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Abstract

Laser induced fluorescence (LIF) is a widely used technique for both laboratory-based and ambient atmospheric chemistry measurements. However, LIF instruments require calibrations in order to translate instrument response into concentrations of chemical species. Calibration of LIF instruments measuring OH and HO₂ (HO_X), typically involves the photolysis of water vapor by 184.9 nm light thereby producing quantitative amounts of OH and HO₂. For ground-based systems HO_X instruments, this method of calibration is done at one pressure (typically ambient pressure) at the instrument inlet. However, airborne HO_X instruments can experience varying cell pressures, internal residence times, temperatures, and humidity during flight. Therefore, replication of such variances when calibrating are essential to acquire the appropriate sensitivities. This requirement resulted in the development of the APACHE (All Pressure Altitude-based Calibrator for HO_X Experimentation) chamber. It utilizes photolysis of water vapor, but has the additional ability to alter the pressure at the inlet of the HO_X instrument thus relating instrument sensitivity to the external pressure ranges experienced during flight (275 to 1000 mbar). Measurements supported by COMSOL multiphysics and its computational fluid dynamics calculations revealed that, for all pressures explored in this study, APACHE is capable of initializing homogenous flow and maintain near uniform flow speeds across the internal cross-section of the chamber. This reduces the uncertainty regarding average exposure times across the mercury (Hg) UV ring lamp. Two different actinometrical approaches characterized the APACHE UV ring lamp flux as 6.3 x 10¹⁴ (± 0.9 x 10¹⁴) s^-1 depending on pressure. Data presented in this study are the first direct calibrations, performed in a controlled environment using APACHE of an airborne HO_X system instrument.