Atmospheric uptake is reduced if modified seawater (e.g. seawater with enhanced alkalinity or depleted dissolved organic carbon) is transferred to the deep ocean before either increasing the uptake of atmospheric CO₂ or decreasing the outgassing of CO₂ to the atmosphere. The precise dispersal of modified seawater, its residence time at the surface, and hence the associated completeness of air-sea gas equilibration is dependent on the location and timing of the modification, and is hard to directly observe. In-situ measurements can potentially track post-deployment changes in pCO₂, pH, and DIC and deploy water mass tracers to help validate models of plume dispersal and atmospheric CO₂ drawdown, though the sensitivity and cost of sensors may be a challenge. Further, oceanographic models can be used to develop probabilistic estimates of equilibration efficiencies for particular injection sites, contingent on seasonal or interannual variations in forcing regimes. Typical gas exchange timescales of equilibration for the surface ocean and atmosphere are on the order of months to years.