EMEP Contribution to the Preparatory Work for the Review of the CLRTAP Protocol on POPs. New Substances: Model Assessment of Potential for Long-range Transboundary Atmospheric Transport and Persistence of Hexachlorobutadiene

MSC-E Information Note 14/2005

N. Vulykh, E. Mantseva, V. Shatalov

ABSTRACT

Hexachlorobutadiene (HCBD) is a chlorinated aliphatic compound widely used in industry. It is a by-product at manufacturing synthetic caoutchouc, rubber and lubrication materials. It is added to transformer oils, hydraulic liquids, and heat carriers. Previously HCBD was widely used in agriculture. Mainly it was used as fumigant and besides as herbicide and insecticide. In the early 80s HCBD production in the USA, Japan and Western Europe was 10 thous.t/y [Filov, 1990].

HCBD as a potential candidate for the inclusion into the UN-ECE Protocol on POPs was addressed by the Ad Hoc Expert Group on POPs [Lerche et al., 2002]. This year the European Commission has submitted a proposal for amendment to the Protocol with regard to this chemical. In accordance with a suggestion of the Working Group on Strategies and Review that during the 90-day period between the submission of dossier and the twenty-third session of the Executive Body, the dossier on HCBD prepared by national experts [van de Plassche and Schwegler, 2005] is available on the Convention's website for comment and submission of additional information by all interested persons or groups. Additional information for the evaluation of hexachlorobutadiene as a potential new POP in accordance with the two criteria: potential for long-range transboundary atmospheric transport (LRTP) and persistence can be provided by modelling.

A certain experience in the model assessment of the long-range transboundary atmospheric transport and persistence in the environment for a wide range of typical POPs already included in the Protocol on POPs and some potential new POPs [Shatalov et al., 2003; Dutchak et al., 2004; Mantseva et al., 2004; Vulykh et al., 2004, 2005] is accumulated in the Meteorological Synthesizing Centre-East of EMEP. For the evaluation of LRTP and persistence of pollutants, EMEP/MSCE-POP multicompartment hemispheric transport model is used. To estimate the considered parameters for HCBD, the model calculation of its atmospheric transport from a conventional emission point source located in Europe (10°Å; 47.5°N) is made for one-year period. Information on physical-chemical properties and degradation rates of HCBD used for modelling is included in Annex A.

To illustrate LRTP of HCBD, two numerical characteristics are calculated. The first one is residence time in the atmosphere (Half-life in aircalc (T1/2air)) obtained with allowance of all processes removing the considered pollutant from the atmosphere. The second is Transport Distance (TD) that is the distance from the source at which annual mean atmospheric concentration of a chemical in question drops 1000 times compared with the concentration near the source. Additional information on this criterion for HCBD is provided by the spatial distribution of air pollution caused by the considered conventional point source.

Persistence in the environment is evaluated by Half-life in the environment (T1/2env) estimated for HCBD on the basis of the model simulation of its atmospheric transport taking into account deposition processes, degradation and exchange of the pollutant between main environmental media.

To diminish uncertainties in evaluating LRTP and persistence of HCBD with the help of the model dependent TD and T1/2env, a comparison of their numerical values against those obtained for B[a]P and HCB as adequately studied "benchmark substances" is presented.

In conclusion, the calculated numerical characteristics of LRTP and persistence of HCBD are given in relation to the indicative criteria outlined in Executive Body Decision 1998/2.

Detailed information on the structure of the model and parameterisation of the media processes can be found in the EMEP/MSC-E reports [Gusev et al., 2005].