An indirect-calibration method for non-target quantification of trace gases applied to a time series of fourth-generation synthetic halocarbons at the Taunus Observatory (Germany)

Abstract

Abstract. Production and use of many synthetic halogenated trace gases are regulated internationally due to their contribution to stratospheric ozone depletion or 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 In many applications they have been replaced by shorter-lived compounds, which have become measurable in the atmosphere as emissions increased.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 acquires data over a continuous mass range that enables a retrospective analysis of the dataset, which can be considered a type of digital air archive. This archive can be used 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 measured substances or the amounts in the calibration gas may 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, select periods of stable instrument performance and determine well suited reference compounds. The method is applied to three short-lived synthetic halocarbons: HFO-1234yf, HFO-1234ze(E), and HCFO-1233zd(E). They represent replacements for longer-lived hydrofluorocarbons (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 dataset. The application of the indirect calibration method on several test cases can result in uncertainties of around 6 % to 11 %. For hydro(chloro-)fluoroolefines (denoted H(C)FOs), uncertainties up to 23 % are achieved. The indirectly calculated mole fractions of the investigated H(C)FOs at Taunus Observatory range between measured mole fractions at urban Dübendorf and Jungfraujoch stations in Switzerland.

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2021

“…Uses of third-generation CFC-replacement compounds—hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs)—are increasing in commercial applications. Ambient concentrations of HFO-1234yf and HCFO-1233zd(E) have been measured in central Europe at urban, semi-urban, and remote sites from 2011 onwards [ 44 , 45 ]. In general, the concentrations of HFOs increased by a factor of 10 from 2011 to 2019, but the concentrations are low (0.7 parts per trillion (ppt) for HFO-1234yf in 2019, [ 45 ]) in comparison with the CFCs and HFCs they replace; e.g.…”

“…Ambient concentrations of HFO-1234yf and HCFO-1233zd(E) have been measured in central Europe at urban, semi-urban, and remote sites from 2011 onwards [ 44 , 45 ]. In general, the concentrations of HFOs increased by a factor of 10 from 2011 to 2019, but the concentrations are low (0.7 parts per trillion (ppt) for HFO-1234yf in 2019, [ 45 ]) in comparison with the CFCs and HFCs they replace; e.g. values for CFC-11 and HFC-134a were 494 and 124 ppt, respectively [ 46 ].…”

“…Uses of third-generation CFC-replacement compoundshydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs)-are increasing in commercial applications. Ambient concentrations of HFO-1234yf and HCFO-1233zd(E) have been measured in central Europe at urban, semi-urban, and remote sites from 2011 onwards [44,45].…”

“…In general, the concentrations of HFOs increased by a factor of 10 from 2011 to 2019, but the concentrations are low (0.7 parts per trillion (ppt) for HFO-1234yf in 2019, [45]) in comparison with the CFCs and HFCs they replace; e.g. values for CFC-11 and HFC-134a were 494 and 124 ppt, respectively [46].…”

Automated fragment formula annotation for electron ionisation, high resolution mass spectrometry: application to atmospheric measurements of halocarbons

“…In the early 2000s, fast response, large coverage and high accuracy mass analyser, such as Orbitrap and time-offlight (TOF) mass spectrometers, were introduced for water analysis, but only recently, first approaches have been made to use these powerful detectors also for organic atmospheric trace gases [36,37]. Due to the challenge of identifying compounds, EI-HRMS are currently used as large mass-range coverage target or suspect screening instruments [e.g., 38,39] but only rarely as NTS instruments. For state-of-the art EI-HRMS, there is currently a huge divide between what it can deliver in terms of sample coverage, throughput and mass resolution, compared to what identification tools can provide.…”