Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NO_x, and Degree of Oxygenation

Abstract

Oxidation of volatile organic compounds (VOCs) forms oxygenated organic molecules (OOMs), which contribute to secondary pollution. Herein, we present measurement results of OOMs using chemical ionization mass spectrometry with nitrate as the reagent ion in Shanghai. Compared to those in forests and laboratory studies, OOMs detected at this urban site were of relatively lower degree of oxygenation. This was attributed to the high NO x concentrations (∼44 ppb), which overall showed a suppression on the propagation reactions. As another result, a large fraction of nitrogenous OOMs (75%) was observed, and this fraction further increased to 84% under a high NO/VOC ratio. By applying a novel framework on OOM categorization and supported by VOC measurements, 50 and 32% OOMs were attributed to aromatic and aliphatic precursors, respectively. Furthermore, aromatic OOMs are more oxygenated (effective oxygen number, nOeff = 4–6) than aliphatic ones (nOeff = 3–4), which can be partly explained by the difference in initiation mechanisms and points to possible discrimination in termination reactions. This study highlights the roles of NO x in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs.

Molecular Characterization of Oxygenated Organic Molecules and Their Dominating Roles in Particle Growth in Hong Kong

“…7,37−41 Recent field studies highlighted such suppression effects of NOx on the oxidation degree of OOMs in the urban atmosphere in East China. 12,42 Moreover, though the molecular features of OOMs in terms of element numbers have been explored in previous studies, 12,14,36 limited information is available on their multifunctional groups, 24,30,43,44 which hinder the complete understanding of their atmospheric fates and impacts.…”

“…In urban environments, high level of NO x could outcompete the autoxidation propagation reactions, affect the termination pathway of RO 2 radicals, and also change the atmospheric oxidants (e.g., affect OH/HO 2 radical levels and produce NO 3 radicals). It will eventually influence the OOM products types, distributions, and contributions to SOA. ,− Recent field studies highlighted such suppression effects of NOx on the oxidation degree of OOMs in the urban atmosphere in East China. , Moreover, though the molecular features of OOMs in terms of element numbers have been explored in previous studies, ,, limited information is available on their multifunctional groups, ,,, which hinder the complete understanding of their atmospheric fates and impacts.…”

Low-NO-like Oxidation Pathway Makes a Significant Contribution to Secondary Organic Aerosol in Polluted Urban Air

“…26,27 The latest study in Shanghai has found that the NO/VOC ratios (1.7−2.8) during polluted episodes were much higher than those (0.01−0.2) generally recorded in laboratory experiments, which greatly impeded the formation of multigenerational OOMs through the autoxidation of RO 2 intermediates. 13 However, the current knowledge on the formation mechanisms of urban SOA is still limited, mainly because of the intricate and chemical complexity in the urban atmosphere. Moreover, certain reaction systems failed to accurately replicate the varying polluted conditions.…”

“…Prior laboratory studies have demonstrated the pivotal role of the NO x level in governing the formation of SOA, as it determines the fate of organic peroxy radicals (RO 2 ) by mediating the branching ratios among multiple competing pathways of RO 2 reacting with NO, NO 2 , HO 2 , and other RO 2 , subsequently leading to different distributions of oxidized products and varying SOA yields. Generally, SOA yields decrease dramatically with increasing NO x concentrations for most of the reactive VOCs. − Under high-NO x conditions, the RO 2 + NO reactions are preferred; despite the minor production of low-volatility organic nitrates (ONs), , the predominant generation of chain-scission products with higher volatility through decomposition of alkoxy radicals (RO) , effectively inhibits SOA formation. , Conversely, the reactions involving RO 2 with HO 2 , RO 2 , or autoxidation are more favorable under low-NO x conditions. , These pathways tend to preserve the original carbon backbone of precursors and form more highly oxidized products with lower volatility, which are prone to efficiently condense onto particles forming SOA. , Additionally, aerosol composition and properties, including aerosol liquid water content (ALWC), inorganic salts, and aerosol acidity, can also influence the SOA formation through multiphase reactions, which generally lead to the formation of low-volatile organics, e.g., organosulfates (OSs), − and oligomeric organics. , …”

“…Molecular tracer products formed from the oxidation of biogenic VOCs have been successfully identified in SOA constituents from diverse locations worldwide using advanced high-resolution chemical ionization mass spectrometry (CIMS). Recent field observations conducted in China’s megacities have provided insights into the chemical composition, temporal variability, and formation pathways of gas-phase oxygenated organic molecules (OOMs) primarily derived from anthropogenic VOCs by the use of the nitrate-CIMS technique. , The latest study in Shanghai has found that the NO/VOC ratios (1.7–2.8) during polluted episodes were much higher than those (0.01–0.2) generally recorded in laboratory experiments, which greatly impeded the formation of multigenerational OOMs through the autoxidation of RO 2 intermediates . However, the current knowledge on the formation mechanisms of urban SOA is still limited, mainly because of the intricate and chemical complexity in the urban atmosphere.…”