Current sources of carbon tetrachloride (CCl₄) in our atmosphere

Carbon tetrachloride (CCl₄ or CTC for short and used hereafter) is an ozone-depleting substance (ODS), a greenhouse gas (WMO 2014), a toxic substance at high concentrations, and a carcinogen (MAK coll 2012). Being a major contributor to stratospheric ozone depletion, its production and consumption are controlled under the Montreal Protocol's Article 2D, which was agreed to at the June 1990 Meeting of the Parties in London and came into force in 1995. Further adjustments to the Protocol led to a 100% reduction of CTC production and consumption for emissive uses from 2010 onwards. As a consequence of the Montreal Protocol, global atmospheric levels of CTC are steadily decreasing (Carpenter et al 2014).

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Although CTC production and consumption for emissive uses is now fully controlled, Carpenter et al (2014) reported that CTC emissions to the atmosphere seem to have continued since the full controls were implemented in 2010. This 2014 report, which estimated a 57 Gg year⁻¹ CTC discrepancy between report based emissions (i.e. bottom-up) and emissions derived from atmospheric observations (i.e. top-down), was the most recent from a number of reports in the UNEP/WMO Scientific Assessments of Ozone Depletion (1999, 2003, 2007, 2011). These large observation-based emissions contrast to small emissions (3(0−8) Gg year⁻¹) inferred from mandated reports to the United Nations Environment Programme (UNEP) under Article 7 of the Montreal Protocol from its signatory parties.

As a consequence of this discrepancy, and the potential omissions to reporting of CTC emissions under the Montreal Protocol, a focused report was instigated to identify the cause of the CTC emissions discrepancy (SPARC 2016). In this report, new findings led to lower calculated CTC losses in soils and the ocean, reducing the global top-down emissions estimate to about 40 Gg year⁻¹. Furthermore, global CTC emissions of about 30 Gg year⁻¹ were estimated by the difference in CTC levels between the Northern and Southern hemispheres. SPARC (2016) merged these results to estimate that current top-down CTC emissions to the atmosphere are 35 ± 16 Gg year⁻¹.

Under Article 7, the Montreal Protocol requires each Party to report annually on CTC amounts produced to be used as feedstocks, amounts destroyed, imports and exports. It is important to note that these reported amounts do not represent emissions, but can be used (with reasonable assumptions) to estimate emissions. Montzka et al (2011) used the UNEP reports to derive bottom-up CTC emissions by differencing the feedstock sum and amounts destroyed, with estimated emissions as a small proportion of both feedstock usage and reported destruction. In SPARC (2016), a re-evaluation of these Article 7 reports yielded CTC emissions of approximately 3 (0−8) Gg year⁻¹ for the post-2010 period.

The difference between the SPARC (2016) top-down 35 ± 16 Gg year⁻¹ and the Article 7 report-based 3 (0–8) Gg year⁻¹ emissions estimates suggests that there are on-going unidentified CTC emissions sources. Herein, we seek to identify current CTC sources and evaluate their potential magnitudes. The paper elaborates these sources and potential emissions pathways in the following order: (A) production and use of chlorine gas, (B) production of CTC, (C) usage of CTC, and D) contained stocks or legacy emissions. A summary of the different sources is shown in figure 1.

The first potential CTC emissions pathway is related to the production of chlorine in chlor-alkali plants (see figure 1, left side in green). This is due to the relative ease with which hydrocarbons are chlorinated, thus CTC might be formed in many chlorination procedures and released into the environment, to the atmosphere or into surface water.

These chemical production facilities produce chlorine, hydrogen and alkali (sodium or potassium hydroxide) by electrolysis of a salt solution. The main technologies applied to chlor-alkali production are mercury, diaphragm and membrane cell electrolysis, mainly using sodium chloride, or to a lesser extent potassium chloride, as feed. Currently, the chlor-alkali process produces 97% of the world's chlorine (Brinkmann et al 2014) and the process technologies employed are not specific to geographical regions. In 2012, global chlorine production capacity was estimated to be 76 800 Gg (Brinkmann et al 2014). Of the world chlor-alkali capacity, more than 80% is concentrated in three regions; East Asia (48%), North America (19%), and Western Europe (16%) (from Brinkmann et al (2014) figure 1).

