IN BRIEF DETECTION OF SHORT ATMOSPHERIC LIFETIME REFRIGERANT GASES
Now that HFOs are becoming more widely used in the EU, very low atmospheric concentrations (typically below 1 ppt parts per trillion) can be detected [1]. Background concentrations at end of 2017 for 3 HFOs/HCFOs are less than 0.1 ppt [2]. The HFOs have similar safety and technical properties to the HFCs, but are designed to breakdown rapidly in the atmosphere, which means they have very low Global Warming Potentials (<7 with, most = 1) similar to or lower than propane or butane. Substances having short atmospheric lifetimes (typically measured in days) are transported to some extent in the atmosphere as they breakdown. The detection of HFOs in the atmosphere, with lifetimes of about 10 to 40 days, is no different to the detection of other very short lifetime substances such as propane, isobutane, pentane (non-methane hydrocarbons NMHCs, a sub-set of non-methane volatile organic compounds NMVOCs). These hydrocarbons also have similar atmospheric lifetimes (for example 13 days for propane) [3]. The much wider sources of the NMHCs typically lead to substantially higher atmospheric concentrations that can be in the ppb (parts per billion, about 1000 times greater than the HFOs) range [4], particularly in urban areas. In general terms the HFCs [and HFOs] have very low Photochemical Ozone Creation Potential (POCP) values and take no part in ground-level ozone formation (commonly known as “smog”). In contrast, the alkanes (ethane, propane and isobutane) and alkenes (ethylene and propylene) exhibit steadily increasing reactivities from ethane, which is unreactive, to propylene, which is highly reactive. [5]
Explanatory note: 1 part per trillion is about the same as 1 cm2 compared to the area of Paris (105.4 sq km)
References
1. Scientific Assessment of Ozone Depletion: 2018 Executive Summary page 40: Some next-generation substitute chemicals with very low GWPs (unsaturated HCFCs and unsaturated HFCs, also known as hydrofluoroolefins, or HFOs) have now been detected in ambient air, consistent with the transition to these compounds being underway. Unsaturated HFCs and HCFCs are replacement compounds for some long-lived HCFCs and HFCs. Because unsaturated HFCs have short atmospheric lifetimes (days) and GWPs typically less than 1 they are not included as controlled substances in the Kigali Amendment to the Montreal Protocol. Very low mole fractions (typically below 1 ppt) of two unsaturated HFCs (HFC-1234yf and HFC-1234ze(E)) have been measured at a continental background European site.
2. Continuous measurement of non-CO2 greenhouse gases on the Jungfraujoch (HALCLIM-2015-18), EMPA Materials Science and Technology, August 2018. The table on page 20 gives background atmospheric concentrations for 2017 at the Jungfraujoch station in Switzerland. These are HFO-1234yf < 0.005 ppt; HFO-1234ze(E) 0.018 ppt; and HCFO-1233zd(E) 0.045 ppt. Charts on pages 4 and 28 show concentrations due to sporadic pollution events and are less than 1 ppt for HFO-1234yf and HCFO- 1233zd(E), with sporadic higher concentrations for pollution events up to 14 ppt for HFC-1234ze (E).
3. Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10), Øivind Hodnebrog, Stig B. Dalsøren, Gunnar Myhre, Atmospheric Science Letters, 26 February 2018, https://doi.org/10.1002/asl.804
4. See for example Sources and variability of non-methane hydrocarbons in the Eastern Mediterranean, Cecilia Arsene, A. Bougiatioti, N. Mihalopoulos, Global Nest Journal 11(3):333-340 · November 2009
5. IPCC/TEAP Special Report Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons Chapter 2 page 169.