IN BRIEF: WHAT ARE CRIEGEE INTERMEDIATES AND HOW DO THEY REACT WITH TFA IN THE ATMOSPHERE?

Until recently, atmospheric models for trifluoroacetic acid (TFA) incorporated surface deposition and rain-out as the main loss processes. This is because TFA reacts only slowly with hydroxyl radicals and does not photolyse at actinic wavelengths (ie UV spectrum). The hydroxyl radical (OH) is the primary reactive agent of the lower atmosphere and reacts with many organic species including hydrocarbons, HFCs and HFOs to remove them from the atmosphere.

Criegee intermediates are named after the chemist Rudolf Criegee, who first postulated their formation in the 1950s. They are formed from the reaction of ozone with alkenes, which are both present naturally in the atmosphere. Alkenes (including C2 to C5 alkenes eg isoprene) are emitted in forested areas. In the troposphere near the Earth’s surface, the natural concentration of ozone is about 10 parts per billion (0.000001 percent). Once formed Criegee intermediates are very reactive especially towards highly polar molecules such as TFA and other carboxylic acids.

The loss rate of TFA in the atmosphere by reaction with the most tropospherically abundant Criegee Intermediates (such as those formed from isoprene) has been predicted and mapped globally. The outcomes suggest that rapid reactions with Criegee intermediates are the dominant sink for tropospheric TFA in forested regions around the world. Inclusion of this new gas-phase chemistry in atmospheric models could decrease the projected deposition of TFA over landmasses (by as much as a factor of two).

The reaction of one Criegee intermediate with TFA results in the formation of a hydroperoxyester (CF3C(O)OCH2OOH, hydroperoxymethyltrifluoroacetate) which is believed to rapidly decompose in the atmosphere via reaction with OH radicals or uptake on atmospheric particles and subsequent processing. The atmospheric fate and products of hydroperoxyesters is an active area of research.

Explanatory Note: The environmental effects of TFA from HFCs and HFOs have been thoroughly investigated without taking into account any effects of reactions with Criegee intermediates. The 2018 Environmental Effects Assessment Report [2] stated that “Overall, there is no new evidence that contradicts the conclusion of our previous Assessments that exposure to current and projected concentrations of salts of TFA in surface waters present a minimal risk to the health of humans and the environment. A recent review of this topic reached a similar conclusion.”[3]

References
[1] Chhantyal-Pun, R., Rotavera, B., McGillen, M. R., Khan, M. A. H., Eskola, A. J., Caravan, R. L., et al. (2018). Criegee Intermediate Reactions with Carboxylic Acids: A Potential Source of Secondary Organic Aerosol in the Atmosphere. ACS Earth and Space Chemistry, 2(8), 833–842. https://doi.org/10.1021/acsearthspacechem.8b00069
[2] Environmental Effects and Interactions of Stratospheric Ozone Depletion, UV Radiation, and Climate Change 2018 Assessment Report UN Environment Programme Section 5.1 Trifluoroacetic acid from replacements of ODS and refrigerants with large GWPs.
[3] The recent review is the Norwegian Environment Agency, 2017, Study on Environmental and Health Effects of HFO Refrigerants, Norwegian Environment Agency Report No. No. M-917|2017, Oslo, Norway.