Tropospheric air pollution has impacts on scales ranging from local to global, Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) play central roles : the hydroxyl radical (OH), during the day; the nitrate radical (NO3), at night; and ozone (O-3), which contributes during the day and night, Halogen atoms can also play a role during the day, Here the implications of the complex VOC-NOx chemistry for O-3 control are discussed, In addition, OH, NO3, and O-3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.