Articles | Volume 17, issue 12
Atmos. Chem. Phys., 17, 7593–7603, 2017
Atmos. Chem. Phys., 17, 7593–7603, 2017

Research article 22 Jun 2017

Research article | 22 Jun 2017

Particle size dependence of biogenic secondary organic aerosol molecular composition

Peijun Tu and Murray V. Johnston Peijun Tu and Murray V. Johnston
  • Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA

Abstract. Formation of secondary organic aerosol (SOA) is initiated by the oxidation of volatile organic compounds (VOCs) in the gas phase whose products subsequently partition to the particle phase. Non-volatile molecules have a negligible evaporation rate and grow particles at their condensation rate. Semi-volatile molecules have a significant evaporation rate and grow particles at a much slower rate than their condensation rate. Particle phase chemistry may enhance particle growth if it transforms partitioned semi-volatile molecules into non-volatile products. In principle, changes in molecular composition as a function of particle size allow non-volatile molecules that have condensed from the gas phase (a surface-limited process) to be distinguished from those produced by particle phase reaction (a volume-limited process). 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 analyzer, and individual particle sizes between 35 and 110 nm in diameter were characterized by on- and offline 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 primarily in the particle phase (accretion reactions, which become more favored as the volume-to-surface-area ratio of the particle increases). Analysis of a series of polydisperse SOA samples showed similar dependencies: as the mass loading increased (and average volume-to-surface-area ratio increased), 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.

Short summary
In this study, we determined the particle-size-dependent molecular composition of secondary organic aerosol (SOA) that was produced from β-pinene, an important biogenic precursor. We find that the composition changes significantly with particle size, and these changes can be linked to changes in the chemical processes that contribute to particle growth. Measurements of this type can aid the modeling and prediction of SOA formation.
Final-revised paper