The m,p-xylenes

Measurement of the relative concentrations of the aromatic isomers m, p-xylenes and ethylbenzene is proposed as the basis of a technique for estimating hydrocarbon age in ambient atmospheres. These compounds are shown to be present in constant proportion in the major anthropogenic sources of hydrocarbon emission in Sydney and this is probably also the case in at least some other urbanized regions. Differences in photochemical reactivity cause the isomers to disappear from the atmosphere at notably different rates, a fact which is confirmed by experiments with large outdoor smog chambers. Application of the technique is illustrated by field measurements designed to clarify the importance of recirculation of previous day's emissions on the formation of photochemical smog in the Sydney area.