A recently developed, organic tracer-based method was used to estimate the secondary contributions of biogenic and anthropogenic precursor hydrocarbons to ambient organic carbon concentrations in PM2.5 during 2006 in Research Triangle Park, North Carolina, USA. Forty-six ambient PM2.5 samples were collected on a one in six schedule and analyzed for (1) secondary organic aerosol tracer compounds, and (2) levoglucosan, a compound used as a tracer for biomass burning. For isoprene, α-pinene, β-caryophyllene, and toluene, the secondary contributions to ambient organic carbon concentrations (OC) were estimated using measured tracer concentrations and previously established, laboratory-determined mass fractions. The estimates show secondary formation from these four hydrocarbons contributes up to 55% of the ambient organic carbon concentrations (Julian day 197) when OC was 5.98 μg C/m3. The relative contributions are highly temperature dependent; estimates of particulate carbon from isoprene and α-pinene precursors peaked during the warmest days, and represented up to 40% and 10% of the measured OC, respectively (Julian days 197 and 191). Conversely, biomass burning represented up to 21% of the organic carbon concentrations on the coldest day sampled, Julian day 329, while contributions of secondary organic carbon from these four precursor hydrocarbons remained low at 4% of the measured 2.55 μg C/m3 OC.