Abstract
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.
Highlights
Ultrafine particles, defined here as smaller than 100 nm in diameter, constitute the largest number of particles in the atmosphere and are of interest owing to their disproportionate influence on climate and human health (Bzdek et al, 2012; Zhang et al, 2012)
The greatest uncertainty associated with particle growth and its impact on radiative forcing is the contribution of secondary organic matter (Carslaw et al, 2013), which is formed by oxidation of volatile compounds in the gas phase followed by subsequent migration of the products to the particle phase
The trend of decreasing O / C ratio with increasing particle size is consistent with the expectation that lower-volatility molecules are preferentially found in small particles, where the high surface-tovolume ratio favors condensation of non-volatile molecules over partitioning of semi-volatile molecules, while highervolatility molecules are preferenwww.atmos-chem-phys.net/17/7593/2017/
Summary
Ultrafine particles, defined here as smaller than 100 nm in diameter, constitute the largest number of particles in the atmosphere and are of interest owing to their disproportionate influence on climate and human health (Bzdek et al, 2012; Zhang et al, 2012). Processes limited by the amount of available volume, such as partitioning, are favored in larger particles, where the surface-to-volume ratio is low Superimposed on these dependencies is the radiusof-curvature (Kelvin) effect on molecular volatility, which favors the incorporation of lower-volatility species into smaller-diameter particles. This dissimilarity could arise from subsequent reaction of ELVOCs after they enter the particle phase, or by the formation of completely new oligomers in the particle phase through accretion chemistry In principle, these two sources can be distinguished through the size dependence of particle composition, since molecular species derived from condensation (surface area limited) should be more strongly represented in smaller particles while those derived from accretion chemistry (volume limited) should be more strongly represented in larger particles. The results provide direct evidence for accretion chemistry as a significant source of oligomers in SOA from this precursor
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