The first calibration of a FAGE HOx instrument over a range of external pressures using HIRAC

OH and HO2 (HOx) are two of the most critical radical species in the atmosphere. This paper describes the first calibration of a FAGE (Fluorescence Assay by Gas Expansion) HOx instrument over a range of external pressures simulating the pressure conditions experienced during aircraft measurements at different altitudes. FAGE is a low pressure, on resonance, laser induced fluorescence technique employed for the detection of OH and HO2 radicals from both ground-based and airborne platforms. The sensitivity of FAGE instruments is known to be strongly dependent upon the internal detection cell pressure and hence external pressure, amongst other environmentally and experimentally induced parameters. The calibration of FAGE instruments over a range of external pressures has not previously been possible. Instead, the pressure dependence of FAGE instruments has been determined by invoking a range of internal detection cell pressures using different inlet pinhole diameters and calibrating the instrument on the ground using the traditional FAGE HOx calibration method of 185 nm photolysis of H2O vapour in air. This method potentially introduces a number of unquantifiable variables when compared to ambient measurement such as changes in the losses of radicals upon sampling and the flow dynamics within the FAGE inlet. The Highly Instrumented Reactor for Atmospheric Chemistry, HIRAC, has offered the first opportunity to develop an independent method for calibrating FAGE instruments over a range of external pressures. HIRAC is an atmospheric simulation chamber which allows researchers the unique ability to simultaneously control temperature, pressure and photolysis rates during experiments. Pressure dependent calibrations to determine the sensitivity of a FAGE instrument to OH between 300 - 1000 mbar were carried out inside HIRAC by measuring the decay of multiple hydrocarbons in the presence of OH produced from the photolysis of methyl nitrite. The results from these experiments were found to be in excellent agreement with those determined using the traditional H2O photolysis method with a variety of pinhole diameters thus validating previous measurements of FAGE instrument sensitivity to OH as a function of pressure. Pressure dependent HO2 calibrations of a FAGE instrument inside HIRAC are currently underway. The methodology for these calibrations involves measuring the decay in the FAGE HO2 signal produced from HCHO photolysis resulting solely from HO2 self-reaction.