Bridging advection and diffusion in the encounter dynamics of sedimenting marine snow - Oceans are vital carbon sinks, absorbing approximately 30% of anthropogenic carbon emissions. A fraction of this carbon settles to the seafloor, reducing its presence in the short-term global cycle. The primary drivers of this vertical transport are sinking aggregates of dead phytoplankton, known as marine snow. To predict the sedimentation dynamics, it is first necessary to understand and quantify the processes affecting the aggregates.  
 
One of the key physical processes is the encounter between sinking particles. Collisions between such particles promote further aggregation, increasing their sedimentation velocity, while interactions with free-floating bacteria enhance dissolution, potentially slowing their descent. Such collision rates are typically calculated using two types of models focusing either on direct (ballistic) interception with a finite interaction range, or advective-diffusive capture with a zero interaction range. Since the range of applicability of the two models is unclear, and many relevant marine encounter scenarios span across both regimes, quantifying such encounters remains challenging, as the two approaches yield asymptotically different predictions. Consequently, many existing studies either neglect one of these processes or simply superimpose them, raising concerns about their accuracy. 
 
Here, we present a systematic approach to modeling collision rates as a function of particle size and Péclet number. Interestingly, we find that the model for pure direct interception, even under conditions that would suggest its applicability, can lead to errors of up to two orders of magnitude.