suspended matter

in atmospheric chemistry
https://doi.org/10.1351/goldbook.S06197
All particulate material which persists in the atmosphere or in a flue gas stream for lengthy periods because the particles are too small in size to have an appreciable falling velocity.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2217 (https://doi.org/10.1351/pac199062112167)