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.
Highlights
Halogenated trace gases play an important role in atmospheric chemistry: they contribute to the depletion of stratospheric ozone and directly or indirectly to the radiative forcing of the atmosphere (Carpenter et al, 2014; IPCC, 2013)
Air sample collection at Mace Head is restricted to times when air masses approach from the clean air sector and the data represent a baseline case for the time series of halogenated compounds
Remarkable in the time series is an episode in the time series in September 2016 with exceptionally low mixing ratios occurring at both Taunus Observatory and Mace Head, which is apparent for CFC-11 and CFC-12 but is more pronounced for CFC-12
Summary
Halogenated trace gases play an important role in atmospheric chemistry: they contribute to the depletion of stratospheric ozone and directly or indirectly to the radiative forcing of the atmosphere (Carpenter et al, 2014; IPCC, 2013). Use of long-lived compounds from the generation replacements, hydrofluorocarbons (HFCs), has only recently been included in the Montreal Protocol, and atmospheric mixing ratios of halocarbons from this group are currently still increasing (Simmonds et al, 2017; Montzka et al, 2015). Several recent studies have combined station measurements of halocarbons and atmospheric transport models to inversely estimate emissions on different geographical scales (e.g. Keller et al, 2012; Maione et al, 2014; O’Doherty et al, 2014; Simmonds et al, 2016; Brunner et al, 2017; Hu et al, 2017) This approach has the potential to improve existing emission inventories and can serve for the verification of emissions reported to the United Nations Framework Convention on Climate Change (UNFCCC) on the European level.
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