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Published June 3, 2019 | Published + Supplemental Material
Journal Article Open

Low-volatility compounds contribute significantly to isoprene secondary organic aerosol (SOA) under high-NO_x conditions


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 reevaluating SOA yields from isoprene. In particular, SOA from isoprene oxidation under high-NOx 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 or global modeling. Results suggest that low-volatility nitrates and dinitrates produce significantly more aerosol than previously thought; the experimentally measured 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. The isoprene SOA mass yield from the LV pathway measured in this work is significantly higher than previous studies have reported, suggesting that low-volatility compounds such as organic nitrates and dinitrates may contribute to isoprene SOA under high-NOx conditions significantly more than previously thought and thus deserve continued study.

Additional Information

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 29 Dec 2018 – Discussion started: 11 Jan 2019 – Revised: 11 Apr 2019 – Accepted: 08 May 2019 – Published: 03 Jun 2019. We thank Dennis Fitz for assistance with maintenance and data analysis of the luminol NO_2/acyl peroxynitrate analyzer. This research has been supported by the National Science Foundation, Division of Atmospheric and Geospace Sciences (grant no. AGS-1523500) and the National Science Foundation (grant no. 1745301). This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement no. 1852977. Author contributions. RHS designed the experiments. RHS and SMC performed the experiments. RHS analyzed the data with help from SMC, KHB, TBN, JHS, and YH. RHS did the kinetic modeling. YH, HM, WK, and RCF assisted RHS with DMA operation and data analysis. RHS wrote the paper with assistance from KHB, TBN, JHS, SMC, and YH. Data availability. We welcome future collaboration with those who wish to use this data set for additional modeling purposes (e.g., creating volatility basis set parameters for global or regional models or for evaluating the results with a more complex box model that includes aerosol chemistry). Please contact Rebecca H. Schwantes (rschwant@ucar.edu). The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-7255-2019-supplement. The authors declare that they have no conflict of interest. Review statement. This paper was edited by Jacqui Hamilton and reviewed by two anonymous referees.

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