Secondary Formation and Impacts of Gaseous Nitro-Phenolic Compounds in the Continental Outflow Observed at a Background Site in South China

Nitro-phenolic compounds (NPs) have attracted increasing attention because of their health risks and impacts on visibility, climate, and atmospheric chemistry. Despite many measurements of particulate NPs, the knowledge of their gaseous abundances, sources, atmospheric fates, and impacts remains incomplete. Here, 18 gaseous NPs were continuously measured with a time-of-flight chemical ionization mass spectrometer at a background site in South China in autumn and winter. Abundant NPs were observed in the continental outflows from East Asia, with a total concentration up to 122.1 pptv. Secondary formation from the transported aromatics dominated the observed NPs, with mono-NPs exhibiting photochemical daytime peaks and nighttime enrichments of di-NPs and Cl-substituted NPs. The budget analysis indicates that besides the •OH oxidation of aromatics, the NO3• oxidation also contributed significantly to the daytime mono-NPs, while the further oxidation of mono-NPs by NO3• dominated the nocturnal formation of di-NPs. Photolysis was the main daytime sink of NPs and produced substantial HONO, which would influence atmospheric oxidation capacity in downwind and background regions. This study provides quantitative insights on the formation and impacts of gaseous NPs in the continental outflow and highlights the role of NO3• chemistry in the secondary nitro-aromatics production that may facilitate regional pollution

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

Oxygenated organic molecules (OOMs) are critical intermediates linking volatile organic compound oxidation and secondary organic aerosol (SOA) formation. Yet, the understanding of OOM components, formation mechanism, and impacts are still limited, especially for urbanized regions with a cocktail of anthropogenic emissions. Herein, ambient measurements of OOMs were conducted at a regional background site in South China in 2018. The molecular characteristics of OOMs revealed dominant nitrogen-containing products, and the influences of different factors on OOM composition and oxidation state were elucidated. Positive matrix factorization analysis resolved the complex OOM species to factors featured with fingerprint species from different oxidation pathways. A new method was developed to identify the key functional groups of OOMs, which successfully classified the majority species into carbonyls (8%), hydroperoxides (7%), nitrates (17%), peroxyl nitrates (10%), dinitrates (13%), aromatic ring-retaining species (6%), and terpenes (7%). The volatility estimation of OOMs was improved based on their identified functional groups and was used to simulate the aerosol growth process contributed by the condensation of those low-volatile OOMs. The results demonstrate the predominant role of OOMs in contributing sub-100 nm particle growth and SOA formation and highlight the importance of dinitrates and anthropogenic products from multistep oxidation

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

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

Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NO_<i>x</i>, and Degree of Oxygenation

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 NOx 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 NOx in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs