Global modeling of trifluoroacetic acid surface concentration and deposition from the gas-phase oxidation of a wide range of precursor hydrofluoroolefins

Abstract

Oxidation of hydrofluoroolefins (HFOs) is a source of trifluoroacetic acid (TFA) in the lower atmosphere. TFA is deposited in precipitation and accumulates in water bodies and at land surfaces and concerns have been raised over its environmental impact. The formation and distribution of atmospheric TFA from the gas-phase oxidation of fifteen HFOs were studied. The deposition of TFA associated with regional emissions of HFOs were examined using a global three-dimensional chemical transport model, STOCHEM-CRI, where hypothetical scenarios with annual emissions of 1, 10 and 100 Gg for each of the HFOs were modelled. Globally, between 54 and 78 Gg year⁻¹ of TFA are produced in scenarios using lower and upper limit TFA yields, respectively. The most significant contributors to the TFA formation are found to be HFO-1234yf (9.9 Gg year⁻¹, 13–18%), HFO-1225yeZ (8.5 Gg year⁻¹, 11–16%), HFO-1225yeE (8.6 Gg year⁻¹, 11–16%) and HFO-1216 (7.5 Gg year⁻¹, 10–14%). The tropospheric global burden and lifetime of TFA are found to be 0.54–0.78 Gg and 3.8 days, respectively. Atmospheric levels of TFA from HFO oxidation are highest in northern mid-latitudes, with up to 1.5–2.0 ppt in Europe, 0.5–0.7 ppt in Asia, and 0.5–0.7 ppt in North America during the northern hemispheric summer. TFA is mainly deposited in North America, Europe, and Asia, with deposition rates of up to 0.5 × 10⁻³ Mg km⁻² years⁻¹, 1.0 × 10⁻³ Mg km⁻² years⁻¹, and 1.0 × 10⁻³ Mg km⁻² years⁻¹, respectively. A metric called the TFA deposition potential (TDP) is proposed that quantifies the extent to which different HFOs contribute towards enhanced environmental TFA deposition, relative to that from the oxidation of the most widely used HFO (HFO-1234yf).