Underestimated Contribution of Heavy Aromatics to Secondary Organic Aerosol Revealed by Comparative Assessments Using New and Traditional Methods

Abstract

Oxygenated organic molecules (OOMs) from oxidation of volatile organic compounds (VOCs) are important contributors to secondary organic aerosol (SOA) formation. Recent field studies showed that anthropogenic precursors significantly contributed to OOMs and subsequent SOA formation in urban areas. We conducted collocated OOM measurements with nitrate-ion chemical ionization mass spectrometry and SOA molecular tracer measurements with thermal desorption aerosol gas chromatography–mass spectrometry in Shanghai. Using the newly developed OOM-based method, we found that OOMs derived from aromatic VOCs (aromatic OOMs) dominated the local SOA production with a contribution of 52%. The traditional SOA tracer-based method estimated a consistent fraction of 49% from monoaromatics and polyaromatics (e.g., naphthalene and methylnaphthalene). We further categorized the aromatic OOMs into heavy (carbon number: nC > 9) and light (nC = 6–9) ones primarily based on the ring number. Surprisingly, the contribution of heavy aromatic OOMs to SOA formation (25%) was more than twice of the naphthalene-derived SOA from the tracer-based method (10%). The gap could be explained by the fact that the OOM-based method also counted the contributions from other polyaromatic VOCs that are beyond methyl-/naphthalene. The high degrees of oxygenation caused by multistep oxidation and the higher carbon number (nC > 9) in heavy aromatic OOMs lead to their lower volatility and higher contributions to SOA. Our study provides previously unavailable linkage between the aromatic SOA with its precursors via simultaneous measurements of OOMs and molecular tracers, revealing the overlooked contribution from heavy aromatic VOCs to SOA formation.

Oxygenated organic molecules produced by low-NOx photooxidation of aromatic compounds: contributions to secondary organic aerosol and steric hindrance

“…The volatility distribution of OOMs from naphthalene has a greater ULVOC fraction than that of benzene, which is consistent with the greater estimated contributions of naphthalene-derived OOMs to the SOA mass. Field measurements in urban Shanghai suggest that heavy aromatic VOCs are important SOA precursors (Tian et al, 2022). The results herein provide laboratory evidence that heavy (e.g., double-ring) aromatic VOCs might be more important than light (i.e., monocyclic) aromatic ones in initial particle growth during SOA formation.…”

Fostering a Holistic Understanding of the Full Volatility Spectrum of Organic Compounds from Benzene Series Precursors through Mechanistic Modeling

“…We design simulations under typical urban conditions to investigate the full volatility spectrum of products from benzene series precursors in the atmosphere and to evaluate contributions of different pathways and the effects of atmospheric conditions. Because benzene series precursors are a major source of SOA and OOMs in urban areas, , we simulate the conditions of a typical polluted city, Beijing. The concentrations of precursors (8.5 ppb in total), OH (1.5 × 10 6 #/cm 3 ), and NO x (10 ppb), as well as the temperature (300 K) and reaction time (6 h) of the base case, are determined from previous studies. , We also design sensitivity cases to discuss the effect of different reaction pathways (dimerization, autoxidation, and multigenerational oxidation), NO x (0 and 3 ppb), OH (5 × 10 5 and 5 × 10 6 #/cm 3 ), precursor concentration (4.25 and 17 ppb in total), and temperature (270 and 310 K).…”

“…15,16 In more polluted city areas, although monoterpene still contributed to OOM formation, 8,17,18 benzene series precursors, as important anthropogenic VOCs, have been recognized as a major contributor to OOMs in urban areas. 19,20 A few studies have shown that the growth rate of the newly formed particle can be largely explained by considering benzene series OOMs in some cities. 6,8 Furthermore, benzene series OOMs can contribute to 30−50% of SOA in several cities.…”

“…The VOC precursors, on one hand, undergo regular multigenerational oxidation reactions induced by hydroxyl (OH), and, on the other hand, the peroxyl radicals (RO 2 ) from VOC precursors undergo irregular radical reactions like autoxidation, dimerization, etc. , Oxidation products with low volatility and a high oxygen atom number are named oxygenated organic molecules (OOMs), which are potential driving factors of NPF and SOA formation. Earlier studies mainly focused on OOM formation from monoterpene in relatively clean areas. , In more polluted city areas, although monoterpene still contributed to OOM formation, ,, benzene series precursors, as important anthropogenic VOCs, have been recognized as a major contributor to OOMs in urban areas. , A few studies have shown that the growth rate of the newly formed particle can be largely explained by considering benzene series OOMs in some cities. , Furthermore, benzene series OOMs can contribute to 30–50% of SOA in several cities. , Hence, a holistic understanding of benzene series of OOMs in the full volatility range is vital in urban areas.…”

“…6,8 Furthermore, benzene series OOMs can contribute to 30−50% of SOA in several cities. 8,19 Hence, a holistic understanding of benzene series of OOMs in the full volatility range is vital in urban areas. OOMs from the benzene series have been investigated by chamber experiments and theoretical calculations.…”