This study investigates the spatiotemporal evolution of hypopycnal intrusive particle-laden flows to determine how particle concentration and size regulate flow instabilities, horizontal propagation, and vertical transport. These processes are directly related to the effectiveness of ocean alkalinity enhancement (OAE), where alkaline mineral particles are added to the ocean surface. Using high-resolution pseudospectral simulations, we systematically vary particle concentration and diameter to quantify the entrained area and effective settling velocity. Following particle release, the flow dynamics is governed by the competition between horizontal spreading and vertical sedimentation processes. We find that the terminal propagation distance decreases monotonically with increasing the particle concentration and diameter. This terminal distance is limited by collective vertical sedimentation driven by convective instabilities at the base of the sediment layer. Consequently, the effective vertical particle settling speed can exceed the Stokes settling speed of an individual particle by one to two orders of magnitude. The settling enhancement factor, defined as the ratio of the effective settling velocity to the Stokes settling velocity, reaches up to 100 for fine particles (i.e., particle diameter $5$µm) and increases with particle concentration. However, a large fraction of particles in the upper part of the particle-laden flow still settle according to the Stokes settling velocity, as they are not influenced by convective instabilities. This leads to a bulk centroid velocity that is much smaller than that of the vertical sedimentation front driven by convective instabilities. We categorize the flow evolution into three morphological regimes, governed by the balance between spreading and settling. We find that intermediate particle concentration and size can maximize the surface entrainment area and homogeneity, offering an optimal configuration for OAE. These results demonstrate that collective instabilities, rather than individual Stokes settling, control particle residence times, potentially limiting the carbon removal efficiency of surface-release OAE strategies.

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• Geophysical fluid dynamics
• Mixing in geophysical flows
• Sediment transport
• Stratified geophysical flows