From AMS Glossary
Reaction of a substance with oxygen, or incorporation of oxygen into a molecule.
Examples are combustion and rust formation. In solution-phase ionic chemistry, oxidation is formally defined as a process that reduces the electron density at a given site. Since oxygen is a very electronegative species, incorporation of oxygen into a molecule results in a reduction in electron density. Due to the high proportion of oxygen, the atmosphere is intrinsically oxidative in nature, and photochemical degradation of pollutants usually involves an increased degree of oxygenation, and a consequent reduction in the number of hydrogen atoms in a molecule, for example,
Hence, the driving force in the atmosphere is from reduced emissions to oxidized products.