Abstract
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 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.
Highlights
In the atmosphere, submicrometer particulate matter is composed of a significant fraction of organic aerosol (Zhang et al, 2007)
Because larger seed particle number and surface area concentrations were used in these experiments, corrections to β(Dp, t) that account for coagulation are needed (Pierce et al, 2008; Nah et al, 2017)
Considering that many multifunctional isoprene-derived organic nitrates have been detected in ambient aerosol (Lee et al, 2016), all secondary organic aerosol (SOA) precursors in Table S2 with FP > 0.05 at 26 ◦C are combined and converted to mass. Extrapolating these to ambient organic aerosol concentrations is more difficult because these compounds are more likely to exist in the particle phase because of accretion reactions and not volatility. When these products are assumed to exist entirely in the particle phase and no factor is applied to correct for differences in organic aerosol concentration or for these products only representing about one-third of the isoprene SOA yield measured in this study (Fig. S4), the LV pathway is estimated to contribute to ∼ 0.6 of the SOA formed under high-NOx conditions
Summary
Submicrometer particulate matter is composed of a significant fraction of organic aerosol (Zhang et al, 2007). There are two forms of organic aerosol: primary, which is directly emitted into the atmosphere, and secondary, which is formed when gas-phase compounds partition to the particle phase. Schwantes et al.: Low-volatility compounds contribute significantly to isoprene SOA aerosol (SOA) formation are complex (Kroll and Seinfeld, 2008; Hallquist et al, 2009). SOA yields, the ratio of the mass of SOA formed to the mass of the parent volatile organic compound (VOC) reacted, are measured in environmental chambers and are used in models to reduce the complexity of SOA formation
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