The production of ozone in the troposphere is closely related to the reactions of two important families of radicals: the HOx family (OH+HO2) and the NOx family (NO+NO2). These radicals have very short lifetimes due to fast interconversion reactions, e.g. CO+OH(+O2) --> CO2+HO2. The main production process for the HOx family is the photolysis of O3 to the excited state of the oxygen atom, O(1D), followed by the reaction of O(1D) with water vapor. The main source of NOx is provided by anthropogenic emissions, biomass burning, soils, and lightning. When NO2 photolyses, it produces an oxygen atom O which reacts immediately with molecular oxygen to form ozone. However NO2 is mostly produced by the NO+O3 reaction which consumes one ozone molecule. Net ozone production is possible only when NO is converted to NO2 without consuming ozone, i.e. by the reaction of NO with peroxy radicals (HO2 and the organic peroxy radicals generated in the oxidation of volatile organic compounds (VOC).