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Secondary organic aerosol (SOA) particles are ubiquitous in air and understanding the mechanism by which they grow is critical for predicting their effects on visibility and climate. The uptake of three organic nitrates into semi-solid SOA particles formed by α-pinene ozonolysis either with or without an OH scavenger was investigated. Four types of experiments are presented here. In Series A, uptake of the selected organic nitrates (2-ethylhexyl nitrate (2EHN); β-hydroxypropyl nitrate (HPN); β-hydroxyhexyl nitrate (HHN)) into impacted SOA particles was interrogated by attenuated total reflectance (ATR)-FTIR. In this case, equilibrium was reached and partition coefficients ( K SOA = [–ONO 2 ] SOA /[–ONO 2 ] air ) were measured to be K 2EHN = (3.2–11) × 10 4 , K HPN = (4.4–5.4) × 10 5 , and K HHN = (4.9–9.0) × 10 6 . In Series B, SOA particles were exposed on-the-fly to gas phase organic nitrates for comparison to Series A, and uptake of organic nitrates was quantified by HR-ToF-AMS analysis, which yielded similar results. In Series C (AMS) and D (ATR-FTIR), each organic nitrate was incorporated into the SOA as the particles formed and grew. The incorporation of the RONO 2 was much larger in Series C and D ( during growth ), exceeding equilibrium values determined in Series A and B ( after growth ). This suggests that enhanced uptake of organic nitrates during SOA formation and growth is due to a kinetically controlled “burying” mechanism, rather than equilibrium partitioning. This has important implications for understanding SOA formation and growth under conditions where the particles are semi-solid, which is central to accurately predicting properties for such SOA.
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Understanding interactions of organic nitrates with the surface and bulk of organic films: implications for particle growth in the atmosphere
Understanding impacts of secondary organic aerosol (SOA) in air requires a molecular-level understanding of particle growth via interactions between gases and particle surfaces. The interactions of three gaseous organic nitrates with selected organic substrates were measured at 296 K using attenuated total reflection Fourier transform infrared spectroscopy. The organic substrates included a long chain alkane (triacontane, TC), a keto-acid (pinonic acid, PA), an amorphous ester oligomer (poly(ethylene adipate) di-hydroxy terminated, PEA), and laboratory-generated SOA from α-pinene ozonolysis. There was no uptake of the organic nitrates on the non-polar TC substrate, but significant uptake occurred on PEA, PA, and α-pinene SOA. Net uptake coefficients ( γ ) at the shortest reaction times accessible in these experiments ranged from 3 × 10 −4 to 9 × 10 −6 and partition coefficients ( K ) from 1 × 10 7 to 9 × 10 4 . Trends in γ did not quantitatively follow trends in K , suggesting that the intermolecular forces involved in gas–surface interactions are not the same as those in the bulk, which is supported by theoretical calculations. Kinetic modeling showed that nitrates diffused throughout the organic films over several minutes, and that the bulk diffusion coefficients evolved as uptake/desorption occurred. A plasticizing effect occurred upon incorporation of the organic nitrates, whereas desorption caused decreases in diffusion coefficients in the upper layers, suggesting a crusting effect. Accurate predictions of particle growth in the atmosphere will require knowledge of uptake coefficients, which are likely to be several orders of magnitude less than one, and of the intermolecular interactions of gases with particle surfaces as well as with the particle bulk.
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- PAR ID:
- 10092195
- Date Published:
- Journal Name:
- Environmental Science: Processes & Impacts
- Volume:
- 20
- Issue:
- 11
- ISSN:
- 2050-7887
- Page Range / eLocation ID:
- 1593 to 1610
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/C8EM00348C