2.1. Unreported inadvertent emissions from chlorine production and usage (figure 1, Pathway A)

CTC is produced by two methods (see figure 1, middle, Pathway B). First, CTC is a co-product of the industrial production of chloromethanes, including the mono-, di- and tri- chloromethanes (CH₃Cl, CH₂Cl₂ and CHCl₃, hereafter referred to as 'CMs') in CM plants. Second, CTC is also co-produced with perchloroethylene (PCE) in PCE/CTC plants.

CM plants: CMs are produced by hydrochlorinating methanol (CH₃OH) with HCl (CH₃OH + HCl → CH₃Cl + H₂O) to form methyl chloride (CH₃Cl), and with subsequent chlorination (e.g. CH₃Cl + Cl₂ → CH₂Cl₂ + HCl) to produce a series of other important chloromethanes (CH₂Cl₂ and CHCl₃) as well as CTC. Global chloromethanes (CMs) production in 2014 was 2785 Gg. Older technology tended to produce ~6%–8% CTC of the total CMs, together with some heavier chlorocarbons, specifically hexachlorobenzene (C₆Cl₆) and hexachlorobutadiene (C₄Cl₆). These heavy products, together with some CTC, accumulated as tars, can be refined to produce CTC for chemical intermediate use. Newer technology (e.g. as used in China) generally yields about 4% CTC. An average 4% on the CTC/CM production number is the absolute minimum for CTC generation and is used as the default. In some plants/regions, the CTC/CM production percentage is greater than the default 4%, either because of older equipment or because CTC product is actually required from the process.

PCE/CTC plants: The PCE/CTC process made both products until the mid-1990s, when low CTC demand and strict regulations resulted in the complete closing of plants in developed countries, or investments in plants to run to 100% PCE and zero CTC. The reaction to generate PCE and CTC, by chlorinating a hydrocarbon such as propene, is a chemical equilibrium, and any CTC produced can be separated and either recycled into the reactor to be converted to PCE or can be sent directly to on-line thermal oxidizers or kilns for destruction (Source: multiple discussions over years with producers). The global capacity for PCE/CTC from the seven or eight PCE/CTC plants is 360 Gg. In 2014, production of PCE from this process was 300 Gg, along with 39 Gg of CTC. PCE is also produced in one PCE/trichloroethylene plant (NAFTA), which uses Cl₂ as feedstock. This route does not involve CTC production or emissions. Furthermore, a small amount of PCE is made from the production of acetylene, but this process is unlikely to create CTC either.

Figure 1 shows the CTC production from CM plants (164 Gg in 2014 in blue: capacity via this route is ~190 Gg and will probably increase in the future) and PCE/CTC plants (39 Gg in blue). Combining these two routes results in a 2014 estimated production of ~203 Gg. This is consistent with 2013 Technology and Economic Assessment Panel (TEAP) reported estimated production of 200 Gg (UNEP 2013).

3.1. Unreported non-feedstock emissions from CTC production (figure 1, Pathway B)

CTC had a number of uses in the past, primarily as a feedstock for the production of chlorofluorocarbons. Current uses are now confined by the Montreal Protocol to be in contained processes. As shown in table 2, industrial production of CTC in 2014 was consumed in: (i) incineration (29 Gg); (ii) as a PCE feedstock (64 Gg); (iii) as HFC feedstock (58 Gg); in (iv) methyl chloride (MeCl) production (26 Gg); (v) in divinyl acid chloride (DVAC) production (23 Gg); and (vi) for use as process agents and laboratory purposes (3 Gg). This consumption is illustrated in figure 1 with the lighter blue bars. As CTC is used as feedstock (>95% of total production), most gets converted to other products, like HFCs, PCE, etc. CTC is still used as a process agent and in laboratory applications (figure 1 vi), but in relatively small amounts of 3 Gg year⁻¹ globally.

