Abstract
Abstract. The International Halocarbons in Air Comparison Experiment (IHALACE) was conducted to document relationships between calibration scales among various laboratories that measure atmospheric greenhouse and ozone depleting gases. This study included trace gases such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), as well as nitrous oxide, methane, sulfur hexafluoride, very short-lived halocompounds, and carbonyl sulfide. Many of these gases are present in the unpolluted atmosphere at pmol mol−1 (parts per trillion) or nmol mol−1 (parts per billion) levels. Six stainless steel cylinders containing natural and modified natural air samples were circulated among 19 laboratories. Results from this experiment reveal relatively good agreement (within a few percent) among commonly used calibration scales. Scale relationships for some gases, such as CFC-12 and CCl4, were found to be consistent with those derived from estimates of global mean mole fractions, while others, such as halon-1211 and CH3Br, revealed discrepancies. The transfer of calibration scales among laboratories was problematic in many cases, meaning that measurements tied to a particular scale may not, in fact, be compatible. Large scale transfer errors were observed for CH3CCl3 (10–100%) and CCl4 (2–30%), while much smaller scale transfer errors (< 1%) were observed for halon-1211, HCFC-22, and HCFC-142b. These results reveal substantial improvements in calibration over previous comparisons. However, there is room for improvement in communication and coordination of calibration activities with respect to the measurement of halogenated and related trace gases.
Highlights
Halogenated trace gases, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and chlorinated solvents, are involved in stratospheric ozone depletion (Montzka et al, 2011)
We report the variation of results exclusively from laboratories that maintain independent scales (Table 5)
We compare the results of select gases from this experiment with those derived from estimates of global mean mole fractions
Summary
Halogenated trace gases, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and chlorinated solvents, are involved in stratospheric ozone depletion (Montzka et al, 2011). In an effort to characterize global distributions and sources/sinks of these gases, several international research groups measure the atmospheric abundance of CFCs, HCFCs, HFCs, and halogenated solvents on a routine basis. Collaborative efforts utilizing measurements from multiple groups have led to more robust estimates of the global distributions and emissions of N2O (Huang et al, 2008; Saikawa et al, 2013), CCl4 (Xiao et al, 2010a), CH3Cl (Xiao et al, 2010b), HCFC-22 (Saikawa et al, 2012) and SF6 (Rigby et al, 2010). Integrating results from different research groups to produce a consistent picture of the global or regional atmospheric distribution can be challenging. Perhaps the most fundamental of these is the calibration scale upon which the measurements are based
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