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.

The energy flows in Earth's natural and modified climate systems are strongly influenced by the concentrations of atmospheric particulate matter (PM). For predictions of concentration, equilibrium partitioning of semivolatile organic compounds (SVOCs) between organic PM and the surrounding vapor has widely been assumed, yet recent observations show that organic PM can be semisolid or solid for some atmospheric conditions, possibly suggesting that SVOC uptake and release can be slow enough that equilibrium does not prevail on timescales relevant to atmospheric processes. Herein, in a series of laboratory experiments, the mass labilities of films of secondary organic material representative of similar atmospheric organic PM were directly determined by quartz crystal microbalance measurements of evaporation rates and vapor mass concentrations. There were strong differences between films representative of anthropogenic compared with biogenic sources. For films representing anthropogenic PM, evaporation rates and vapor mass concentrations increased above a threshold relative humidity (RH) between 20% and 30%, indicating rapid partitioning above a transition RH but not below. Below the threshold, the characteristic time for equilibration is estimated as up to 1 wk for a typically sized particle. In contrast, for films representing biogenic PM, no RH threshold was observed, suggesting equilibrium partitioning is rapidly obtained for all RHs. The effective diffusion rate D org for the biogenic case is at least 10 3 times greater than that of the anthropogenic case. These differences should be accounted for in the interpretation of laboratory data as well as in modeling of organic PM in Earth's atmosphere.atmospheric chemistry | secondary organic aerosol | evaporation A tmospheric particulate matter (PM) has significant effects on climate, air quality, and human health (1). Globally, organic compounds constitute 20 to 90% of the submicron PM mass. Most of this mass is produced by the oxidation and subsequent condensation of biogenic and anthropogenic gaseous precursors as secondary organic material (SOM) (2). Parameterizations of produced mass have been developed based on the oxidation of different types of volatile organic compounds (VOCs) in environmental chambers (3, 4). Chemical transport models (CTMs) based on these parameterizations, however, do not successfully account for the PM concentrations measured in the atmosphere (5, 6), and closing the gap between model and measurements is a major research goal (7-10).The parameterizations embedded in the CTMs largely assume rapid mass transfer for the partitioning of semivolatile organic compounds (SVOCs) between the particle and vapor phases (11,12). Without considering the kinetic factors, this equilibrium approach may not, however, always accurately represent processes on the timescales of the laboratory experiments or of atmospheric transformations. Observations suggest that atmospheric organic PM and laboratory-generated SOM can be liquid, semisolid, or solid, depen...

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.

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