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 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 contributing significantly. Additionally, positive saturation anomalies and correlations with chlorophyll a indicated the marine biological source for atmospheric CHCl3 and C2HCl3. However, the atmospheric concentration variability of these gases was not fully consistent with oceanic emissions indicators (saturation anomalies and sea-air fluxes) and showed strong correlations with terrestrial tracers, indicating that land-derived atmospheric transport primarily influenced atmospheric CHCl3 and C2HCl3. The estimated sea-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 14.3 ± 6.8 % of CCl4 emissions from Eastern China, 5.6 ± 2.5 % from Eastern Asia, and 2.1 ± 1.1 % of global emissions. Additionally, this region accounted for 6.3 ± 2.8 % of the global oceanic absorption of CCl4. These findings underscore 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.