To better understand the sources of organic PM and the effect of secondary organic aerosols (SOA) on PM levels, samples of fine particulate matter were collected outside Durham, NC in the Duke Research Forest as part of the CELTIC study in July 2003. Among compounds analyzed, oxidation products of α-pinene, namely pinic acid and pinonic acid, were identified in all samples. Pinic acid and pinonic acid have low vapor pressures, of the order of 10 -7 Torr, and are expected to contribute significantly to secondary organic aerosol (SOA) formation from the oxidation of α-pinene (Koch et al. , 2000). Source contribution estimates from primary organic aerosol emissions were computed using measured organic species as molecular markers with the chemical mass balance (CMB) model. The unapportioned organic carbon (OC) was determined as the difference between measured OC and OC apportioned to primary sources. This unapportioned OC was then correlated with pinic and pinonic acid. The correlations between unapportioned OC and pinic and pinonic acids were r 2 = 0.33 (p-value = 0.64) and r 2 = 0.42 (p-value = 0.16), respectively. Given the large number of possible SOA precursors this moderate to good fit between pinic acid and pinonic acid concentrations with unapportioned OC is indicative of the major contribution of α—pinene oxidation products to secondary formation at this sampling site. The results are significant considering the role of monoterpene emissions to global atmospheric chemistry. These results were used as a basis of a second study investigating the use of pinic acid and pinonic acid as molecular markers for secondary formation using source apportionment. This hypothesis was explored for an urban and a rural site in Texas as part of the second sampling campaign, the Texas Air Quality (TexAQS) II study. In the TexAQS II study, fine PM sources were first characterized more broadly, creating a framework to evaluate the effectiveness of pinic and pinonic acid as SOA markers. Five factors were identified for both Dallas and San Augustine with differing contributions to PM 2.5 mass and OC. The factors identified are (1) motor vehicles, including gasoline and diesel-powered vehicles, (2) wood combustion, (3) secondary organic aerosols (SOA) identified using pinic and pinonic acids as markers, (4) plant wax and (5) meat cooking. PMF parameters and residuals were within the acceptable Q-values, indicating valid model performance. The Chemical Mass Balance (CMB) model was used to estimate sources contributions to OC in Dallas and San Augustine airsheds. These source contribution estimates were in turn used to calculate the unapportioned OC based on OC contributions to each source. A strong correlation was observed between measured ambient concentrations and concentrations calculated using CMB. The average r 2 of 0.74 and chi-squared value of 7.8 was acceptable. Mobile sources were a major contributor at Dallas, which is expected for an urban site, followed by meat cooking, plant wax and wood combustion. In these two studies, pinic acid and pinonic acid were effective as molecular markers for secondary formation in North Carolina where α-pinene emissions were dominant among all monoterpenes, but not in Dallas and San Augustine, where other sources dominated SOA mass. (Abstract shortened by UMI.)