Abstract
Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces – ∼45% of reaction products are obtained via intersystem crossing of a pre-product complex – which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.
Highlights
CH3OH CH3OO (×10) HO2 (×10)500 1000 1500 2000 2500 3000 Kinetic time (s)0 500 1000 1500 2000 2500 3000 Kinetic time (s)(same as HO2, blue open squares and blue solid diamonds, Fig. 3b) at a level where its reaction rate with OH was competitive with the reaction rate of OH with CH4
We report direct determinations of the methanol yield using two different experimental approaches: isotopologues of OH + CH3OO via multiplexed photoionization mass spectrometry (MPIMS) and a chamber study coupled to proton-transfer reaction time-of-flight mass spectrometry (PTR-TOFMS)
The products of the OH + CH3OO reaction were quantified at 30 Torr in pulsed photolytic experiments using the Sandia multiplexed photoionization mass spectrometer, and at 740 Torr using a new high-pressure reactor, both interfaced with the tuneable-VUV-output of the Chemical Dynamics Beamline (9.0.2) at the Advanced Light Source of Lawrence Berkeley National Laboratory
Summary
The relevant differences between the experiments lie in the sampling, detection, and residence time Based on these three factors we conclude that the stabilised trioxide (5), with a predicted[18] yield of ~11% at atmospheric pressure, and observed in the 740 Torr MPIMS experiments (Supplementary Note 6 and Figures therein), could undergo water-assisted heterogeneous conversion to methanol (a pathway discussed by Müller et al.18) in the chamber or sampling line, or fragment upon protonation in the PTR-TOFMS detection system, as has been observed for many organic species[29,30,31]. H2O is present in close proximity to the newly protonated trioxide as a result of the proton-transfer reaction in the PTR-TOFMS detection system and is present in appreciable concentrations as a reaction precursor (2.5–3.8 × 1016 molecule cm−3 and higher in the PTR-TOFMS chamber due to the injection of water to produce H3O+). Because there is no method for calibrating the PTR-TOFMS for trioxide, the degree of interference cannot be directly determined
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