Non-target analysis using gas chromatography with time-of-flight mass spectrometry: application to time series of fourth generation synthetic halocarbons at Taunus Observatory (Germany)

Abstract. Production and use of many synthetic halogenated trace gases are regulated internationally because of their contribution to stratospheric ozone depletion or to climate change. In many applications they have been replaced by shorter-lived compounds which have become measurable in the atmosphere as emissions increased. Non-target monitoring of trace gases rather than targeted measurements of well-known substances is needed to keep up with such changes in the atmospheric composition. We regularly deploy gas chromatography (GC) coupled to time-of-flight mass spectrometry (TOF-MS) for analysis of flask air samples and in situ measurements at the Taunus Observatory, a site in central Germany. TOF-MS record data over a continuous mass range enable a retrospective analysis of the data set, which can thus be considered a type of digital air archive. This archive can be made use of if new substances come into use and their mass spectrometric fingerprint is identified. However, quantifying new replacement halocarbons can be challenging, as mole fractions are generally low, requiring high measurement precision and low detection limits. In addition, calibration can be demanding, as calibration gases may not contain sufficiently high amounts of newly used substances or the amounts in the calibration gas have not been quantified. This paper presents an indirect data evaluation approach for TOF-MS data, where the calibration is linked to another compound which could be quantified in the calibration gas. We also present an approach to evaluate the quality of the indirect calibration method and to select periods of stable instrument performance and well suited reference compounds. The method is applied to three short-lived synthetic halocarbons: HFO-1234-yf, HFO-1234ze(E), and HCFO-1233zd(E). They represent replacements for longer-lived HFCs and exhibit increasing mole fractions in the atmosphere. The indirectly calibrated results are compared to directly calibrated measurements using data from TOF-MS canister sample analysis and TOF-MS in situ measurements, which are available for some periods of our data set. The application of the indirect calibration method on several test cases can result into accuracies around 13 % to 20 %. For H(C)FOs accuracies up to 25 % are achieved. The indirectly calculated mole fractions of the investigated H(C)FOs at Taunus Observatory range between measured mole fractions at urban Dubendorf and Jungfraujoch stations in Switzerland.