Influence of oceanic ventilation and terrestrial transport on the atmospheric volatile chlorinated hydrocarbons over the Western Pacific

Abstract. Volatile chlorinated hydrocarbons (VCHCs), key ozone-depleting substances and greenhouse gases, depend on oceanic emission and uptake for their atmospheric budget. However, data on VCHCs in the Western Pacific remain limited. This study investigated the distribution and sources of VCHCs (CHCl₃, C₂HCl₃, CCl₄, and CH₃CCl₃) in the Western Pacific during 2019–2020. Elevated seawater concentrations of CHCl₃ and C₂HCl₃ in the Kuroshio-Oyashio Extension (KOE) were driven by mesoscale eddies enhancing primary productivity, while CCl₄ and CH₃CCl₃ concentrations were mainly influenced by atmospheric inputs. Atmospheric concentrations of VCHCs decreased from coastal to open ocean areas, with terrestrial air masses from Eastern Asia contributing significantly. Additionally, atmospheric CHCl₃ and C₂HCl₃ concentrations were positively correlated with Chl-a in the KOE region. These findings suggested that both atmospheric transport from the continent and ocean emissions could influence CHCl₃ and C₂HCl₃ levels. However, analysis of sea-to-air fluxes and saturation anomalies showed that atmospheric transport primarily influenced atmospheric CHCl₃ and C₂HCl₃ concentrations. The estimated sea-to-air flux indicated that the Western Pacific acted as a source for CHCl₃ and C₂HCl₃ but a sink for CCl₄ and CH₃CCl₃, with the potential to absorb 17 ± 2 % of CCl₄ emissions from Eastern China, 7 ± 5 % from Eastern Asia, and 3 ± 1 % of global emissions. Additionally, this region accounted for 8 ± 4 % of the global oceanic absorption of CCl₄. These findings underscored the Western Pacific’s key role in regulating atmospheric CCl₄ concentrations and mitigating its accumulation in Eastern Asia, providing essential data for global VCHCs emission and uptake estimates.