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 Polychlorinated Naphthalenes

MSC-E Information Note 16/2005

N. Vulykh, E. Mantseva, V. Shatalov

ABSTRACT

Polychlorinated naphthalenes (PCNs) are a group of compounds composed of two fused benzene rings with one to eight chlorine substitutions. Their molecular structures are similar to those of PCDD/Fs and PCBs. Several of the higher chlorinated congeners are found to elicit biochemical responses similar to the toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) [Lundgren et al., 2003]. A quantitative assessment of dioxin-like potential of PCN congeners has been performed in several studies [Hanberg et al., 1990, Blankenship et al., 1999; Villeneuve et al., 2000; Blankenship et al., 2000,etc].

Due to chemical and thermal stability, good electrical insulation properties, excellent weather resistance and low flammability, PCNs were manufactured as several different technical formulations and widely used in electrical and electronic equipment [Jarnberg et al., 1997]. Besides, several emission sources of PCNs in the environment have been suggested apart from those associated with the production and continued use of these formulations. In particular, PCNs are usually common impurities found in technical PCB mixtures and they are formed in thermal (e.g. combustion, roasting, metal reclamation) and other processes (e.g. chloro-alkali industry) in the presence of chlorine [Falandysz, 1998].

PCNs are included into the OSPAR List of Chemicals for Priority Action (Update 2004) [Meeting of the OSPAR Commission (OSPAR), 2004]. As potential candidates for the inclusion into the UN ECE Protocol on POPs this group of compounds 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 PCNs. 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 PCNs prepared by national experts [van de Plassche and Schwegler, 2002] 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 PCNs as potential new POPs 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 PCNs, EMEP/MSCE-POP multicompartment hemispheric transport model is used. Taking into account that PCN group includes 75 possible congeners; this model assessment is carried out for PCN-47. This congener was selected as a representative of the most widespread group of tetra-chlorinated naphthalenes (tetra-CNs) in PCN congener profiles of environmental and source related samples. The homological group of tetra-CNs was detected in most types of samples in prevalent quantities (see Annex A). To estimate LRTP and persistence for PCN-47, 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 this pollutant used for modelling is presented in Annex B.

To illustrate LRTP of the considered congener of tetra-CNs, 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 PCN-47 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 PCN-47 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 the considered congener of PCNs with the help of the model dependent TD and T1/2env, a comparison of their numerical values against those obtained for B[a]P as an adequately studied "benchmark substance" is presented.

In conclusion, the calculated numerical characteristics of LRTP and persistence of PCN-47 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]