Loading-dependent elemental composition of α-pinene SOA particles
The chemical composition of secondary organic aerosol (SOA) particles, formed by the dark ozonolysis of α-pinene, 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 >140μg/m3 by adjusting the concentration of reacted-pinene from 0.9 to 91.1 ppbv. The mass spectra of the organic material changed with loading. For loadings below 5μg/m3 the unit-mass-resolution m/z 44 (CO<sup>+</sup><sub>2</sub>) signal intensity exceeded that of m/z 43 (predominantly C<sub>2</sub>H<sub>3</sub>O<sup>+</sup>), suggesting more oxygenated organic material at lower loadings. The composition varied more for lower loadings (0.5 to 15μg/m3) compared to higher loadings (15 to >140μg/m3). The high-resolution mass spectra showed that from >140 to 0.5μg/m3 the mass percentage of fragments containing carbon and oxygen (C<sub>xH</sub>yO<sup>+</sup><sub>z</sub>+) monotonically increased from 48% to 54%. Correspondingly, the mass percentage of fragments representing C<sub>x</sub>H<sup>+</sup><sub>y</sub>+ decreased from 52% to 46%, and the atomic oxygen-to-carbon ratioincreased 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, 100μg/m3) employing products having empirical formulas of C<sub>1</sub>H<sub>1.32</sub>O<sub>0.48</sub>, C<sub>1</sub>H <sub>1.36</sub>O<sub>0.39</sub>, C<sub>1</sub>H<sub>1.57</sub>O<sub>0.24</sub>, and C<sub>1</sub>H<sub>1.76</sub>O<sub>0.14</sub>. These findings suggest considerable caution is warranted in the extrapolation of laboratory results that were obtained under conditions of relatively high loading (i.e., >15μg/m3) to modeling applications relevant to the atmosphere, for which loadings of 0.1 to 20μg/m3 are typical. For the lowest loadings, the particle mass spectra resembled observations reported in the literature for some atmospheric particles.