4.1. Fugitive emissions from usage of CTC (figure 1, Pathway C)

While the current CTC uses are detailed in section 4, historic uses of CTC for various processes were widespread, and contaminated CTC was typically disposed of in an environmentally careless manner. Historic CTC applications included metal degreasing, dry-cleaning fluid, fabric spotting, fire extinguishing, grain fumigation, and uses as a reaction medium (Holbrook 2000). These early disposal practices led to a number of 'legacy' sites that have continued emissions (e.g. see Truex et al (2001)). These legacy emissions (i.e. emissions from old industrial sites and landfills) may also be important to the global overall CTC emissions (figure 1, Pathway D). Fraser et al (2014) estimated that unaccounted CTC emissions (legacy emissions and from chlor-alkali plants) could potentially contribute 10–30 Gg year⁻¹ globally. Based on newly available information, SPARC (2016) revised this estimate to 5–10 Gg year⁻¹. This estimate is highly uncertain, however, as it is upscaled from Australian emissions to the global level.

Recent CTC emissions estimates using atmospheric observations over the United States (US) by Hu et al (2016) suggest that the US emissions distribution is very similar to the distribution of chlor-alkali plants in the US. However, these results could not be used to identify the importance of these plants for CTC emissions (Pathway A) relative to other industry-related sources such as CM and PCE (Pathway B). Inadvertent CTC emissions from chlor-alkali plants have not been rigorously assessed globally. Furthermore, a similar study of European CTC emissions by Graziosi et al (2016) showed these to be about 4% of global emissions and suggested that regions with large producers of basic organic chemicals were significant sources of CTC emissions. Combined emissions from legacy sources and chlor-alkali plants have been estimated to be ~10 Gg in 2014.

CTC emissions have been estimated using top-down techniques from observations (35 Gg year⁻¹) and from a bottom-up technique that utilizes Montreal Protocol based reports to UNEP (3 Gg year⁻¹) in SPARC (2016). This large discrepancy between the top-down and bottom-up estimates suggests uncontrolled CTC emissions.

An analysis of CTC sources from consideration of industrial processes in 2014 was completed as part of SPARC (2016). In that report, four potential sources were identified; Pathway (A) unreported but inadvertent emissions from chlorine gas usage and chlor-alkali plants; Pathway (B) unreported non-feedstock emissions from industrial production that includes CTC: ~13 Gg; Pathway (C) fugitive emissions from known feedstock usages, incineration, and process agents/laboratory: ~2 Gg; and Pathway (D) legacy emissions from landfills and contaminated soils: together (A) and (D) could be as high as 10 Gg. In sum, these four sources were estimated to be up to 25 Gg in 2014, an amount that is considerably larger than 3 Gg of emissions inferred from UNEP reported CTC production.

The non-feedstock emissions from CM and PCE/CTC production and the inadvertent emissions associated with the production and use of chlorine gas have so far not been included in the bottom-up estimates for the WMO/UNEP Scientific Assessments of Ozone Depletion (e.g. WMO 2014) and hence contribute considerably to closing the gap between bottom-up estimates and top-down calculated emissions.

This amount of 25 Gg year⁻¹ is however still lower than the average of the estimated emissions of 35 ± 16 Gg year⁻¹ related to global trends of CTC in the atmosphere and current best estimates of CTC lifetimes in air, sea water and soil. Narrowing this budget gap will require additional research as noted in the Research Suggestions section of SPARC (2016).

Qing Liang and Paul A Newman were supported by the NASA Atmospheric Composition Campaign Data Analysis and Modeling program under NNH13ZDA001N-ACCDAM. We also thank Dr Helen Tope for her extremely helpful comments on the manuscript.