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Discussion papers
https://doi.org/10.5194/acp-2018-1358
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1358
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Jan 2019

Research article | 11 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Low-volatility compounds contribute significantly to isoprene SOA under high-NO conditions

Rebecca H. Schwantes1,a, Sophia M. Charan2, Kelvin H. Bates2,b, Yuanlong Huang1, Tran B. Nguyen3, Huajun Mai1, Weimeng Kong2, Richard C. Flagan2, and John H. Seinfeld2,4 Rebecca H. Schwantes et al.
  • 1Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States
  • 2Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States
  • 3Department of Environmental Toxicology, University of California – Davis, Davis, California 95616, United States
  • 4Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA
  • acurrent affiliation: National Center for Atmospheric Research, Boulder, Colorado, 80307, USA
  • bcurrent affiliation: Faculty of Arts and Sciences, Harvard University, Cambridge, MA 02138, USA

Abstract. Recent advances in our knowledge of the gas-phase oxidation of isoprene, the impact of chamber walls on secondary organic aerosol (SOA) mass yields, and aerosol measurement analysis techniques warrant re-evaluating SOA yields from isoprene. In particular, SOA from isoprene oxidation under high-NO conditions forms via two major pathways: (1) low-volatility nitrates and dinitrates (LV pathway) and (2) hydroxymethyl-methyl-α-lactone (HMML) reaction on a surface or the condensed phase of particles to form 2-methyl glyceric acid and its oligomers (2MGA pathway). These SOA production pathways respond differently to reaction conditions. Past chamber experiments generated SOA with varying contributions from these two unique pathways, leading to results that are difficult to interpret. This study examines the SOA yields from these two pathways independently, which improves the interpretation of previous results and provides further understanding of the relevance of chamber SOA yields to the atmosphere and regional/global modeling. Results suggest that low-volatility nitrates and dinitrates produce significantly more aerosol than previously thought; the SOA mass yield from the LV pathway is ≃ 0.15. Sufficient seed surface area at the start of the reaction is needed to limit the effects of vapor wall losses of low-volatility compounds and accurately measure the complete SOA mass yield. Under dry conditions, substantial amounts of SOA are formed from HMML ring-opening reactions with inorganic ions and HMML organic oligomerization processes. However, the lactone organic oligomerization reactions are suppressed under more atmospherically relevant humidity levels, where hydration of the lactone is more competitive. This limits the SOA formation potential from the 2MGA pathway to HMML ring-opening reactions with water or inorganic ions under typical atmospheric conditions. Due to the high isoprene SOA mass yield from the LV pathway measured in this work, we now roughly estimate that the LV pathway produces moderately more SOA mass than the 2MGA pathway under typical atmospheric conditions (RH = 70 %, T = 298 K, NO2/NO = 6, NO = 0.05 ppb, isoprene = 5 ppb, and OH = 1.5 × 106 molec cm−3). This suggests that in the atmosphere low-volatility compounds such as organic nitrates and dinitrates may contribute to isoprene SOA under high-NO conditions significantly more than previously thought, and thus deserve continued study.

Rebecca H. Schwantes et al.
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Rebecca H. Schwantes et al.
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Short summary
Oxidation of isoprene, the dominant non-methane biogenic volatile organic compound emitted into the atmosphere, is a significant source of secondary organic aerosol (SOA). Here formation of SOA from isoprene oxidation by the hydroxyl radical (OH) under high-NO conditions is measured. This work improves our understanding of isoprene SOA formation by demonstrating that low-volatility compounds formed under high-NO conditions produce significantly more aerosol than previously thought.
Oxidation of isoprene, the dominant non-methane biogenic volatile organic compound emitted into...
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