pressure. During descent the tube fills with ambient air and captures a continuous vertical profile. The length of the tubing and the laminar flow during sampling play a crucial role in preserving the continuous sample. The stratospheric air is stored near the sealed end of the AirCore, whereas the tropospheric air is stored near the open end.

Over time, the sampled air mixes due to molecular diffusion. Albeit, one can reconstruct the vertical profile if the AirCore sample is measured directly after landing. For analysis, we use a Picarro G2401 Cavity Ring-Down Spectrometer, which measures CO2, CH4, CO and water vapor in a continuous gas flow. The AirCore system of the Goethe University Frankfurt is described in detail in Engel et al., 2017. The total weight of our AirCore including the electronic system and the protective casing is approximately 2.5 kg – little enough to be deployed to small and economic weather balloons.

Images by Andreas Engel, Johannes Degen, GUF

Air collection is carried out according to the principle of whole air sampling, wherein the collectors are cooled down to 27K using liquid neon. The inflowing air freezes on the inside of the sample containers. After thawing of the samples, an overpressure of up to 70 bar builds up inside of the containers. In cooperation with the French space agency CNES (Centre National d'Études Spatiales), the air samplers are flown at heights of up to 40 km with the help of stratospheric balloons.

Two collectors were developed at the research center of Jülich between 1980 and 1985. These two collectors (BONBON) each have 15 sample containers with volumes of 580 or 350 ml. In 2006, a lighter collector called CLAIRE (Cryogenic Lightweight AIR Sampling Experiment) with 26 sample containers (volume of 140ml per container) was developed at the University of Frankfurt. Both collector types have internally electropolished stainless steel containers in order to achieve a passive inner surface and a good sample stability. The inlets consist of a thin-walled gold tube closed with a glass cap. The sample containers are opened by triggering a spring pin, which deflects the glass cap. The sample containers are closed by crushing the glass tube (cold welding) with the aid of an electrically ignited detonator. As a result, on the one hand a high flow rate can be achieved, on the other hand, any kind of plastic gaskets can be avoided, which prevents sample contamination. Due to this high flow rate, a low collection time, which equals a high time resolution (a few minutes), is achieved even at low external pressures.

The group uses two different systems of gas chromatography (GC) coupled with mass spectrometry (MS) for ground-based measurements of halogenated compounds. Both contain a cryogenic enrichment-unit, a gas chromatograph and one or two mass spectrometers. Due to the comparatively small mixing ratios of the halogenated substances present in the atmosphere (parts-per-trillion), the enrichment unit is of high importance. For enrichment, halogenated substances get frozen onto an adsorbent agent. Following desorption and gas chromatographic separation, the substances are detected in the mass spectrometer.

At the Taunus Observatory air is measured in situ several times per day with a time-of-flight mass spectrometer coupled to a gas chromatograph. This mass spectrometer-type is sampling over a certain mass range continuously, without the need to pre-define target substances. Thus, a digital archive is generated, which allows future retrospective analysis of yet unidentified substances or substances which are not yet in the scientific focus.

For the offline analysis of air samples which are collected in canisters, a GC/MS-system with two mass spectrometers is available in the working group's laboratory. This system contains, next to another time-of-flight-MS, a quadrupole mass spectrometer. The quadrupole-MS exclusively detects defined target substances with high technical reliability, measurement accuracy and linearity.