All substances in ambient air exist as either particulate matter, gases, or vapors. In general, the distinction is easily made; gases and vapors consist of substances dispersed as molecules in the atmosphere, while particulate matter consists of aggregates of molecules large enough to behave like particles. Particulate matter, or particulates, are filterable, can be precipitated, and can settle out in still air. By contrast, gases and vapors do not behave in this way and are homogeneously mixed with the air molecules.
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