The toluene/benzene, (m, p)-xylene/benzene, o xylene/benzene and(m, p) xylene/ethylbenzene concentration ratios are useful for characterizing the distance of vehicular emission sources and for estimating the photochemical age of the air mass The initial step in the production of ozone is the reaction between an NMHC and the hydroxyl radical(OH). Therefore, the relative contributions of individual NMHC species to ozone formation vary depending on each compound's rate of reaction with OH. The oxi- dation process usually starts with reaction[1], from OH attack on a VOC(RH):