CIMS Detection of Hydroxymethyl Hydroperoxide: Insights into Alkene Ozonolysis in a Variety of Environments

Abstract

Hydroxymethyl hydroperoxide (HMHP, CH₂OHOOH) is produced from the reaction of water vapor with CH₂OO, which is the simplest Criegee intermediate and is produced by the reaction of ozone (O₃) with a terminal alkene. After formation, HMHP is thought to have a considerable lifetime upward of 1 day due to its reaction with OH, which produces formic acid and formaldehyde. HMHP measurements can therefore provide meaningful insight into atmospheric oxidation processes, especially alkene oxidation. We demonstrate the ability of iodide adduct chemical ionization mass spectrometry (I^–CIMS) to detect HMHP and show data from three measurement studies that span a range of chemical environments. Measurements of HMHP are presented from a controlled chamber-based biomass burning (BB) O₃ oxidation experiment, from outdoor sampling periods in Boise, Idaho, during the summer with mixed rural/biogenic influences, and from Philadelphia during summer and late fall during time periods of BB impact. Ambient HMHP concentrations ranged up to 100 pptv during summer at both field sites but rarely exceeded 10 pptv in the late fall. HMHP is often but not always correlated with OH oxidation products detected by I^–CIMS, including I(C₅H₁₀O₃)⁻ from isoprene hydroxy hydroperoxides (ISOPOOH) and isoprene epoxydiols (IEPOX), providing insight into the relative importance of O₃ versus OH oxidation and alkene composition. The greatest observed HMHP mixing ratios exceeded 300 pptv during an extended BB episode at the Philadelphia site caused by the historic Canadian wildfires of 2023 and were considerably greater than the chamber-simulated BB experiment. Given the age of the sampled air masses, these elevated concentrations suggest sustained production and long-range transport of HMHP, highlighting that HMHP can accumulate, persist, and act as an important precursor to its degradation products over long distances. Given how commonplace I-CIMS measurements have become in the past decade, measurements of HMHP may serve as a useful tool for assessing the budgets and chemistry of several important tropospheric compounds, including Criegee intermediates, formaldehyde, and formic acid.