A new method for atmospheric detection of the CH 3O 2 radical
A new method for measurement of the methyl peroxy (CH 3O 2) radical has been developed using the conversion of CH 3O 2 into CH 3O by excess NO with subsequent detection of CH 3O by fluorescence assay by gas expansion (FAGE) with laser excitation at ca. 298 nm. The method can also directly detect CH 3O, 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 CH 3O 2 via reaction with CH 4 in the presence of O 2. Detection limits of 3.8 × 10 8 and 3.0 × 10 8 molecule cm −3 were determined for CH 3O 2 and CH 3O respectively for a signal-to-noise ratio of 2 and 5 min averaging time. Averaging over 1 h reduces the detection limit for CH 3O 2 to 1.1 × 10 8 molecule cm −3, which is comparable to atmospheric concentrations. The kinetics of the second-order decay of CH 3O 2 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 1 σ 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 CH 3O in chamber experiments is demonstrated via observation in HIRAC of CH 3O formed as a product of the CH 3O 2 self-reaction.