Biomass burning releases highly reactive methoxyphenols into the atmosphere, which can undergo heterogeneous oxidation and act as precursors for secondary organic aerosol (SOA) formation. Understanding the reactivity of such methoxyphenols at the air−water interface is a matter of major atmospheric interest. Online electrospray ionization mass spectrometry (OESI-MS) is used here to study the oxidation of two methoxyphenols among three phenolic aldehydes, 4-hydroxybenzaldehyde, vanillin, and syringaldehyde, on the surface of water. The OESI-MS results together with cyclic voltammetry measurements at variable pH are integrated into a mechanism describing the heterogeneous oxidative processing of methoxyphenols by gaseous ozone (O 3 ) and hydroxyl radicals (HO • ). For a low molar ratio of O 3 ≤ 66 ppbv, the OESI-MS spectra show that the oxidation is dominated by in situ produced HO • and results in the production of polyhydroxymethoxyphenols. When the level of O 3 increases (i.e., ≥78 times), the ion count of polyhydroxymethoxyphenols increases, while new ring fragmentation products are generated, including conjugated aldehydes and double bonds as well as additional carboxylic acid groups. The interfacial reactivity of methoxyphenols with O 3 and HO • is enhanced as the number of methoxy (−OCH 3 ) groups increases (4-hydroxybenzaldehyde < vanillin < syringaldehyde). The experimental observations are summarized in two reaction pathways, leading to the formation of (1) hydroxylated methoxyphenols and (2) multifunctional carboxylic acids from fragmentation of the aromatic ring. The new highly oxygenated products with low volatility are excellent precursors for aqueous SOA formation.

Abundant substituted catechols are emitted to, and created in, the atmosphere during wildfires and anthropogenic combustion and agro-industrial processes. While ozone (O 3 ) and hydroxyl radicals (HO • ) efficiently react in a 1 μs contact time with catechols at the air−water interface, the nighttime reactivity dominated by nitrate radicals (NO 3 ) remains unexplored. Herein, online electrospray ionization mass spectrometry (OESI-MS) is used to explore the reaction of NO 3 (g) with a series of representative catechols (catechol, pyrogallol, 3-methylcatechol, 4-methylcatechol, and 3-methoxycatechol) on the surface of aqueous microdroplets. The work detects the ultrafast generation of nitrocatechol (aromatic) compounds, which are major constituents of atmospheric brown carbon. Two mechanisms are proposed to produce nitrocatechols, one (equivalent to H atom abstraction) following fast electron transfer from the catechols (QH 2 ) to NO 3 , forming NO 3 − and QH 2 •+ that quickly deprotonates into a semiquinone radical (QH • ). The second mechanism proceeds via cyclohexadienyl radical intermediates from NO 3 attack to the ring. Experiments in the pH range from 4 to 8 showed that the production of nitrocatechols was favored under the most acidic conditions. Mechanistically, the results explain the interfacial production of chromophoric nitrocatechols that modify the absorption properties of tropospheric particles, making them more susceptible to photooxidation, and alter the Earth's radiative forcing.

Nitrogen-containing organic carbon (NOC) in atmospheric particles is an important class of brown carbon (BrC). Redox active NOC like aminophenols received little attention in their ability to form BrC. Here we show that iron can catalyze dark oxidative oligomerization of o- and p-aminophenols under simulated aerosol and cloud conditions (pH 1–7, and ionic strength 0.01–1 M). Homogeneous aqueous phase reactions were conducted using soluble Fe(III), where particle growth/agglomeration were monitored using dynamic light scattering. Mass yield experiments of insoluble soot-like dark brown to black particles were as high as 40%. Hygroscopicity growth factors (κ) of these insoluble products under sub- and super-saturated conditions ranged from 0.4–0.6, higher than that of levoglucosan, a prominent proxy for biomass burning organic aerosol (BBOA). Soluble products analyzed using chromatography and mass spectrometry revealed the formation of ring coupling products of o- and p-aminophenols and their primary oxidation products. Heterogeneous reactions of aminophenol were also conducted using Arizona Test Dust (AZTD) under simulated aging conditions, and showed clear changes to optical properties, morphology, mixing state, and chemical composition. These results highlight the important role of iron redox chemistry in BrC formation under atmospherically relevant conditions.

Substantial amounts of phenolic aldehydes, represented by the structures of syringaldehyde, vanillin, and 4hydroxybenzaldehyde, are emitted to the atmosphere during biomass burning. The oxidative transformation of phenolic aldehydes during atmospheric transport has the potential to modify the physicochemical properties of particulates, which play a vital role in Earth's climate and human health. Herein, thin solid films made of syringaldehyde, vanillin, and 4-hydroxybenzaldehyde are oxidized in contact with O 3 (g) under a relative humidity of 74% representative of average global conditions. New physical insights into the surface reactions are achieved by analyzing isopropanolextracted films before and during oxidation by multiple techniques. Changes in electronic transitions at 220, 310, and 350−400 nm registered by UV−vis spectroscopy show that the oxidized films have enhanced mass absorption coefficients at λ > 300 nm. Electrospray ionization (ESI) mass spectrometry (MS) and ion chromatography with conductivity and MS detection of extracted oxidized films confirm aromatic ring cleavage of syringaldehyde and vanillin by O 3 (g) with the production of carboxylic acids. Carboxylic acids were observed as anions ([M − H] − ) at m/z 45 (formic acid), 73 (glyoxylic acid), 75 (glycolic acid), 89 (oxalic acid), 115 (maleic acid), 117 (mesoxalic acid), 119 (tartronic acid), and 129 (maleic acid monomethyl ester), while other polyfunctional products were registered by ultrahigh-pressure liquid chromatography with UV−vis and MS detection. In situ production of hydroxyl radicals is confirmed by demethoxylation products and ipso attack at the C 1 ring position holding the −C(H)�O group. The order of reactivity increased with the number of methoxy substituents that donate electron density to the aromatic ring. Combined oxidation mechanisms for the three compounds are proposed based on all of the experimental observations and explain the contribution of aged biomass burning material to secondary organic aerosol formation.

Biomass burning emissions contain abundant phenolic aldehydes (e.g., syringaldehyde, vanillin, and 4-hydroxybenaldehyde) that are oxidized during atmospheric transport, altering the physicochemical properties of particulates. Herein, the oxidative processing of thin films made of syringaldehyde, vanillin, and 4hydroxybenaldehyde is studied at the air−solid interface under a variable O 3 (g) molar ratio (410 ppbv−800 ppmv) and relative humidity (0−90%). Experiments monitored the absorption changes of C�C, C�O, and �COOH vibration changes during the oxidation of thin films by transmission Fourier transform infrared spectroscopy (FTIR). Selected spectroscopic features of aromatic ring cleavage by O 3 (g) revealed the production of carboxylic acids. Instead, monitoring O�H stretching provided a comparison of a hydroxylation channel from in situ produced hydroxyl radical. The overall oxidation reactivity trend syringaldehyde > vanillin > 4hydroxybenzladehyde can be explained based on the additional electron density from methoxide substituents to the ring. The reactive uptake coefficient of O 3 (g) increases for higher relative humidity, e.g., for syringaldehyde by 18 and 215 times at 74% and 90% relative humidity (RH), respectively, as compared to dry conditions. A Langmuir−Hinshelwood mechanism fits well the kinetics of oxidation under a variable O 3 (g) molar ratio at 74% RH, providing useful information that should be included in atmospheric chemistry models.