Particle Size Dependence of Biogenic Secondary Organic Aerosol Molecular Composition
Peijun Tu and Murray V. Johnston
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA
Received: 20 Jan 2017 – Accepted for review: 27 Jan 2017 – Discussion started: 30 Jan 2017
Abstract. Formation of secondary organic aerosol (SOA) is initiated by the oxidation of volatile organic compounds (VOCs) in the gas phase. Mass transfer to the particle phase is thought to occur primarily by a combination of condensation of non-volatile products and partitioning of semi-volatile products, though particle phase chemistry may also play a role if it transforms semi-volatile reactants into non-volatile products. In principle, changes in particle composition as a function of particle size allow the relative contributions of e.g. condensation (a surface-limited process) and particle phase reaction (a volume-limited process) to be distinguished. In this work, SOA was produced by β-pinene ozonolysis in a flow tube reactor. Aerosol exiting the reactor was size-selected with a differential mobility analyser, and individual particle sizes between 35 and 110 nm in diameter were characterized by on- and off- line mass spectrometry. Both the average oxygen-to-carbon (O / C) ratio and carbon oxidation state (OSc) were found to decrease with increasing particle size, while the relative signal intensity of oligomers increased with increasing particle size. These results are consistent with oligomer formation in the particle phase i.e. accretion reactions, which become more favoured as the surface-to-volume ratio of the particle decreases. Analysis of a series of polydisperse SOA samples showed similar dependencies: as the mass loading increased (and average surface-to-volume ratio decreased), the average O / C ratio and OSc decreased while the relative intensity of oligomer ions increased. The results illustrate the potential impact that particle phase chemistry can have on biogenic SOA formation and the particle size range where this chemistry becomes important.
Tu, P. and Johnston, M. V.: Particle Size Dependence of Biogenic Secondary Organic Aerosol Molecular Composition, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-53, in review, 2017.