The Carbonaceous Aerosols and Radiative Effects Study (CARES) took place in the Sacramento Valley of California in summer 2010. We present results obtained at Cool, CA, the T1 site of the project (~40 km downwind of urban emissions from Sacramento), where we deployed an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) in parallel with complementary instrumentation to characterize the sources and processes of submicron particles (PM₁). Cool is located at the foothill of the Sierra Nevada Mountains, where intense biogenic emissions are periodically mixed with urban outflow transported by daytime southwesterly winds from the Sacramento metropolitan area. The particle mass loading was low (3.0 μg m⁻³ on average) and dominated by organics (80 % of the PM₁ mass) followed by sulfate (9.9 %). Organics and sulfate appeared to be externally mixed, as suggested by their different time series (r² = 0.13) and size distributions. Sulfate showed a bimodal distribution with a droplet mode peaking at $\sim400$ nm in vacuum aerodynamic diameter (D_va), and a condensation mode at ~150 nm, while organics generally displayed a broad distribution in 60–600 nm (D_va). New particle formation and growth events were observed almost every day, emphasizing the roles of organics and sulfate in new particle growth, especially that of organics. The organic aerosol (OA) had a~nominal formula of C₁H_1.38N_0.004O_0.44, thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Two different oxygenated OA (OOA, 90 % of total OA mass) and a hydrocarbon-like OA (HOA, 10 %) were identified by Positive matrix factorization (PMF) of the high-resolution mass spectra. The more oxidized MO-OOA (O/C = 0.54) corresponded to secondary OA (SOA) primarily influenced by biogenic emissions, while the less oxidized LO-OOA (O/C = 0.42) corresponded to SOA associated with urban transport. The HOA factor corresponded to primary emissions mainly due to local traffic. Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool) were identified using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The average PM₁ mass loading was much higher in urban plumes (3.9 μg m⁻³) than in air masses dominated by biogenic SOA (1.8 μg m⁻³). The change in OA mass relative to CO (Δ OA/Δ CO) varied in the range of 5–196 μg m⁻³ ppm⁻¹, reflecting large variability in SOA production. The highest Δ OA/Δ CO were reached when urban plumes arrived at Cool in the presence of a~high concentration of biogenic volatile organic compounds (BVOCs = isoprene + monoterpenes + 2-methyl-3-buten-2-ol [MBO] + methyl chavicol). This ratio, which was 77 μg m⁻³ ppm⁻¹ on average when BVOCs > 2 ppb, is much higher than when urban plumes arrived in a low biogenic VOCs environment (28 μg m⁻³ ppm⁻¹ when BVOCs < 0.7 ppb) or during other periods dominated by biogenic SOA (40 μg m⁻³ ppm⁻¹). The results from this study demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors.