Calibration of an airborne HO<sub><i>x</i></sub> instrument using the All Pressure Altitude-based Calibrator for HO<sub><i>x</i></sub> Experimentation (APACHE)

Abstract

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 (HOx) typically involves the photolysis of water vapor by 184.9 nm light, thereby producing quantitative amounts of OH and HO2. For ground-based HOx instruments, this method of calibration is done at one pressure (typically ambient pressure) at the instrument inlet. However, airborne HOx 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 HOx Experimentation) chamber to characterize the sensitivity of the airborne LIF-FAGE (fluorescence assay by gas expansion) HOx 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 OH (26.1–7.8 cts s−1 pptv−1 mW−1, ±22.6 % 1σ; cts stands for counts by the detector) and HO2 (21.2–8.1 cts s−1 pptv−1 mW−1, ±22.1 % 1σ) 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 6.37×1014(±1.3×1014) photons cm−2 s−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 6.9(±1.1)×1014 photons cm−2 s−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 6.11(±0.8)×1014 photons cm−2 s−1. Data presented in this study are the first direct calibrations of an airborne HOx instrument, performed in a controlled environment in the lab using APACHE.

New photolytic converter for improving aircraft measurements of NO<sub>2</sub> via chemiluminescence

“…Final NO 2 concentrations were determined via Equation ( 4) which includes subtraction of the calculated (and interpolated) NO c background concentration and the NO concentration in the NO channel, and dividing by the conversion efficiency of the blue light converter which was 24.2 ± 2.8 % during the campaign. (Novelli et al, 2014;Marno et al, 2020). Please note that these data are still preliminary and the measurement uncertainty is estimated at 50 %.…”

Central role of nitric oxide in ozone production in the upper tropical troposphere over the Atlantic Ocean and western Africa

“…CO and CH 4 were measured by mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) with TRISTAR, a multichannel spectrometer (Schiller et al, 2008;Tadic et al, 2017). OH and HO 2 radicals were measured by laser-induced fluorescence with the custom-built HORUS instrument (Marno et al, 2020). Note that both OH and HO 2 data are preliminary.…”

“…Important for this work, the NO x emissions from lightning activity have been estimated using the submodel LNOX (Tost et al, 2007), where the parameterization by Grewe et al (2001) was applied. The global NO x emissions from lightning were scaled to 6.3 Tg(N) yr −1 , following Miyazaki et al (2014). Tracer and aerosol wet and dry deposition were estimated following Tost et al (2006) and Kerkweg et al (2006), respectively.…”

Central role of nitric oxide in ozone production in the upper tropical troposphere over the Atlantic Ocean and West Africa

“…CO and CH4 were measured by mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) with TRISTAR, a multi-channel spectrometer (Schiller et al, 2008;Tadic et al, 2017). OH and HO2 radicals were measured by laser-induced fluorescence with the custom-built HORUS instrument (Marno et al, 2020). Note that both OH and HO2 data are preliminary.…”