Observation of Concurrent Latitudinal and Longitudinal Fractionations of α- and β-HCH in Chinese Rural Surface Soil: Implications for Global Lifecycle

In this study, for the first time, the distribution and fractionation of persistent organic pollutants (POPs) along both latitudinal and longitudinal transects are investigated simultaneously in surface soil. Distributions and fractionations of two isomers of hexachlorocyclohexane (HCH), α- and β-HCH, were analyzed in Chinese rural surface soil using a set of soil samples collected in 2005, 2012, and 2019 to study the lifecycle of the global/latitudinal fractionation of POPs. The primary longitudinal and secondary latitudinal distributions for each of α- and β-HCH and the primary longitudinal and secondary latitudinal fractionations formed together by these two isomers were observed concurrently in these three years. Based on these observations, we identified three stages for the lifecycle of global/latitudinal fractionation for α- and β-HCH. In Stage I, the primary fractionation of the two isomers mainly due to the distance from the source regions begins and intensifies with the fractionation pattern of the enrichment of the compounds having lower K_OA (octanol-air partition ratio) values and the depletion of the compounds with higher K_OA values. In Stage II, the secondary fractionation caused mainly by the air temperature gradient along latitudinal directions begins and intensifies, keeping the fractionation patterns appearing in the Stage I unchanging. In Stage III, the secondary fractionation is weakened primarily caused by local factors, degradation in particular, until its disappearance. The fractionation patterns appearing in the Stages I and II may change. Although the present study on the three stages for the development of latitudinal fractionation is for α- and β-HCH in Chinese surface soil. We believe that the findings presented in this study can provide instructive implications for the lifecycle of all polar cold-trapped persistent organic pollutants (PCT‑POPs) on a global scale.