Correspondence/Rebuttal pubs.acs.org/est
Comment on “Environmental Fate of the Next Generation Refrigerant 2,3,3,3-Tetrafluoropropene (HFO-1234yf)″.
I
research reported by Im et al.1 is of general relevance from a perspective of understanding the potential for abiotic degradation of halocarbons but it is not relevant to the environmental fate of HFO-1234yf. The dominant environmental fate of HFO-1234yf is gas-phase reaction with OH radicals.6 HFO-1234yf has a negligible photochemical ozone creation potential, negligible global warming potential, an ozone depletion potential which does not differ materially from zero, and in the concentrations expected in the environment its degradation products will have a negligible impact on ecosystems.7
n their recent paper on the environmental fate of HFO1234yf (CF3CFCH2), Im et al.1 make the claim that “high production volumes and widespread handling are expected to introduce this hydrofluoroolefin into aquatic environments, including aquifers”. This claim is unsubstantiated and incorrect. It is well established that HFO-1234yf is a highly volatile gas (normal boiling point = −29 °C, vapor pressure = 607 kPa at 21 °C) with limited water solubility (198 mg/L at 24 °C).2 Im et al. reported a dimensionless Henry’s Law constant (39.7 at 20 °C; mass in air/mass in water) which when converted into the units typically used by atmospheric chemists is approximately 1 × 10−3 M atm−1. It is well established that species with Henry’s Law constants below approximately 103 M atm−1 partition strongly into the atmospheric gas phase.3 The Henry’s Law constant for HFO-1234yf is approximately one million times lower than needed for it to partition appreciably into aquatic environments. With its low boiling point and high vapor pressure a leak of liquid HFO-1234yf would quickly evaporate. It is well established that reaction with OH radicals provides a rapid gas-phase removal mechanism for HFO-1234yf resulting in an atmospheric lifetime of approximately 11 days.4 Strong partitioning into the atmospheric gas phase combined with a rapid gas phase removal mechanism precludes any contamination of aquatic environments by HFO-1234yf. Im et al. tested an aqueous concentration of HFO-1234yf in simulated groundwater (100 μM, ∼5% of its solubility in water at 20 °C) that corresponds to an equilibrium gas phase concentration of 0.1 atm. Assuming global emissions of HFO-1234yf reach 100 ktonne per year (similar to current hydrofluorocarbon emissions) with a lifetime of 11 days it can be estimated that background atmospheric concentrations of HFO-1234yf would be of the order of 0.1−1.0 ppt (parts per trillion, 10−12). Local concentrations near sources would be higher. Vollmer et al.5 have reported annual average levels of HFO-1234yf in 2014 of 0.136 ppt at a suburban site in Zurich and 0.003 ppt in a remote alpine site (Jungfraujoch). Atmospheric concentrations of HFO-1234yf are currently, and expected to remain, extremely low (
