Loading-dependent elemental composition of α-pinene SOA particles

Abstract

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+2) signal intensity exceeded that of m/z 43 (predominantly C2H3O+), 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 (CxHyO+z+) monotonically increased from 48% to 54%. Correspondingly, the mass percentage of fragments representing CxH+y+ 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 C1H1.32O0.48, C1H 1.36O0.39, C1H1.57O0.24, and C1H1.76O0.14. These findings suggest considerable caution is warranted in the extrapolation of laboratory results that were obtained under conditions of relatively high loading (i.e., >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.