The product of a VOC’s concentration and its OH rate constant wiltdetermine in an approximate manner its relative role as an ozone precursor.• A species with a large concentration will not necessarily be an important Oprecursor if it is unreactive conversely, another compound with a smallconcentration can be important if it is extremely reactive.• An airmass can have a large total VOC concentration but a lowozoneproducing capacity if the VOCs present are relatively unreactive.