Time-resolved Absorption Spectroscopy Applied to Tropospheric Chemistry

A Time-Resolved UV-Vis Absorption Spectrometer (TRUVAS) has been constructed with the ability to record time-resolved absorption spectra between ~250-800 nm, with a maximum time-resolution of 0.78 ms. The rate of reaction of the C1 Criegee Intermediate (formaldehyde oxide) with water vapour has been studied. The removal rate constant shows a quadratic dependence on [H2O], implying reaction with the water dimer, (H2O)2. The removal rate of the Criegee intermediate via. reaction with the water dimer ((H2O)2) was discovered to be 4.0 ± 1.2 × 10−12 cm3 molecule−1 s−1, and sufficiently fast to dominate the species’ removal in the troposphere. This result was published in PCCP3. The primary motivation for building the TRUVAS instrument was to measure the absorption cross-sections of isoprene-derived peroxy radicals in the actinic region, with a view to calculating the total atmospheric photolysis rates. The total atmospheric photolysis rates of the isoprene, 2,3-dimethylbutadiene and butadiene hydroxyperoxy (HP) radicals were calculated at noon in Borneo, and were found to be 1.27, 1.81 and 1.01 × 10-3 s-1 respectively. The absorption spectra of ethylene, but-2-ene and 2,3-dimethylbut-2-ene (TetraMethyl Ethylene – TME) HP radicals were measured. The measured absorption spectra show a marked broadening effect, with the Gaussian width increasing 11% from ethylene to but-2-ene HP radical absorption spectrum, and 26% to the TME HP radical, which translate to atmospheric photolysis rates of 0.552, 1.21 and 1.68 × 10-3 s-1. The absorption spectra of the methyl, tertiary-butyl and cyclohexyl peroxy radicals are presented. The methyl peroxy absorption spectrum measured in this study largely agree with the numerous existing studies. Atmospheric photolysis of methyl peroxy radicals comprises around 1% of the species’ removal in the marine boundary layer.