Hydrocarbons are a class of naturally occurring or man-made liquid or gaseous organic compounds that consist of primarily carbon and hydrogen in an arrangement in which the hydrogen is attached to a carbon framework in either a chain or ring. The densities, boiling points and freezing points of hydrocarbons increase with their molecular weight, making prediction of their chemical and physical behavior possible. Types of hydrocarbons include aliphatic, aromatic, chlorinated, nonmethane, petroleum and polyaromatic. They are the major constituents of fossil fuels including petroleum, natural gas and coal. Their combustion byproducts contribute to air pollution by reacting with nitrogen oxides and sunlight to form ozone. Naturally occurring hydrocarbons are emitted by trees and produced by plankton and various decomposition processes.