Comparison of temperature dependent calibration methods of an instrument to measure OH and HO2 radicals using laser-induced fluorescence spectroscopy.

Laser Induced Fluorescence (LIF) spectroscopy has been widely applied to fieldwork measurements of OH radicals, and of HO₂, following conversion to OH, over a wide variety of conditions, on different platforms, and in simulation chambers. Conventional calibration of HO_x (OH + HO₂) instruments has mainly relied on a single method, generating known concentrations of HO_x from H₂O vapour photolysis in a flow of zero air impinging just outside the sample inlet (SHO_x = CHO_x.[HO_x], where SHO_x is the observed signal and CHO_x is the calibration factor). The FAGE (Fluorescence Assay by Gaseous Expansion) apparatus designed for HO_x measurements in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) at the University of Leeds has been used to examine the sensitivity of FAGE to external gas temperatures (266 -- 348 K). The conventional calibration methods give the temperature dependence of C_OH (relative to the value at 293 K) of (0.0059 ± 0.0015) K^-1 and CHO₂ of (0.014 ± 0.013) K^-1. Errors are 2σ. C_OH was also determined by observing the decay of hydrocarbons (typically cyclohexane) caused by OH reactions giving C_OH (again, relative to the value at 293 K) of (0.0038 ± 0.0007) K^-1. Additionally, CHO₂ was determined based on the second order kinetics of HO₂ recombination with the temperature dependence of CHO₂, relative to 293 K being (0.0064 ± 0.0034) K^-1. The temperature dependence of CHO_x depends on HO_x number density, quenching, relative population of the probed OH rotational level and HO_x transmission from inlet to detection axis. The first three terms can be calculated and, in combination with the measured values of CHO_x, show that HO_x transmission increases with temperature. Comparisons with other instruments and the implications of this work are discussed. [ABSTRACT FROM AUTHOR]