Modeling historical budget for β-Hexachlorocyclohexane (HCH) in the Arctic Ocean: A contrast to α-HCH

Abstract

The historical annual loading to, removal from, and cumulative burden in the Arctic Ocean for β-hexachlorocyclohexane (β-HCH), an isomer comprising 5-12% of technical HCH, is investigated using a mass balance box model from 1945 to 2020. Over the 76 years, loading occurred predominantly through ocean currents and river inflow (83%) and only a small portion via atmospheric transport (16%). β-HCH started to accumulate in the Arctic Ocean in the late 1940s, reached a peak of 810 t in 1986, and decreased to 87 t in 2020, when its concentrations in the Arctic water and air were ∼30 ng m^-3 and ∼0.02 pg m^-3, respectively. Even though β-HCH and α-HCH (60-70% of technical HCH) are both the isomers of HCHs with almost identical temporal and spatial emission patterns, these two chemicals have shown different major pathways entering the Arctic. Different from α-HCH with the long-range atmospheric transport (LRAT) as its major transport pathway, β-HCH reached the Arctic mainly through long-range oceanic transport (LROT). The much higher tendency of β-HCH to partition into the water, mainly due to its much lower Henry's Law Constant than α-HCH, produced an exceptionally strong pathway divergence with β-HCH favoring slow transport in water and α-HCH favoring rapid transport in air. The concentration and burden of β-HCH in the Arctic Ocean are also predicted for the year 2050 when only 4.4-5.3 t will remain in the Arctic Ocean under the influence of climate change.

Keywords: Air-water exchange; Arctic Ocean; Budget; Mass balance model; β-Hexachlorocyclohexane.

Graphical abstract

Fig. 1

a, Spatial design of AMBBM 2.0. b, Temporal design of AMBBM 2.0.

Fig. 2

a, Annual concentrations of α-HCH in the Arctic air predicted by both models (Blue line: results provided by AMBBM 1.0; pink line: results provided by AMBBM 2.0; grey dots: monitoring data [32] (https://ebas.nilu.no/). b, Annual water concentrations of α-HCH in the Arctic Ocean predicted by AMBBM 1.0 and 2.0 with the monitoring data. (Sources of the monitoring data from Li et al. [10] and Wang et al. [33]). c, Annual gas-ocean exchange of α-HCH in the Arctic from 1945 to 2000 predicted by both models.

Fig. 3

Annual inputs of α-HCH in the Arctic Ocean through ocean currents (a) and river inflows (b) from 1945 to 2000 predicted by AMBBM 1.0 and 2.0.

Fig. 4

The annual budgets of α-HCH in the Arctic Ocean are predicted by AMBBM 1.0 (left) and AMBBM 2.0 (right).

Fig. 5

Loading to, removal from, and burden of α-HCH in the Arctic waters: a, AMBBM 1.0; b, AMBBM 2.0. The square indicates the estimated burden of 2910 t in the early 1990s by Macdonal et al. [7] and the round indicates the estimated burden of 7680 t in the early 1990s by Wania and Mackay [14].

Fig. 6

a, Air concentration of β-HCH [32] (Monitoring data: https://ebas.nilu.no/). b, Ocean concentration of β-HCH (Sources of the monitoring data: Li et al. [16] and Cai et al. [36]). c, Soil concentration of β-HCH (Sources of the monitoring data: Casal et al. [37]).

Fig. 7

a, Distribution of β-HCH's burdens in the Arctic air, water, soil, sea ice, and snowpack; b, β-HCH's burden in the Arctic Ocean.

Fig. 8

a–c, Loading and removal of β-HCH in the ocean: total period (a), accumulation period (AP, b), and decay period (DP, c). d–e, Loading and removal of β-HCH in the total period in the air (d) and soil (e).

Fig. 9

Burden of α-HCH and β-HCH after normalization.

Fig. 10

Predicted concentration of β-HCH and α-HCH in the Arctic Ocean. The pink lines represent β-HCH; the blue lines represent α-HCH; the solid lines represent scenario 1, and the dashed lines represent scenario 2.