The reaction of Criegee intermediate CH₂OO with water dimer: primary products and atmospheric impact

The rapid reaction of the smallest Criegee intermediate, CH₂OO, with water dimers is the dominant removal mechanism for CH₂OO in the Earth’s atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10 – 90%, relative to previous assumptions, which indicates that the reaction CH₂OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.