The chemical composition of secondary organic aerosol (SOA) particles, formed by the dark ozonolysis of αpinene, was characterized by a high-resolution time-of-flight aerosol mass spectrometer. The experiments were conducted using a continuous-flow chamber, allowing the particle mass loading and chemical composition to be maintained for several days. The organic portion of the particle mass loading was varied from 0.5 to >140 µg/m3 by adjusting the concentration of reacted α-pinene from 0.9 to 91.1 ppbv. The mass spectra of the organic material changed with loading. For loadings below 5 µg/m3 the unit-mass-resolution m/z 44 (CO+ ) signal intensity exceeded that of m/z 43 (predomi2 nantly C2 H3 O+ ), suggesting more oxygenated organic material at lower loadings. The composition varied more for lower loadings (0.5 to 15 µg/m3 ) compared to higher loadings (15 to >140 µg/m3 ). The high-resolution mass spectra showed that from >140 to 0.5 µg/m3 the mass percentage of fragments containing carbon and oxygen (Cx Hy O+ ) monoz tonically increased from 48% to 54%. Correspondingly, the mass percentage of fragments representing Cx H+ decreased
y from 52% to 46%, and the atomic oxygen-to-carbon ratio increased from 0.29 to 0.45. The atomic ratios were accurately parameterized by a four-product basis set of decadal volatility (viz. 0.1, 1.0, 10, 100 µg/m3 ) employing products having empirical formulas of C1 H1.32 O0.48 , C1 H1.36 O0.39 , C1 H1.57 O0.24 , and C1 H1.76 O0.14 . These findings suggest considerable caution is warranted in the extrapolation of laboratory results that were obtained under conditions of relatively high loading (i.e., >15 µg/m3 ) to modeling applications relevant to the atmosphere, for which loadings of 0.1 to 20 µg/m3 are typical. For the lowest loadings, the particle mass spectra resembled observations reported in the literature for some atmospheric particles.