particulate matter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Material suspended in the air in the form of minute solid particles or liquid droplets, especially when considered as an atmospheric pollutant.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any solid or liquid in a finely subdivided state, especially when considered as an atmospheric pollutant.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emissions (usually considered to be an atmospheric pollutant)