Assessing Halogenated Greenhouse Gas Emissions from Regional Atmospheric Measurements

Human-made halocarbons contribute about 11 % of the anthropogenically caused radiative forcing by long-lived greenhouse gases. Moreover, chlorinated or brominated halocarbons cause stratospheric ozone depletion. Synthetic halocarbons are emitted to the atmosphere by a wide range of production or consumption-related activities, being used as foam blowing, cooling, or fire extinguishing agents for example. To derive observation-based estimates of halocarbon emissions so-called "top-down" inverse modeling methods have been developed. These methods rely on global atmospheric observations from long-term halocarbon measurement networks such as the Advanced Global Atmospheric Gases Experiment (AGAGE) and the National Oceanic and Atmospheric Administration (NOAA). However, to assess halocarbon emissions on a country to regional level and to complement national emission inventories by top-down methods, measurements are required, which capture regional pollution events.

We present 18 months of continuous, high-frequency, high-precision halocarbon measurements from the Beromünster and Sottens tall towers (Swiss Plateau). Together, the two sites are sensitive to the most densely populated and industrialized region of Switzerland and parts of southeastern France. For analysis, hourly two-liter air samples were pre-concentrated at low temperatures (down to -165 ^oC), before the analytes were separated by gas chromatography and detected by quadrupole mass spectrometry (GC-MS).

Based on the measured concentration records, we assessed Swiss emissions and source regions of 28 halocarbons, covering the halocarbons of the Montreal and Kyoto Protocols. This includes the banned chlorofluorocarbons (CFCs) and halons, the regulated hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), as well as the recently introduced unregulated hydrofluoroolefins (HFOs). The emissions were quantified using two independent top-down methods: a tracer ratio method and a Bayesian inversion based on regional atmospheric transport modeling.

We found good agreement between our top-down results and the emissions reported in the Swiss national greenhouse gas inventory for the major HFCs, HFC-125 and HFC-32, for which we calculated emissions of 100 Mg yr^-1 and 45 Mg yr^-1, respectively. For HFC-134a, our calculated emissions of 280 Mg yr^-1 hint at an overestimation of the Swiss national inventory. For the CFCs and HCFCs, we observed moderately elevated atmospheric concentrations with the corresponding emissions likely being related to the ongoing outgassing from existing banks. For the recently phased-in HFOs HFO-1234yf, HFO-1234ze(E), and HCFO-1233zd(E), we report the first national emission numbers, totaling to 56 Mg yr^-1. In addition, we present the first quantitative atmospheric measurements of the newly marketed HFO-1336mzz(Z), belonging to the group of emerging unsaturated halocarbons, of which the future environmental impacts are yet unclear.

To continue resolving the picture for Europe, another 6 months (December 2021 to May 2022) measurement campaign is currently being conducted in the Netherlands. The aim is to investigate local halocarbon emissions and locate regional emission sources with the above-described methods.