Establishing long-term measurements of halocarbons at Taunus Observatory

Abstract

Abstract. In late 2013, a whole air flask collection programme was started at Taunus Observatory (TO) in central Germany. Being a rural site in close proximity to the Rhine–Main area, Taunus Observatory allows assessment of emissions from a densely populated region. Owing to its altitude of 825 m, the site also regularly experiences background conditions, especially when air masses approach from north-westerly directions. With a large footprint area mainly covering central Europe north of the Alps, halocarbon measurements at the site have the potential to improve the database for estimation of regional and total European halogenated greenhouse gas emissions. Flask samples are collected weekly for offline analysis using a GC/MS system simultaneously employing a quadrupole as well as a time-of-flight mass spectrometer. As background reference, additional samples are collected approximately once every 2 weeks at the Mace Head Atmospheric Research Station (MHD) when air masses approach from the site's clean air sector. Thus the time series at TO can be linked to the in situ AGAGE measurements and the NOAA flask sampling programme at MHD. An iterative baseline identification procedure separates polluted samples from baseline data. While there is good agreement of baseline mixing ratios between TO and MHD, with a larger variability of mixing ratios at the continental site, measurements at TO are regularly influenced by elevated halocarbon mixing ratios. Here, first time series are presented for CFC-11, CFC-12, HCFC-22, HFC-134a, HFC-227ea, HFC-245fa, and dichloromethane. While atmospheric mixing ratios of the chlorofluorocarbons (CFCs) decrease, they increase for the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs). Small unexpected differences between CFC-11 and CFC-12 are found with regard to frequency and relative enhancement of high mixing ratio events and seasonality, although production and use of both compounds are strictly regulated by the Montreal Protocol, and therefore a similar decrease in atmospheric mixing ratios should occur. Dichloromethane, a solvent about which recently concerns have been raised regarding its growing influence on stratospheric ozone depletion, does not show a significant trend with regard to both baseline mixing ratios and the occurrence of pollution events at Taunus Observatory for the time period covered, indicating stable emissions in the regions that influence the site. An analysis of trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model reveals differences in halocarbon mixing ranges depending on air mass origin.

Comment on amt-2020-488

“…To the north and west, the site is surrounded mainly by forest and agricultural areas. The site is often exposed to European background air approaching at higher altitudes mainly from northern directions, but also to local and regional pollution events (Schuck et al, 2018). Trace gas mole fractions therefore exhibit a high variability with somewhat higher baseline mole fractions compared to clean air sites.…”

“…Whole air canister sampling was started at Taunus Observatory in October 2013. Details of the sample collection and the analytical procedure were described in detail by Schuck et al (2018) and Hoker et al (2015) and are only briefly summarised here. Air is collected pairwise approximately weekly using stainless steel flasks.…”

“…For both measurement set-ups, the integration of the chromatographic peaks is performed in a similar way as described in Schuck et al (2018). The signal areas A of each substance are divided by the enriched sample Volume V to yield a response R. A relative response rR of each analysed substance is calculated by dividing the response of a substance in an air sample measurement (R air ) by the linearly interpolated response of the bracketing calibration gas measurements (R cal ):…”

“…For the measurements of the weekly whole air sampling programme, an automated procedure is used to filter the data based on the double analysis of samples and parallel sampling into two canisters to ensure a high-quality dataset, as described in Schuck et al (2018). For the in situ measurements, only one measurement and one preceding and subsequent calibration gas measurement are available.…”

Stability of halocarbons in air samples stored in stainless- steel canisters

“…CARIBIC air sampling was initially limited to 28 glass flasks per series of flights. Taking advantage of the large spatial coverage of commercial long-distance flights, measurements from these CARIBIC glass flask samples have provided valuable information on the distribution of halogenated trace gases in the upper troposphere and lowermost stratosphere, for example regarding HFC-227ea (Laube et al, 2010), SF 5 CF 3 (Sturges et al, 2012), perfluorocarbons (Laube et al, 2012), dichloromethane and other shortlived chlorocarbons (Leedham-Elvidge et al, 2015;Oram et al, 2017) and bromocarbons (Wisher et al, 2014), CFC-114 (Laube et al, 2016), and CFC-113a (Adcock et al, 2018). Complementing the two existing sampling units with glass cylinders, the new high-resolution sampler (HIRES) unit with 88 stainless-steel cylinders was built in 2010 for the automated collection of whole-air samples at a higher frequency (Schuck et al, 2012).…”