Modelling study of TFA deposition and transport in Rhine River basin

This modelling study, conducted by Ramboll, assessed the fate and transport of Trifluoroacetic Acid (TFA) in the freshwater aquatic environment resulting from the emission and atmospheric degradation of HFO-1234yf and subsequent atmospheric deposition in the Rhine River basin. The results demonstrate that with constant and ongoing emissions of HFO-1234yf, within a short timeframe (months) a steady state concentration of TFA is reached, which does not further increase over time. In the case study of the Rhine basin, TFA concentrations below 0.6 μg/L are predicted, based on a forecast of the emissions of HFO-1234yf in the EU. The Rhine was selected as a case study as it represents a prominent watershed in western Europe with a large population.

From the model, the steady state TFA concentrations are at least 100 times below the German drinking water guide (60 μg/L) and are multiple orders of magnitude below the no-effects level of the human health risk assessment (No observed adverse effect level, NOAEL = 10 mg TFA/kg bw/day from a 90-day oral study in rats). The study demonstrates that freshwater TFA concentrations would not continue to increase from constant emissions of HFO-1234yf. In addition, the TFA concentrations modelled are well in line with the actual measured TFA concentrations in the Rhine River as reported by UBA (https://gis.uba.de/maps/resources/apps/TFA-Herkunft-und-Belastungen/index.html ) considering that the measurements include all sources of TFA not only from HFC/HFO degradation.

The study concludes that “Importantly, this study contests the assumption made in the REACH PFAS Restriction Proposal that freshwater TFA concentrations would continue to increase until “inevitably” a toxic level would be reached. Moreover, the TFA concentrations modelled are reasonably within the mean and maximum measured TFA concentrations in the Rhine River.”

Model parameters

An atmospheric modelling study was previously conducted by Henne et al. [1] to study the deposition of TFA resulting from emissions of HFO-1234yf from mobile air conditioners in Europe. Taking leakage rate assumptions and predicted vehicle numbers for the year 2020 into account [assuming complete conversion to HFO-1234yf by that date], they estimated an upper limit for total HFO-1234yf emissions to be 19,212 tonnes/yr and assessed resulting TFA wet and dry deposition rates over Europe using established models for atmospheric modelling. Because TFA deposition is expected to vary linearly with emissions of HFO-1234yf, the modelled TFA deposition results from Henne et al. [1] are applied in the current study after reducing by a factor of 2.71 to account for the emissions over-estimate in Henne et al. compared to the current best estimate of 7,090 tonnes reported in the REACH Dossier CSR. See explanatory note.

The annual average atmospheric dry, wet, and total deposition rates (microgram per square metre per year [μg/m2-yr]) of TFA in each of the Rhine River sub-basins were then estimated using the deposition rates at 5° by 5° spatial resolution over Europe. These deposition rates were used as inputs to the subsequent environmental fate and transport modelling of TFA after it deposits on the soil or waterbody in the Rhine River watershed. A series of compartment models that represent the six sub-basins within the Rhine River were modelled to simulate the transport of TFA in a river system from the beginning to the end where it discharges into the ocean.

Other results from the model

The model predicted steady state TFA concentrations in the Rhine River sub-basins range from 0.11 μg/L for the upstream sub-basin to 0.57 μg/L for the Delta Basin where the Rhine River reaches the North Sea. In general, the TFA concentration in the river system increases from the upstream sub-basins to the downstream basins.

The TFA concentrations in surface soil receiving deposition also reach steady state within two to seven months. Based on the simulation results of the TFA mass allocation in soil, river system, and the ocean, a very small percentage (<1 %) of the TFA mass deposited from the air would remain in the river system or the mixing zone of surface soil receiving deposition (i.e., assumed to be the top 2 centimetres of soil for this study) after the TFA concentrations reach steady state in the surface water and in the mixing zone of surface soil, the majority of the TFA would enter the ocean.

The study by Ramboll “Environmental modelling of trifluoroacetic acid (TFA) originating from hydrofluoro-olefins” was submitted to the PFAS restriction proposal public consultation and is available at https://echa.europa.eu/registry-of-restriction-intentions/-/dislist/details/0b0236e18663449b > Document RCOM part 44,  > Contribution No 6608.

Explanatory Note: A key study for HFO-1234yf emissions from MAC in Europe EU is the Henne et al. [1] study. This simulation assumed that all car air conditioning systems are operated with the refrigerant HFO-1234yf. The study included low and high emissions scenarios, which used different annual leakage rates and end-of-life losses. The model assumed complete replacement of HFC-134a by HFO-1234yf and used predicted vehicle numbers for the year 2020 to calculate emissions. In reality, the complete conversion to HFO-1234yf did not occur by 2020 but does not invalidate the simulation. Future changes in vehicle numbers or changes in leakage rates, for example, would affect the derived emissions to some extent. The authors arrived at a total emission level of HFO-1234yf for Europe (EU+ Croatia, Norway, Switzerland and Turkey) for 2020 of 11.0-19.2 kilotonnes per year. About 30-40% of these emissions are deposited within Europe in the form of TFA, the rest is transported in the atmosphere towards the Atlantic, Central Asia and Africa.

However, based on:

• The overstated HFC-134a emissions in the EU submissions to UNFCCC in the National Inventory Reports (NIR and Common Reporting Format (CRF) tables in recent years, used to validate emission models, as MAC is a main use of HFC-134a see newsletter item Global and European HFC reported emissions compared to emissions derived from atmospheric monitoring – Fluorocarbons;
• The recent trends for reduced leakage rates, for example a report from the USA for 50 car and van models, some with dual evaporators, for the model year 2021, reports leakage rates in the range 0.6% to 4.2%, with an average leakage rate of 1.6% and a median leakage rate of 1.25%. MPCA, 2022. Minnesota Pollution Control Agency, Mobile conditioner leakage rates for Model Year 2021. Mobile Air Conditioner Leakage Rates – Model Year 2021 (Alphabetical List) (state.mn.us)
• The transition to electric vehicles, which removes one of the main sources of leakage - the compressor shaft seal for mechanically driven compressors;

Future emissions from MAC in Europe are expected to be at or below the low emission scenario (11,000 tonnes) used in the Henne et al. study. The Rhine River study used the current best estimate of 7,090 tonnes emissions of HFO-1234yf reported in the REACH Dossier CSR.

 

Reference [1] Henne, S.; Shallcross, D. E.; Reimann, S.; Xiao, P.; Brunner, D.; O’Doherty, S.; Buchmann, B., Future emissions and atmospheric fate of HFC-1234yf from mobile air conditioners in Europe, Environmental Science and Technology, 2012, 46, 1650−1658. DOI: 10.1021/es2034608.