HYDROGEN FLUORIDE (HF) EMISSIONS FROM DEGRADATION OF FLUOROCARBON REFRIGERANTS
The recent UBA report [1] estimates the hydrogen fluoride (HF) emissions to air at about 20,000 tonnes/year from 2030 to 2050 due to degradation of HCFCs, HFCs, HFOs and HCFOs emissions from the EU based on its scenario of maximum future use and emissions of halogenated substitutes. However, the UBA report provides no context for the relative magnitude of these extremely low concentration widely dispersed emissions compared to other natural emission sources of fluoride and hydrogen fluoride. Fluorine, in the form of fluoride ion, is found in many rocks and minerals and most soils. It comprises 0.06% of the Earth's crust (which, although it does not seem much, is about twice the known amount of fossil fuel carbon). Fluorides are naturally released into the environment through the weathering and dissolution of minerals, and in emissions from volcanoes and in marine aerosols.
Volcanoes: The major natural source of hydrogen fluoride emissions to the atmosphere is volcanoes. These emissions are estimated to range from 0.6 to
6 million metric tons per year [2]. On average, <10% of these emissions are a result of large eruptions that are efficiently ejected into the stratosphere. Passive degassing is a major source of tropospheric hydrogen fluoride. Mount Etna is the largest known point source of atmospheric fluorine on a global scale, contributing about 70,000 tonnes/year [3].
Soil and sea salt aerosol: Soil naturally contains fluoride, and resuspension of soil by wind also contributes to the atmospheric burden of fluorides in the form of soil minerals [4]. The wide distribution in soils means that there is significant natural movement of fluoride through the atmosphere on wind-borne dust particles (estimates vary from 1 to 10 million tonnes/year). Another source is sea salt aerosol, which releases small amounts of gaseous hydrogen fluoride and fluoride salts into the air. The marine aerosol is potentially a major source of tropospheric hydrogen fluoride [5]. However, these releases would be confined to the air over the oceans.
Exposure to fluorides: The routes for exposure to fluoride are well established globally. including from the addition of fluorides to dental products [6]. The WHO 2004 monograph [7] states that “Levels of daily exposure to fluoride depend mainly on the geographical area. In the Netherlands, the total daily intake is calculated to be 1.4–6.0 mg of fluoride. Food seems to be the source of 80–85% of fluoride intake; intake from drinking-water is 0.03–0.68 mg/day and from toothpaste 0.2–0.3 mg/day.” The WHO 2006 monograph [6] states that waters with high fluoride concentrations occur in large and extensive geographical belts associated with a) sediments of marine origin in mountainous areas, b) volcanic rocks and c) granitic and gneissic rocks. It explains that air is typically responsible for only a small fraction of total fluoride exposure, but it does note that there are some exceptions mainly due to localised anthropogenic sources.
Contribution to acidification in the EU: The main air pollutants contributing to acidification in the EU are nitrous oxides (NOx), sulphur oxides (mainly SO2), ammonia (NH3) as they form acidic species in the atmosphere, which are then rained out (“acid rain”) [8]. In 2016, their emissions in the EU were about 13 million tonnes, with a target of about 9 million tonnes in 2030 [9]. The 20,000 tonnes of HF emissions estimated in the UBA report for emissions in 2030 are less than 0.5% of this 2030 target [10].
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References
[2] Symonds RB, Rose WJ, Reed MH. 1988. Contribution of Cl- and F-bearing gases to the atmosphere by volcanoes. Nature 334:415-418.
[3] Francis, P., Burton, M.R., Oppenheimer, C., 1998. Remote measurements of volcanic gas compositions by solar occultation spectroscopy. Nature 396, 567–570.
[4] Biologic effects of atmospheric pollutants: Fluorides. Washington, DC: National Academy of Sciences, National Research Council, Committee on Biologic Effects of Atmospheric Pollutants, 239.
[5] Friend JP. 1989. Natural chlorine and fluorine in the atmosphere, water and precipitation. United Nations Environmental Programme/World Meteorological Association. Scientific Assessment of Stratospheric Ozone: 1989. Alternative Fluorocarbon Environmental Acceptability Study Report.
[6] 2006 World Health Organization (WHO). Fluoride in Drinking-water by J. Fawell, K. Bailey, J. Chilton, E. Dahi, L. Fewtrell and Y. Magara. ISBN: 1900222965. Published by IWA Publishing, London, UK.
[7] WHO/SDE/WSH/03.04/96 Fluoride in Drinking-water Background document for development of WHO Guidelines for Drinking-water Quality 2004
[8] Air quality in Europe — 2018 European Environment Agency (EEA) report 12/2018
[9] European Environment Agency (EEA) EU progress in meeting 2010 emission ceilings set out in the NEC Directive and the 2020/2030 reduction commitments https://www.eea.europa.eu/themes/air/national-emission-ceilings/nec-directive-reporting-status-2018.
[10] The contribution to acidification is calculated from Acidification Potential (AP) for the contributions of SO2, NOx, HCl, NH3, HF and other acids (or acid precursors) make to the potential acid deposition, which is their potential to form H+ ions and is calculated from the Equivalency Factor and the emissions of each substance. Acidification potentials are expressed as SO2-equivalents (SO2-eq) and the potentials are expressed relative to the potential of SO2. The method of establishing effect factors for acidifying substances is based on stoichiometric considerations and it is internationally accepted.