New methods for the speciated detection of peroxy radicals

Organic peroxy radicals, RO2, are a key component of the reaction cycles that control the chemical composition of the troposphere and how efficiently pollutants like greenhouse gases are removed. They are formed through the oxidation of any biogenic or anthropogenic volatile organic compound (VOC) in the troposphere and lead to the production of ozone, secondary organic aerosols, organic nitrates and more. Despite the significance of RO2 radicals in tropospheric chemistry, there are currently no methods in regular use that can perform speciated measurements of the radicals in the field, instead only the sum of all RO2 can be measured routinely. In this thesis a new method is developed to specifically detect the methyl-peroxy radical, CH3O2, which is the simplest and most abundant RO2 radical in the troposphere. The method is based on Fluorescence Assay by Gas Expansion (FAGE) and detects CH3O2 by converting it to CH¬3O by titration with NO, which is then detected by Laser Induced Fluorescence (LIF) using ~298 nm light to excite the A2A1 (’3 = 3)  X2E (”3 = 0) electronic transition. With a limit of detection of ~2 × 108 molecule cm 3 for a 5-minute averaging time the method could be a viable technique in low NOx environments where CH3O2 levels are estimated to be ~2 - 6 × 108 molecule cm 3. The method was tested using the Highly Instrumented Reaction for Atmospheric Chemistry (HIRAC), an atmospheric simulation chamber that can operate at temperatures between ~250 – 350 K. Using the new FAGE method in conjunction with HIRAC, the temperature dependent kinetics of the CH3O2 self-reaction were measured between 268 – 344 K, giving a temperature dependent rate coefficient of k6.1 = (4.2±3.8) × 10 14 exp[(516±284)/T] cm3 molecule 1 s 1 and a value at 298 K of k6.1 = (2.37 ± 1.09) × 10 13 cm3 molecule 1 s 1. No significant temperature dependence was found, but the rate coefficients are approximately 30 % lower than current literature. A ROxLIF instrument, newly developed for HIRAC, was also used to make preliminary measurements of the CH3O2 self-reaction at 295 K, finding a value that was similarly ~30 % lower than literature. The temperature dependence of the CH3O2 + HO2 reaction was also measured using the new FAGE method, giving a temperature dependent rate coefficient of k6.3 = (1.26 ± 0.38) × 10 13 exp[(1119 ± 89)/T] cm3 molecule 1 s 1 and a value at 298 K of k6.3 = (5.38 ± 0.18) × 10 12 cm3 molecule 1 s 1, agreeing well with literature values.