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The calibration of field instruments used to measure concentrations of OHand HO2 worldwide has traditionally relied on a single methodutilising the photolysis of water vapour in air in a flow tube atatmospheric pressure. Here the calibration of two FAGE (fluorescence assayby gaseous expansion) apparatuses designed for HOx (OH and HO2)measurements have been investigated as a function of external pressure usingtwo different laser systems. The conventional method of generating knownconcentrations of HOx from H2O vapour photolysis in a turbulentflow tube impinging just outside the FAGE sample inlet has been used to studyinstrument sensitivity as a function of internal fluorescence cell pressure(1.8–3.8 mbar). An increase in the calibration constants CHO andCHO2 with pressure was observed, and an empiricallinear regression of the data was used to describe the trends, with ΔCHO = (17 ± 11) % and ΔCHO2 = (31.6 ± 4.4)% increase permillibar air (uncertainties quoted to 2σ). Presented here are the firstdirect measurements of the FAGE calibration constants as a function ofexternal pressure (440–1000 mbar) in a controlled environment using theUniversity of Leeds HIRAC chamber (Highly Instrumented Reactor forAtmospheric Chemistry). Two methods were used: the temporal decay ofhydrocarbons for calibration of OH, and the kinetics of the second-orderrecombination of HO2 for HO2 calibrations. Over comparableconditions for the FAGE cell, the two alternative methods are in goodagreement with the conventional method, with the average ratio ofcalibration factors (conventional : alternative) across the entire pressurerange, COH(conv)/COH(alt) = 1.19 ± 0.26 andCHO2(conv)/CHO2(alt) = 0.96 ± 0.18 (2σ). These alternative calibration methods currently have comparablesystematic uncertainties to the conventional method: ~ 28% and ~ 41% for the alternative OH and HO2calibration methods respectively compared to 35% for the H2O vapourphotolysis method; ways in which these can be reduced in the future arediscussed. The good agreement between the very different methods ofcalibration leads to increased confidence in HOx field measurements andparticularly in aircraft-based HOx measurements, where there aresubstantial variations in external pressure, and assumptions are maderegarding loss rates on inlets as a function of pressure

atmospheric measurement techniques

Pressure-dependent calibration of the OH and HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; channels of a FAGE HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)