Abstract
Abstract. A new method for measurement of the methyl peroxy (CH3O2) radical has been developed using the conversion of CH3O2 into CH3O by excess NO with subsequent detection of CH3O by fluorescence assay by gas expansion (FAGE) with laser excitation at ca. 298 nm. The method can also directly detect CH3O, when no nitric oxide is added. Laboratory calibrations were performed to characterise the FAGE instrument sensitivity using the conventional radical source employed in OH calibration with conversion of a known concentration of OH into CH3O2 via reaction with CH4 in the presence of O2. Detection limits of 3.8 × 108 and 3.0 × 108 molecule cm−3 were determined for CH3O2 and CH3O respectively for a signal-to-noise ratio of 2 and 5 min averaging time. Averaging over 1 h reduces the detection limit for CH3O2 to 1.1 × 108 molecule cm−3, which is comparable to atmospheric concentrations. The kinetics of the second-order decay of CH3O2 via its self-reaction were observed in HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) at 295 K and 1 bar and used as an alternative method of calibration to obtain a calibration constant with overlapping error limits at the 1σ level with the result of the conventional method of calibration. The overall uncertainties of the two methods of calibrations are similar – 15 % for the kinetic method and 17 % for the conventional method – and are discussed in detail. The capability to quantitatively measure CH3O in chamber experiments is demonstrated via observation in HIRAC of CH3O formed as a product of the CH3O2 self-reaction.
Highlights
Methyl peroxy (CH3O2) radicals are critical intermediates in the atmospheric oxidation (Orlando and Tyndall, 2012) and combustion of hydrocarbons (Zador et al, 2011)
As methoxy (CH3O) radicals can be generated by techniques such as pulsed laser photolysis and microwave discharge and detected with high sensitivity by laser-induced fluorescence (LIF) (Shannon et al, 2013; Chai et al, 2014; Albaladejo et al, 2002; Biggs et al, 1993, 1997), the method has been used in kinetic studies of a range of CH3O reactions
This paper reports the development of a new method for the selective and sensitive detection of CH3O2 radicals using fluorescence assay by gas expansion (FAGE) by titrating CH3O2 to CH3O by reaction with added NO (Reaction R3) and detecting the resultant CH3O by off-resonant LIF with laser excitation at ca. 298 nm
Summary
Methyl peroxy (CH3O2) radicals are critical intermediates in the atmospheric oxidation (Orlando and Tyndall, 2012) and combustion of hydrocarbons (Zador et al, 2011). As methoxy (CH3O) radicals can be generated by techniques such as pulsed laser photolysis and microwave discharge and detected with high sensitivity by LIF (Shannon et al, 2013; Chai et al, 2014; Albaladejo et al, 2002; Biggs et al, 1993, 1997), the method has been used in kinetic studies of a range of CH3O reactions. These studies used the electronic excitation of the methoxy radical from the ground state to the first electronically excited state (A2A1 ← X2E). This work enhances the capability of HIRAC to measure short-lived radical species by the addition of both CH3O2 and CH3O detection, and we discuss the potential of the method for detection of CH3O2 in the atmosphere itself
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