What does the mean of the fine particulate matters

Particulate matter 2.5 known as the particle size of the atmosphere, which is less than or equal to 2.5 microns in diameter.
Its radius is less than the thickness of human hair silk 1/400.
PM2.5 is only a few of the components of the earth's atmosphere,
but has important influence on air quality and visibility.
We use handheld particle counter to check the air quality and know what kind of air we breath.
This way we can know how to change air quality under the help of the particle counter.