Pressure and temperature dependence of OH and HO2 detection sensitivities for an LIF based FAGE instrument – a comparison of calibration methods.

Abstract:

Instruments based on the Laser Induced Fluorescence (LIF) of OH radicals have been widely applied to fieldwork measurements – on the ground and in aircraft – as well as in atmospheric simulation chambers. The calibration of instruments used to measure concentrations of HO_x worldwide have traditionally relied on a single method of generating known concentrations of HO_x from H₂O vapour photolysis in a turbulent flow tube impinging just outside the sample inlet. The FAGE (Fluorescence Assay by Gaseous Expansion) apparatus designed for HO_x measurements in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) has been calibrated using the conventional method over a range of internal cell pressures (1.8 – 3.8 mbar) and external cell temperatures (270 – 340 K). Two alternative calibration techniques for OH and HO₂ were developed using the HIRAC chamber over the same pressure and temperature range to verify the observations. The alternative methods, agreement and disagreement with the conventional calibration methods will be discussed. The outcome of these studies will enable future operation of the FAGE instrument in the HIRAC chamber away from STP conditions.