中央研究院 原子與分子科學研究所

Recent reports [Jara‐Toro et al., Angew. Chem. Int. Ed. 2017, 56, 2166 and PCCP 2018, 20, 27885] suggest that the rate coefficient of OH reactions with alcohols would increase by up to two times in going from dry to high humidity. This finding would have an impact on the budget of alcohols in the atmosphere and it may explain differences in measured and modeled methanol concentrations. The results were based on a relative technique carried out in a small Teflon bag, which might suffer from wall reactions. The effect was reinvestigated using a direct fluorescence probe of OH radicals, and no catalytic effect of H2O could be found. Experiments in a Teflon bag were also carried out, but the results of Jara‐Toro et al. were not reproducible. Further theoretical calculations show that the water‐mediated reactions have negligible rates compared to the bare reaction and that even though water molecules can lower the barriers of reactions, they cannot make up for the entropy cost.

Carbonyl oxides, or Criegee intermediates, are important transient species formed in
the reactions of unsaturated hydrocarbons with ozone. Although direct detection of
Criegee intermediates has recently been realized, the main atmospheric sink of
Criegee intermediates remains unclear. We report ultraviolet absorption spectroscopic
measurements of the lifetime of the simplest Criegee intermediate, CH₂OO, at various
relative humidity levels up to 85% at 298 kelvin. An extremely fast decay rate of
CH₂OO was observed at high humidity. The observed quadratic dependence of the
decay rate on water concentration implied a predominant reaction with water dimer.
On the basis of the water dimer equilibrium constant, the effective rate coefficient of
the CH₂OO + (H₂O)₂ reaction was determined to be 6.5(+/-0.8)×10⁻¹² cubic centimeters
per second. This work would help modelers to better constrain the atmospheric
concentrations of CH₂OO.

SO₂ scavenging and self-reaction of CH₂OO were utilized for the decay of CH₂OO to extract the absorption spectrum of CH₂OO under bulk conditions. Absolute absorption cross sections of CH₂OO at 308.4 and 351.8 nm were obtained from laser-depletion measurements in a jet-cooled molecular beam. The peak cross section is (1.23 ± 0.18) × 10⁻¹⁷ cm² at 340 nm.

Institute of Atomic and Molecular Sciences (IAMS) have made precise measurements of chlorine peroxide (ClOOCl), calming recent international debate on exactly how humans are depleting the ozone layer. Their results were published in Science on May 8, 2009.
The chemical under debate, ClOOCl, is formed in the atmosphere due to human emissions of chlorofluorocarbons, major ingredients in refrigerants and propellants. In the presence of sunlight, photolysis of ClOOCl forms Cl atoms; Cl atoms react with ozone (O₃) to form O₂ and ClO, ClO can then dimerize to form ClOOCl again, thus catalytically converting O₃ to O₂. The larger the absorption cross section of ClOOCl, the faster it absorbs sunlight, then the faster the chlorine atoms are generated, and thus the faster the ozone is destroyed.
Dr. Jim J. Lin and his colleagues at IAMS designed an elegant experiment using a chlorine peroxide molecular beam, and determined the photodissociation probability (the probability of the chlorine peroxide being split into chlorine atoms and oxygen by light) by measuring the decrease in beam intensity after laser irradiation. By precisely measuring the ratio of the molecules before and after laser irradiation, they were able to quantify the absorption cross section without knowing the absolute concentration. Using currently accepted kinetic models, their new results can well explain the ozone-hole formation as well as the partition of ClO/ClOOCl from field measurements, indicating that the current atmospheric models are in fact still valid.