Particle size dependence of biogenic secondary organic aerosol molecular composition

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.

Nanoparticle growth by particle-phase chemistry

“…The overall conclusion that accretion chemistry may contribute to nanoparticle growth and/or composition under atmospherically relevant conditions is similar to that reached by Vesterinen et al (2007). The current work provides an additional, fundamental basis for interpreting recent experimental measurements of molecular composition as a function of particle size in the case of monodisperse aerosols (Tu and Johnston, 2017), or more generally, volumeto-surface-area ratio for polydisperse aerosols (Wu and Johnston, 2017). In each study, the relative concentration of accretion products increased approximately linearly with particle size (or volume-to-surface-area ratio) as predicted by Fig.…”

“…In the Tu study (Tu and Johnston, 2017), β-pinene ozonolysis produced SOA having systematic changes in molecular composition as a function of particle size. The relative concentrations (as indicated by the signal intensities of ions produced by electrospray ionization) of higher-order oligomers, i.e., trimers and tetramers, increased linearly with increasing particle size, similar to Fig.…”

“…This work was inspired by recent composition studies of laboratory-generated secondary aerosol spanning several tens of nanometers in diameter, which showed systematic changes in composition with increasing particle size (Tu and Johnston, 2017;Wu and Johnston, 2017). In the first set of calculations, chemical composition and growth rate are determined for particles between 2 and 100 nm assuming that growth occurs by partitioning alone.…”

Sulfur Dioxide Modifies Aerosol Particle Formation and Growth by Ozonolysis of Monoterpenes and Isoprene

“…Offline molecular composition measurements of the particles grown by α-pinene and limonene ozonolysis, both with and without SO 2 present, were conducted using high-resolution electrospray ionization mass spectrometry (ESI-MS) following a previously established method described by Tu and Johnston (2017). For each condition, 10 μg of particles were collected onto a 25-mm quartz microfiber filter (GF/D, Whatman, Maidstone, UK), subsequently extracted using 2 ml of acetonitrile (Optima grade, Fisher Scientific, Hampton, NH) and concentrated under vacuum to a final volume of 40 μl, resulting in a concentration of 0.1 μg/μl.…”

Response to comments by Referee #2

“…Gas-phase oxidation of volatile organic compounds occurs through a complex set of reaction pathways involving both the gas and particle phases to yield particle-phase products that often number in the hundreds or thousands based on accurate mass measurements (Bateman et al, 2008;Mentel et al, 2015;Reinhardt et al, 2007;Tu et al, 2016). Absorptive partitioning (Barsanti et al, 2017;Pankow, 1994) of gasphase products to the particle phase forms secondary organic aerosol (SOA), which includes both non-volatile (NVOCs) and semi-volatile (SVOCs) organic compounds (Kroll and Seinfeld, 2008;Riipinen et al, 2012).…”