The composition of PM can vary considerably from one region to another depending on the sources of particulate emissions in each region.
The PM's composition may vary depending on the sources of emissions.
The composition of PM never varies, they are constant.
a man driving a car
A man plays poker with his friends.
A woman helps a child.
Group of men outside looking.
Parker studied if there are spendthrifts in America.
A boy waits outside.