The chemical compositions of ambient PM2.5 samples, collected in Research Triangle Park, North Carolina, USA, and a sample of secondary organic aerosol, formed by irradiating a mixture of the sesquiterpene (SQT), _-caryophyllene, and oxides of nitrogen in a smog chamber, were chemically analyzed using derivative-based GC-MS methods. The analyses showed the presence of an oxidized compound, tentatively identified as _-caryophyllinic acid, in both the ambient PM2.5 field samples and in the smog chamber sample. The seasonal concentrations of _-caryophyllinic acid in the ambient PM2.5 samples were 0.5, 0.9, 7.0, and 0.5 ng m-3 during the winter, spring, summer and fall respectively. To our knowledge, this is the first time that an oxidation product of a sesquiterpene, a hydrocarbon with high secondary organic aerosol yields and emitted from plants and trees in significant quantities, has been detected in ambient PM2.5 samples.