UC Davis

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 2-methyltetrol, 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 C4 polyol analogue, erythritol. We studied the kinetics and reaction products of the aqueous 2-methyltetrol (2-MT) + OH and erythritol (E) + OH reactions using 1H and 13C 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 (CO2) 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.