Chiral chemicals as tracers of atmospheric sources and fate processes in a world of changing climate

Abstract

Elimination of persistent organic pollutants (POPs) under national and international regulations reduces "primary" emissions, but "secondary" emissions continue from residues deposited in soil, water, ice and vegetation during former years of usage. In a future, secondary source controlled world, POPs will follow the carbon cycle and biogeochemical processes will determine their transport, accumulation and fate. Climate change is likely to affect mobilisation of POPs through e.g., increased temperature, altered precipitation and wind patterns, flooding, loss of ice cover in polar regions, melting glaciers, and changes in soil and water microbiology which affect degradation and transformation. Chiral compounds offer advantages for following transport and fate pathways because of their ability to distinguish racemic (newly released or protected from microbial attack) and nonracemic (microbially degraded) sources. This paper discusses the rationale for this approach and suggests applications where chiral POPs could aid investigation of climate-mediated exchange and degradation processes. Multiyear measurements of two chiral POPs, trans-chlordane and α-HCH, at a Canadian Arctic air monitoring station show enantiomer compositions which cycle seasonally, suggesting varying source contributions which may be under climatic control. Large-scale shifts in the enantioselective metabolism of chiral POPs in soil and water might influence the enantiomer composition of atmospheric residues, and it would be advantageous to include enantiospecific analysis in POPs monitoring programs.

Keywords: air-surface exchange; atmospheric transport; chiral; climate change; persistent organic pollutants.

Fig. 1. Percent of soils showing depletion of the (+) enantiomer (blue, EF <0.5), depletion of the (−) enantiomer (red, EF>0.5) and containing racemic residues (green, EF=0.5) for α-HCH, cis-chlordane (CC), trans-chlordane (TC) and o,p′-DDT (number of soils in parentheses), based on reports from the 1990s to the present where such information is given or can be deduced. Data sources are given in ref. . Figure reproduced with modifications from ref. .

Fig. 2. Range of EFs reported for α-HCH in aquatic systems, red line indicates racemic α-HCH (EF=0.5). Figure reproduced from ref.

Fig. 3. EFs of TC in atmospheric deposition collected in Sweden, Iceland and Slovakia during 1971–1973,⁴⁶⁾ air sampled at Rörvik, Sweden and Pallas, Finland in 1998–2001,⁴⁶⁾ and means of annual maxima and minima at Alert, Canada in 1994–2000 (this study).

Fig. 4. Wide range in EFs of α-HCH in air sampled over the ocean before and after ice breakup in the Canadian Archipelago at Resolute Bay (1999)⁵⁹⁾ and Banks Island (2008),⁶⁰⁾ compared to the narrower range of EFs (means of annual maxima and minima) in air sampled at the land station Alert (this study).