Department of Physics, NCU
Direct Kinetic Measurement of the Reaction of the Simplest Criegee Intermediate CH2OO with Water Vapor
Dr. Jim Jr-Min Lin (林志民)
Institute of Atomic and Molecular Sciences, Academia Sinica
Department of Chemistry, National Taiwan University
Ozonolysis reaction has been seem as important not only because it is a crucial way to diminish alkenes but it generates a series of highly reactive intermediates, called Criegee intermediates (CIs). A lot of issues about atmospheric chemistry had been thought to relate to CIs, such as OH radical concentration or acid rain formation, thus understanding about the reaction rate of CIs with common species in troposphere is important. Despites their importance, direct detection of CIs has been realized only quite recently. A new method, photolysis of diiodoalkanes, is reported to generate sufficient CIs in lab to do direct kinetic measurement in 2012. Rate coefficients of CIs reactions with SO2, NO2, formic acid have been measured with direct detection. However, the rate of CIs reaction with water vapor is still unclear.
We focus on the reaction kinetics of the simplest Criegee intermediate (CH2OO) with water vapor by using ultraviolet absorption spectroscopy. A multiple pass system is build up to make the reaction clear and enhance the signal. The lifetime of CH2OO at various relative humidity levels up to 85% at 298 K was measured. A very fast decay rate (~ 5000 per second) of CH2OO was observed at high humidity. The decay rate has a quadradic dependence of concentration of water monomer, indicating that two water molecules participate in the reaction of CH2OO decay. The effective rate coefficient of CH2OO reaction with water dimer was determined to be (6.5±0.8)x1012 cm3 s1, independent of the total pressure from 100 to 500 Torr. This result offers a new insight about the reactivity of Criegee intermediates and provides a crucial data for better understanding the Criegee chemistry.