Persistent organic pollutants in global surface soils: Distributions and fractionations

Abstract

The distribution and fractionation of persistent organic pollutants (POPs) in different matrices refer to how these pollutants are dispersed and separated within various environmental compartments. This is a significant study area as it helps us understand the transport efficiencies and long-range transport potentials of POPs to enter remote areas, particularly polar regions. This study provides a comprehensive review of the progress in understanding the distribution and fractionation of POPs. We focus on the contributions of four intermedia processes (dry and wet depositions for gaseous and particulate POPs) and determine their transfer between air and soil. These processes are controlled by their partitioning between gaseous and particulate phases in the atmosphere. The distribution patterns and fractionations can be categorized into primary and secondary types. Equations are developed to quantificationally study the primary and secondary distributions and fractionations of POPs. The analysis results suggest that the transfer of low molecular weight (LMW) POPs from air to soil is mainly through gas diffusion and particle deposition, whereas high molecular weight (HMW) POPs are mainly via particle deposition. HMW-POPs tend to be trapped near the source, whereas LMW-POPs are more prone to undergo long-range atmospheric transport. This crucial distinction elucidates the primary reason behind their temperature-independent primary fractionation. However, the secondary distribution and fractionation can only be observed along a temperature gradient, such as latitudinal or altitudinal transects. An animation is produced by a one-dimensional transport model to simulate conceptively the transport of CB-28 and CB-180, revealing the similarities and differences between the primary and secondary distributions and fractionations. We suggest that the decreasing temperature trend along latitudes is not the major reason for POPs to be fractionated into the polar ecosystems, but drives the longer-term accumulation of POPs in cold climates or polar cold trapping.

Keywords: POPs; Primary and secondary distribution patterns; Primary and secondary emissions; Primary and secondary fractionations; Primary and secondary sources.

Graphical abstract

Fig. 1

Fluxes for the major processes (particle deposition, gaseous diffusion, and rain and snow scavenging) for CB-28 and CB-180 as a function of temperature from −30 to 30 °C. The two vertical dashed yellow lines are the temperature thresholds for CB-180, which divide the temperature into three domains (ER, NE, and MP domains) for CB-180 (Assuming TSP = 50 μg m⁻³, C_A + C_P = 100 pg m³, which are concentrations of CB-28 or CB-180 in the gas phase and particle phase).

Fig. 2

BDE-209 concentrations in the soil from a BFR manufacturing plant as a function of distance, showing pulse distribution patterns. a, One-dimensional graph. b, Three-dimensional graph. Data are taken from Li et al. [24].

Fig. 3

a–d, Longitudinal distributions and fractionations based on data from China. a, PCB usage versus longitude. b, PCB concentration versus PCB usage. c, PCB concentration versus longitude. d, Relative compositions of PCBs versus longitude. e–h, Latitudinal distributions and fractionations based on data from Norway. e, PCB usage versus latitude. f, PCB concentration versus PCB usage. g, PCB concentration versus latitude. h, Relative compositions of PCBs versus latitude. Note: PCB concentration data in Chinese soil are taken from Ren et al. [28]; PCB concentration data in Norwegian soil are taken from Meijer et al. [27]; PCB usage data in China are taken from Zhang et al. [72]; and PCB usage data in Norway are taken from Breivik et al. [3,4].

Fig. 4

The total usages and measured soil concentrations of α-HCH in 2005–2006 at five sites from the south (30.1° N) to the north (51.5° N) of China. Data are taken from Liu et al. [40].

Fig. 5

The relative composition of PCB homologs in soil samples at sites U0, R1, R2, and R3. All the values were normalized to the percent composition for Site U0, where all values are unity. Data are taken from Ren et al. [28].

Fig. 6

Latitude distribution of CB-28 and CB-52, showing a secondary fractionation pattern. Data are taken from Wang et al. [73].

Fig. 7

Strong and significant correlation between temperature and latitude from the southern UK to northern Norway. Data are taken from Meijer et al. [27].

Fig. 8

The relative concentrations of α-HCH and β-HCH in the surface water of the Pacific Ocean (a) and the Western Arctic Ocean (b) measured between 1988 and 1999. Although these data span ten years and are collected from a wide range of latitudes, they illustrate well the general increase in α-HCH in cool, northern waters with exceptionally high values under the pack ice of the Canada Basin in contrast to β-HCH, which shows the highest concentrations centered on the Bering Strait (∼65° N). Data are taken from Li et al. [106].