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. Largescale 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.

Annual cycles of organochlorine pesticide enantiomers in Arctic air suggest changing sources and pathways

“…Mixed degradation, though largely of the (+) enantiomer, was found in the equatorial Indian Ocean (Huang et al, 2013). A compilation of degradation preferences for α-HCH in 270 agricultural and background soils showed that (−) degradation was favoured in 50 %, (+) degradation in 20 %, and 30 % contained racemic residues, with an overall mean EF of 0.528 ± 0.095 (reviewed by Bidleman et al, 2012Bidleman et al, , 2013. Regional "footprints" are important in determining the enantiomer composition of α-HCH in air.…”

“…4) suggest that microbial processing plays a role in its transport and fate, but how and where is unclear. Average degradation preferences in soils worldwide are 56 % (+)TC, 29 % (−)TC, with 15 % of soils containing racemic TC; 22 % (+)CC, 64 % (−)CC, and 14 % racemic CC; and average EFs are TC 0.480 ± 0.067, and CC 0.531 ± 0.073 (reviewed by Bidleman et al, 2012Bidleman et al, , 2013. From these general enantiomer profiles, emissions from soils should be depleted in (+)TC and (−)CC.…”

Enantiomer-specific stable carbon isotope analysis (ESIA) to evaluate degradation of the chiral fungicide Metalaxyl in soils

“…Chiral pesticides, i.e., pesticides with at least two enantiomers which are non-superimposable mirror images of each other, may constitute more than 25% of currently used pesticides worldwide [1,2]. Enantiomers of chiral pesticides have identical physico-chemical properties, and transport processes or abiotic reactions appear not changing enantiomers proportions [3]. In contrast, degradation of chiral pesticides may be mainly microbiologically-mediated and enantioselective [4][5][6].…”

Chiral pollutants and their significance in the environment

“…Söz konusu bu çalışmalarda, kiral bileşikler rasemik (henüz yeni kullanılmış veya bozunma proseslerinden etkilenmeden korunmuş) ve rasemik olmayan (mikrobiyal bozunmaya uğramış veya uzun süredir çevresel ortamda bulunan) oranları kullanılarak kirleticilerin taşınım mekanizmalarının ve akıbetlerinin değerlendirilebileceği öngörülmektedir [93], [102].…”