Impact of anticyclonic eddies on the spatial distribution and emission of non-methane hydrocarbons in the northern South China Sea

Non-methane hydrocarbons (NMHCs) are important atmospheric trace active gases that exert significant impacts on global climate. Ubiquitous mesoscale eddies likely act as a key physical process regulating the marine emission of these gases, yet the underlying mechanisms remain unclear. This study investigated the spatial distributions and emissions of NMHCs in the South China Sea during the summer of 2021, with particular emphasis on the impacts of an anticyclonic eddy. The distributions of NMHCs were significantly regulated by the anticyclonic eddy, with substantially lower concentrations observed within the eddy-controlled region (201 ± 101 pmol L−1) compared to the reference sites (433 ± 62.5 pmol L−1). Alkanes and isoprene exhibited a pronounced tendency for subduction within the eddy core, which can be attributed to the convergence and downwelling of upper-layer water associated with the anticyclonic disturbance. This physical process reduces surface nutrient availability, consequently suppressing phytoplankton growth and the biological production of alkanes and isoprene. In contrast, C2–C4 alkenes showed no subsurface accumulation, as their photochemical production was constrained by insufficient light radiation. Relative to the reference sites, diminished photochemical production of C2–C4 alkenes within the eddy core was identified as a crucial mechanism responsible for the lower concentrations of C2–C4 alkenes. Furthermore, the sea-to-air fluxes of NMHCs were reduced by 56% in the regions controlled by the anticyclonic eddy compared to the reference sites. This reduction further decreased the contribution of ocean‑emitted NMHCs to atmospheric ozone and secondary organic aerosol formation by 59% and 60%, respectively, indicating that the anticyclonic eddy weakens the negative climate feedback associated with marine NMHC emission. Overall, our findings elucidate how mesoscale eddies regulate the distribution and emissions of NMHCs, highlighting the pivotal role of mesoscale physical processes in shaping marine trace gas cycling and its environmental consequences. Consequently, oceanic mesoscale processes should be explicitly accounted for when assessing global emissions of marine trace gases and their impacts.