Aqueous Photochemistry of 2-Methyltetrol and Erythritol as Sources of Formic Acid and Acetic Acid in the Atmosphere

Abstract

Atmospheric formic acid (FA) and acetic acid (AA) mixing ratios are often underestimated in atmospheric models, particularly over areas with high biogenic influence. We investigated the aqueous hydroxyl radical (OH) oxidation of 2methyltetrol, one of the largest components of secondary organic aerosols (SOAs) that are produced from the oxidation of isoprene, and compare its chemistry to the non-methylated C 4 polyol analogue, erythritol. We studied the kinetics and reaction products of the aqueous 2-methyltetrol (2-MT) + OH and erythritol (E) + OH reactions using 1 H and 13 C nuclear magnetic resonance spectroscopy and high-performance liquid chromatography coupled with high-resolution mass spectrometry. We found that the aqueous oxidation of aliphatic alcohols, such as E and 2-MT, are strong sources of small acids. Nearly all 2-MT is converted to FA, AA, and carbon dioxide (CO 2 ) under atmospherically relevant OH exposures. Suppression of volatile acid partitioning into the gas phase increased the observed yields of volatile products in solution by up to 80%, as quantified by experiments with low headspace. The influence of solution pH on the yields of FA and AA (or their carboxylates) was also investigated in the range of pH 2−9 for the 2-MT + OH reaction. Solution pH strongly influenced the concentrations of FA and AA via their gas−aqueous partitioning, gross production yields, and radical-induced decarboxylation reactions. The data are adequately reproduced with a kinetic model; however, different reaction mechanisms are needed for the low and high pH chemistries. Fewer stable reaction intermediates were observed for 2-MT compared to E and at high pH compared to low pH, providing insight into the decomposition pathways of 2-MT. On the basis of the substantial production yields and partitioning of FA and AA in the aqueous photooxidation of 2-methyltetrol, aqueous aging of isoprene-derived SOA may contribute to FA and AA emissions to the atmosphere that are currently missing from models.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Hydroxyl Radical (OH) Oxidation of Isoprene-Derived Secondary Organic Aerosol Compounds at 298 K

“…Xcalibur 2.0 software was used for analysis of chromatograms and mass spectra. Separations of MT, MD-1,4-DN, MT-3-N, erythritol, and xylitol were performed using a Shodex Asahipak NH2P-40 2D column (2 mm × 150 mm, 4 μm, 100 Å) using a method recently reported elsewhere . An Agilent Poroshell InfinityLab EC-C18 (2.1 mm × 100 mm, 2.7 μm, 120 Å) column was used to separate MD-3-S and MT-4-S using an isocratic method with a flow rate of 0.270 mL/min, and a mobile phase (60:40) consisting of 0.05% ammonium formate in water (v/v) and acetonitrile.…”

“…Separations of MT, MD-1,4-DN, MT-3-N, erythritol, and xylitol were performed using a Shodex Asahipak NH2P-40 2D column (2 mm × 150 mm, 4 μm, 100 Å) using a method recently reported elsewhere. 28 using an isocratic method with a flow rate of 0.270 mL/min, and a mobile phase (60:40) consisting of 0.05% ammonium formate in water (v/v) and acetonitrile. Separation of MGA was adapted from the HPLC-ACPI-MS method of Liu et al 29 using the same column; the run time was reduced to 3 min as MGA appeared at ∼2 min.…”

Gas-Phase Oxidation Rates and Products of 1,2-Dihydroxy Isoprene

“…We hypothesize that this is due to particle-phase or heterogeneous oxidation of some 1,2-DHI products, including the C 5 trihydroxy-hydroperoxide, in seeded experiments due to partitioning of H 2 O 2 . Another isoprene derivative, 2-methyltetrol, has recently been shown to react rapidly with OH in the aqueous phase to produce large yields of formic and acetic acids, which can subsequently revolatilize and be observed in the gas phase . The more rapid loss of DHMP observed in seeded experiments suggests that it may also take part in this particle-mediated reactive pathway.…”

“…Another isoprene derivative, 2-methyltetrol, has recently been shown to react rapidly with OH in the aqueous phase to produce large yields of formic and acetic acids, which can subsequently revolatilize and be observed in the gas phase. 82 The more rapid loss of DHMP observed in seeded experiments suggests that it may also take part in this particle-mediated reactive pathway. Future work isolating the aqueous reactivity of 1,2-DHI products will be needed to quantify these reaction rates and products.…”

Water enhances the formation of fragmentation products via the cross-reactions of RO<sub>2</sub> and HO<sub>2</sub> in the photooxidation of isoprene

“…The literature has provided evidence for the formation of FA and AA from the aqueous-phase photooxidation of dominant products from the HO2 pathway. The aqueous-phase photodegradation of 2-methyltetrol, a major hydrolysis product of IEPOX, is a strong source of FA and AA, with a yield as high as ~78 % for FA and ~53 % for AA (Cope et al, 2021). The aqueous-phase photooxidation of β-IEPOX produces AA at a yield of ~14 % (Otto et al, 2019).…”