deposition velocity in atmospheric chemistry

The ratio of flux density (often given in units of $g cm −2 s −1$ ) of a substance at a sink surface to its concentration in the atmosphere (corresponding units of $g cm −3$ ). While the units of this ratio are clearly those of velocity (in this case $cm s −1$ ), the ratio is not a flow velocity in the normal sense of the word.

Source:

PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2184