Low-volatility compounds contribute significantly to isoprene secondary organic aerosol (SOA) under high-NO<sub><i>x</i></sub> conditions

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.

A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

“…Global models tend to underestimate the atmospheric burden of organic aerosol (Volkamer et al, 2006;de Gouw and Jimenez, 2009;Tsigaridis et al, 2014). Chamber studies isolating specific isoprene oxidation pathways have measured SOA mass yields in excess of 15 % Schwantes et al, 2019), while top-down (Hallquist et al, 2009;Heald et al, 2010;Spracklen et al, 2011) and mass-balance (Goldstein and Galbally, 2007) assessments of global SOA production consistently arrive at higher estimates than the models. Stadtler et al (2018) found better agreement with the top-down assessments by implementing a new isoprene mechanism (including the major tetrafunctional C 5 compounds) into global simulations, resulting in a 33 % global iSOA mass yield (16 % per carbon).…”

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

“…A strong linear relationship was observed between the 2-MG-OS monomer and dimer concentrations (R 2 =0.83) with a dimer:monomer ratio of 0.02. Formation of oligomers from reactions of 2-MG and HMML has been shown to be reduced in chamber experiments under humid conditions (Schwantes et al, 2019;Nestorowicz et al, 2018). The average RH during the afternoon of the campaign was ~40 %, which may 460 account for the relatively low formation of the dimer OS compared to the monomer (see SI Figure 9).…”

Secondary organic aerosol formation from photooxidation of furan: effects of NO<sub><i>x</i></sub> and humidity

“…Identifying the chemical composition and major precursors of SOA is helpful to better understand their formation mechanism and strategies for the control of PM. Furan is an important five-member heteroatom-containing VOC present in the atmosphere, and it can be produced by wood combustion, oil refining, coal mining and gasification, and pyrolysis of biomass, cellulose, and lignin (Shafizadeh, 1982). As shown in previous studies, furan is one of the most com-Published by Copernicus Publications on behalf of the European Geosciences Union.…”