A series of smog chamber (SC) experiments was conducted to identify driving factors responsible for the discrepancy between ambient and SC aerosol degree of oxygenation. An Aerodyne high resolution time-of-flight aerosol mass spectrometer is used to compare mass spectra from α-pinene photooxidation with ambient aerosol. Composition is compared in terms of the fraction of organic mass measured at <i>m/z</i> 44 (<i>f</i><sub>44</sub>), a surrogate for carboxylic/organic acids as well as the atomic oxygen-to-carbon ratio (O : C), vs. <i>f</i><sub>43</sub>, a surrogate for aldehydes, alcohols and ketones. Low (near-ambient) organic mass concentrations were found to be necessary to obtain oxygenation levels similar to those of low-volatility oxygenated organic aerosol (LV-OOA) commonly identified in ambient measurements. The effects of organic mass loading and OH (hydroxyl radical) exposure were decoupled by inter-experiment comparisons at the same integrated OH concentration. On average, an OH exposure of 2.9 ± 1.3 × 10<sup>7</sup> cm<sup>−3</sup> h is needed to increase <i>f</i><sub>44</sub> by 1% during aerosol aging. For the first time, LV-OOA-like aerosol from the abundant biogenic precursor α-pinene was produced in a smog chamber by oxidation at typical atmospheric OH concentrations. Significant correlation between measured secondary organic aerosol (SOA) and reference LV-OOA mass spectra is shown by Pearson's <i>R</i><sup>2</sup> values larger than 0.90 for experiments with low organic mass concentrations between 1.5 and 15 μg m<sup>−3</sup> at an OH exposure of 4 × 10<sup>7</sup> cm<sup>−3</sup> h, corresponding to about two days oxidation time in the atmosphere, based on a global mean OH concentration of ∼1 × 10<sup>6</sup> cm<sup>−3</sup>. Not only is the α-pinene SOA more oxygenated at low organic mass loadings, but the functional dependence of oxygenation on mass loading is enhanced at atmospherically-relevant precursor concentrations. Since the degree of oxygenation influences the chemical, volatility and hygroscopic properties of ambient aerosol, smog chamber studies must be performed at near-ambient concentrations to accurately simulate ambient aerosol properties.