A versatile, refrigerant- and cryogen-free cryofocusing–thermodesorption unit for preconcentration of traces gases in air

Abstract

We present a compact and versatile cryofocusingthermodesorption unit, which we developed for quantitative analysis of halogenated trace gases in ambient air. Possible applications include aircraft-based in situ measurements, in situ monitoring and laboratory operation for the analysis of flask samples. Analytes are trapped on adsorptive material cooled by a Stirling cooler to low temperatures (e.g. −80 • C) and subsequently desorbed by rapid heating of the adsorptive material (e.g. +200 • C). The set-up involves neither the exchange of adsorption tubes nor any further condensation or refocusing steps. No moving parts are used that would require vacuum insulation. This allows for a simple and robust design. Reliable operation is ensured by the Stirling cooler, which neither contains a liquid refrigerant nor requires refilling a cryogen. At the same time, it allows for significantly lower adsorption temperatures compared to commonly used Peltier elements. We use gas chromatography -mass spectrometry (GC-MS) for separation and detection of the preconcentrated analytes after splitless injection. A substance boiling point range of approximately −80 to +150 • C and a substance mixing ratio range of less than 1 ppt (pmol mol −1 ) to more than 500 ppt in preconcentrated sample volumes of 0.1 to 10 L of ambient air is covered, depending on the application and its analytical demands. We present the instrumental design of the preconcentration unit and demonstrate capabilities and performance through the examination of analyte breakthrough during adsorption, repeatability of desorption and analyte residues in blank tests. Examples of application are taken from the analysis of flask samples collected at Mace Head Atmospheric Research Station in Ireland using our laboratory GC-MS instruments and by data obtained during a research flight with our in situ aircraft instrument GhOST-MS (Gas chromatograph for the Observation of Tracerscoupled with a Mass Spectrometer).

Bromine from short-lived source gases in the extratropical northern hemispheric upper troposphere and lower stratosphere (UTLS)

“…An electron capture detector (ECD) is used in an isothermal channel in a similar set-up as used during the SPURT campaign (Bönisch et al, 2009(Bönisch et al, , 2008Engel et al, 2006) to measure SF 6 and CFC-12 with a time resolution of 1 min. The second channel is temperature programmed and uses a cryogenic pre-concentration system (Obersteiner et al, 2016;Sala et al, 2014) and a mass spectrometer (MS) for detection. It is similar to the set-up described by Sala et al (2014) and measures halocarbons in the chemical ionization mode (e.g.…”

“…Therefore, brominated VSLS are particularly effective with respect to ozone chemistry in the lower and lowermost stratosphere, below about 20 km, with the associated ozone decreases exerting a significant radiative effect . It has been shown that observed and modelled ozone show a better agreement if bromine from shortlived species is included in models (Sinnhuber and Meul, 2015;Fernandez et al, 2017;Oman et al, 2016). In particular for the Antarctic ozone hole, an enhancement in size by 40 % and an enhancement in mass deficit by 75 % was simulated due to VSLS (Fernandez et al, 2017) in comparison with a model run without VSLS.…”

“…In particular for the Antarctic ozone hole, an enhancement in size by 40 % and an enhancement in mass deficit by 75 % was simulated due to VSLS (Fernandez et al, 2017) in comparison with a model run without VSLS. A delay in polar ozone recovery by about a decade has also been reported due to the inclusion of brominated VSLS (Oman et al, 2016). In order to have solid projections on the effect of VSLS on ozone and climate, a good knowledge of their atmospheric distribution is thus needed for models.…”

Stability of Halocarbons in Whole Air Samples from the Upper Troposphere and Lowermost Stratosphere

“…The halocarbon measurements are based on adsorption-desorption gas chromatography / mass spectrometry (GC/MS). As halocarbon mixing ratios in the atmosphere are in the ppt range, pre-concentration of the sample air prior to gas chromatographic separation and detection is required (Obersteiner et al, 2016). In addition, the sample air is dried by passing over a 10 heated (80 • C) tube filled with magnesium perchlorate Mg(ClO 4 ) 2 .…”

“…experimental setup was described in more details byHoker et al (2015) andObersteiner et al (2016).HIRES samples are measured relative to a laboratory standard which has been collected cryogenically at Jungfraujoch(Switzerland) in December 2007 and has been calibrated against standards of the Advanced Global Atmospheric Gases Experiment (AGAGE) in December 2018. Mixing ratios are reported in ppt on Scripps Institution of Oceanography (SIO) scales.…”