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-formulaHOx) 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-formulaHOx instruments, this method of calibration is done at one pressure (typically ambient pressure) at the instrument inlet. However, airborne inline-formulaHOx 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-formulaHOx Experimentation) chamber to characterize the sensitivity of the airborne LIF-FAGE (fluorescence assay by gas expansion) inline-formulaHOx 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
Vorschau
Zitieren
Marno
Zugriffsstatistik
