ALBERT

Description: For many years, the comparability of measurements obtained with various instruments withing a global-scale air quality monitoring network has been ensured by anchoring all results to a unique suite of reference gas mixtures, also called 'primary calibration scale'. Such suites of reference gas mixtures are usually prepared and then stored over decades in pressurised cylinders by a designated laboratory. For those halogenated gases which have been measured over the last forty years, this anchoring method is highly relevant as measurement reproducibility is currently much better ( 2 %). Meanwhile, newly emitted halogenated gases are already measured in the atmosphere at sub-pmol/mol levels, while still lacking an established reference standard. For compounds prone to adsorption on material surfaces, it is difficult to evaluate mixture stability and thus variations in the molar fractions over time in cylinders at pmol/mol levels. To support atmospheric monitoring of halogenated gases, we create new primary calibration scales for SF6 (sulfur hexafluoride), HFC-125 (pentafluoroethane), HFO-1234yf (or HFC-1234yf, 2,3,3,3-tetrafluoroprop-1-ene), HCFC-132b (1,2-dichloro- 1,1-difluoroethane) and CFC-13 (chlorotrifluoromethane). The preparation method, newly applied to halocarbons, is dynamic and gravimetric: it is based on the permeation principle followed by dynamic dilution and cryo-filling of the mixture in cylinders. The obtained METAS-2017 primary calibration scales are made of 11 cylinders containing these five substances at near ambient and slightly varying molar fractions. Each prepared molar fraction is traceable to the realisation of SI units (Système International d'Unités) and is assigned an uncertainty estimate following international guidelines (JCGM 100:2008), ranging from 0.6 % for SF6 to 1.3 % (k = 2) for all other substances. The smallest uncertainty obtained for SF6 is mostly explained by the high substance purity level in the permeator as well as low SF6 contamination of the matrix gas. The measured internal consistency of the suite ranges from 0.23 % for SF6 to 1.1 % for HFO-1234yf (k = 1). The expanded uncertainty after verification (i.e. measurement of the cylinders vs each others) ranges from 1 % to 2 % (k = 2). This work combines the advantages of SI-traceable reference gas mixture preparation with a calibration scale system for its use as anchor by a monitoring network. Such a combined system allows to maximise the compatibility within the network while linking all reference values to the SI and assigning carefully estimated uncertainties. For SF6, comparison of the METAS-2017 calibration scale with the scale prepared by SIO (Scripps Institution of Oceanography, SIO-05) shows excellent concordance, the ratio METAS-2017/SIO-05 being 1.002. For HFC-125, the METAS-2017 calibration scale is measured as 7 % lower than SIO-14, and for HFO-1234yf 9 % lower than Empa-2013. No other scale for HCFC-132b was available for comparison. Finally, for CFC-13 the METAS-2017 primary calibration scale is 5 % higher that the interim calibration scale (Interim-98) in use within the Advanced Global Atmospheric Gases Experiment (AGAGE) network.

Description: We present the organization, instrumentation, datasets, data interpretation, modeling, and accomplishments of the multinational, global atmospheric measurement program AGAGE (Advanced Global Atmospheric Gases Experiment). AGAGE is distinguished by its capability to measure globally, at high frequency and multiple sites, all the important species in the Montreal Protocol and all the important non-carbon dioxide (CO2) gases assessed by the Intergovernmental Panel on Climate Change (CO2 is also measured at several sites). The scientific objectives of AGAGE are important in furthering understanding of global chemical and climatic phenomena. They are to: (1) measure accurately the temporal and spatial distributions of anthropogenic gases that contribute the majority of reactive halogen to the stratosphere and/or are strong infrared absorbers [chlorocarbons, chlorofluorocarbons (CFCs), bromocarbons, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and polyfluorinated compounds (perfluorocarbons (PFCs), nitrogen trifluoride (NF3), sulfuryl fluoride (SO2F2), and sulfur hexafluoride (SF6)), and use these measurements to determine the global rates of their emission and/or destruction (i.e. lifetimes); (2) measure accurately the global distributions and temporal behaviors and determine sources and sinks of non-CO2 biogenic-anthropogenic gases important to climate change and/or ozone depletion [methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), molecular hydrogen (H2), methyl chloride (CH3Cl) and methyl bromide (CH3Br); (3) identify new long-lived greenhouse and ozone-depleting gases [e.g. SO2F2, NF3, heavy PFCs (C4F10, C5F12, C6F14, C7F16, and C8F18) and hydrofluoro-olefins (HFOs, e.g. CH2 = CFCF3) have been identified in AGAGE], initiate real-time monitoring of these new gases, and reconstruct their past histories from AGAGE, air-archive and firn-air measurements; (4) determine the average concentrations and trends of tropospheric hydroxyl radicals (OH) from the rates of destruction of atmospheric trichloroethane (CH3CCl3), HFCs and HCFCs, and estimates of their emissions; (5) determine from atmospheric observations and estimates of their destruction rates, the magnitudes, and distributions by region of surface sources/sinks of all measured gases; (6) provide accurate data on the global accumulation of many of these trace gases, that are used to test the synoptic/regional/global-scale circulations predicted by three-dimensional models; and (7) provide global and regional measurements of methane, carbon monoxide and molecular hydrogen, and estimates of hydroxyl levels, to test primary atmospheric oxidation pathways at mid-latitudes and the tropics. Network Information and Data Repository: http://agage.mit.edu/data or http://cdiac.esd.ornl.gov/ndps/alegage.html