Much of our understanding of atmospheric secondary organic aerosol (SOA) formation from volatile organic compounds derives from laboratory chamber measurements, including mass yield and elemental composition. These measurements alone are insufficient to identify the chemical mechanisms of SOA production. We present here a comprehensive dataset on the molecular identity, abundance, and kinetics of α-pinene SOA, a canonical system that has received much attention owing to its importance as an organic aerosol source in the pristine atmosphere. Identified organic species account for ∼58-72% of the α-pinene SOA mass, and are characterized as semivolatile/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation states yet different functionalities. Features of the α-pinene SOA formation process are revealed for the first time, to our knowledge, from the dynamics of individual particle-phase components. Although monomeric products dominate the overall aerosol mass, rapid production of dimers plays a key role in initiating particle growth. Continuous production of monomers is observed after the parent α-pinene is consumed, which cannot be explained solely by gasphase photochemical production. Additionally, distinct responses of monomers and dimers to α-pinene oxidation by ozone vs. hydroxyl radicals, temperature, and relative humidity are observed. Gas-phase radical combination reactions together with condensed phase rearrangement of labile molecules potentially explain the newly characterized SOA features, thereby opening up further avenues for understanding formation and evolution mechanisms of α-pinene SOA.secondary organic aerosol | particulate matter | air quality | climate S econdary organic aerosol (SOA), comprising a large number of structurally different organic oxygenates, is a dominant constituent of submicrometer atmospheric particulate matter (1). Molecular characterization of SOA has been a major research goal in atmospheric chemistry for several decades (2), owing to the importance of organic aerosol in air quality and Earth's energy budget. Both biogenic (e.g., isoprene, monoterpenes) and anthropogenic (e.g., aromatics, large alkanes) organic compounds are well-established precursors to SOA. Knowledge of the SOA molecular composition is crucial for elucidation of its underlying formation mechanisms.The most abundant monoterpene in the troposphere is α-pinene (3). The oxidation of α-pinene by ozone has become a canonical SOA system (4-12). Identification of multifunctional particlephase products has been reported, including monomers with carboxylic acid moieties (4, 6) and high-molecular-weight compounds (7,8,12), although molecular structures and formation pathways of oligomers remain uncertain (5). Recently, a class of extremely low-volatility gas-phase organic compounds (ELVOCs) has been identified as an important component in the α-pinene ozonolysis chemistry (13). Identification of the ELVOCs in the particle phase and elucidation of the mechanism of their format...

Abstract. Vanillin (VL), a phenolic aromatic carbonyl abundant in biomass burning emissions, forms triplet excited states (3VL∗) under simulated sunlight leading to aqueous secondary organic aerosol (aqSOA) formation. Nitrate and ammonium are among the main components of biomass burning aerosols and cloud or fog water. Under atmospherically relevant cloud and fog conditions, solutions composed of either VL only or VL with ammonium nitrate were subjected to simulated sunlight irradiation to compare aqSOA formation via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate. The reactions were characterized by examining the VL decay kinetics, product compositions, and light absorbance changes. Both conditions generated oligomers, functionalized monomers, and oxygenated ring-opening products, and ammonium nitrate promoted functionalization and nitration, likely due to its photolysis products (⚫OH, ⚫NO2, and NO2- or HONO). Moreover, a potential imidazole derivative observed in the presence of ammonium nitrate suggested that ammonium participated in the reactions. The majority of the most abundant products from both conditions were potential brown carbon (BrC) chromophores. The effects of oxygen (O2), pH, and reactants concentration and molar ratios on the reactions were also explored. Our findings show that O2 plays an essential role in the reactions, and oligomer formation was enhanced at pH <4. Also, functionalization was dominant at low VL concentrations, whereas oligomerization was favored at high VL concentrations. Furthermore, oligomers and hydroxylated products were detected from the oxidation of guaiacol (a non-carbonyl phenol) via VL photosensitized reactions. Last, potential aqSOA formation pathways via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate were proposed. This study indicates that the direct photosensitized oxidation of VL may be an important aqSOA source in areas influenced by biomass burning and underscores the importance of nitrate in the aqueous-phase processing of aromatic carbonyls.

Heterogeneous oxidation of SO2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO2 oxidation by OH and NO2 produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86–91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO2 – or HNO2, N­(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO2, γSO2 , can be expressed as γSO2 = 1.64 × p NO3–, where p NO3− is the nitrate photolysis rate in the range of (1.0–8.0) × 10–5 M s–1. Our kinetic model with the p NO3− predicts that N­(III) is the main contributor to the SO2 oxidation, followed by NO2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N­(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m–3 h–1 at pH 4–6 and p NO3– = 10–5 M s–1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO2 by reactive nitrogen (NO2 –/HNO2 and NO2) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

Secondary organic aerosol (SOA) contributes a significant fraction to aerosol mass and toxicity.Low-volatility organic vapors are critical intermediates connecting the oxidation of volatile organic compounds (VOCs) to SOA formation. However, the direct measurement of intermediate vapors poses a great challenge, further compounded by the difficulty of linking them to specific precursors from a cocktail of complex emission sources in the vast urbanized areas. Here, we present coordinated measurements of low-volatility oxidation products, termed oxygenated organic molecules (OOMs) in three most urbanized regions in China. With a newly-developed analysis methodology, we are able to assign these OOMs to their likely precursors and ultimately connect SOA formation to various VOCs. At all measurement locations, we find similar OOM