Calibration of an airborne HO _x instrument using the All Pressure Altitude-based Calibrator for HO _x Experimentation (APACHE)

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 inline-formulaOH and inline-formulaHO₂ (inline-formulaHO_x) typically involves the photolysis of water vapor by 184.9 nm light, thereby producing quantitative amounts of inline-formulaOH and inline-formulaHO₂. For ground-based inline-formulaHO_x instruments, this method of calibration is done at one pressure (typically ambient pressure) at the instrument inlet. However, airborne inline-formulaHO_x instruments can experience varying cell pressures, internal residence times, temperatures, and humidity during flight. Therefore, replication of such variances when calibrating in the lab is essential to acquire the appropriate sensitivities. This requirement resulted in the development of the APACHE (All Pressure Altitude-based Calibrator for inline-formulaHO_x Experimentation) chamber to characterize the sensitivity of the airborne LIF-FAGE (fluorescence assay by gas expansion) inline-formulaHO_x instrument, HORUS, which took part in an intensive airborne campaign, OMO-Asia 2015. It utilizes photolysis of water vapor but has the additional ability to alter the pressure at the nozzle of the HORUS instrument. With APACHE, the HORUS instrument sensitivity towards inline-formulaOH (26.1–7.8 cts sinline-formula⁻¹ pptvinline-formula⁻¹ mWinline-formula⁻¹, inline-formula±22.6 % inline-formula1σ; cts stands for counts by the detector) and inline-formulaHO₂ (21.2–8.1 cts sinline-formula⁻¹ pptvinline-formula⁻¹ mWinline-formula⁻¹, inline-formula±22.1 % inline-formula1σ) was characterized to the external pressure range at the instrument nozzle of 227–900 mbar. Measurements supported by a computational fluid dynamics model, COMSOL Multiphysics, revealed that, for all pressures explored in this study, APACHE is capable of initializing a homogenous flow and maintaining 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 inline-formula $M24inlinescrollmathml\mathrm{normal\; 6.37}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 14}}(\pm \mathrm{normal\; 1.3}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 14}}$ 115pt15ptsvg-formulamathimg3d8dda426ca0cacde246df780151d7c0 amt-13-2711-2020-ie00001.svg115pt15ptamt-13-2711-2020-ie00001.png ) photons cminline-formula⁻² sinline-formula⁻¹. One approach used the HORUS instrument as a transfer standard in conjunction with a calibrated on-ground calibration system traceable to NIST standards, which characterized the UV ring lamp flux to be inline-formula $M27inlinescrollmathml\mathrm{normal\; 6.9}(\pm \mathrm{normal\; 1.1})\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 14}}$ 80pt15ptsvg-formulamathimg88d07a903739f020f62bd09a799c9c09 amt-13-2711-2020-ie00002.svg80pt15ptamt-13-2711-2020-ie00002.png  photons cminline-formula⁻² sinline-formula⁻¹. The second approach involved measuring ozone production by the UV ring lamp using an ANSYCO O3 41 M ozone monitor, which characterized the UV ring lamp flux to be inline-formula $M30inlinescrollmathml\mathrm{normal\; 6.11}(\pm \mathrm{normal\; 0.8})\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 14}}$ 86pt15ptsvg-formulamathimg3b646dd948c1c7eed9e1cf82e6531115 amt-13-2711-2020-ie00003.svg86pt15ptamt-13-2711-2020-ie00003.png  photons cminline-formula⁻² sinline-formula⁻¹. Data presented in this study are the first direct calibrations of an airborne inline-formulaHO_x instrument, performed in a controlled environment in the lab using APACHE.