https://doi.org/10.1351/goldbook.D01602
The ratio of flux density (often given in units of \(\text{g}\ \text{cm}^{-2}\ \text{s}^{-1}\)) of a substance at a sink surface to its concentration in the atmosphere (corresponding units of \(\text{g}\ \text{cm}^{-3}\)). While the units of this ratio are clearly those of velocity (in this case \(\text{cm}\ \text{s}^{-1}\)), the ratio is not a flow velocity in the normal sense of the word.