Comparison of formaldehyde measurements by Hantzsch, CRDSand DOAS in the SAPHIR chamber

Abstract. Three instruments using different techniques measuring gaseous formaldehyde (HCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Julich. One instrument detected HCHO by using the wet-chemical Hantzsch reaction for efficient gas-phase stripping, chemical conversion and fluorescence measurement (AL4021, Aero Laser GmbH). An internal permeation HCHO source allows for daily calibrations. It was characterized by sulfuric acid titration (overall accuracy 8.5 %). Measurements have a time resolution of 90 s with a limit of detection (3 σ) of 0.3 ppbv. In addition, a new commercial instrument making use of cavity ring-down spectroscopy (CRDS) determined concentrations of HCHO, water, and methane (G2307, Picarro Inc.). The limit of detection (3 σ) is specified as 0.3 ppbv for an integration time of 300 s and the accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 ppbv). A custom-built, high-resolution laser differential optical absorption spectroscopy (DOAS) instrument provided HCHO measurements with a limit of detection (3 σ) of 0.9 ppbv and an accuracy of 6 % using an optical multiple reflection cell. The measurements were conducted from June to December 2019 in experiments in which either ambient air was flowed through the chamber or the photochemical degradation of organic compounds in synthetic air was investigated. Measured HCHO concentrations were up to 8 ppbv. Various mixtures of organic compounds, water vapour, nitrogen oxides, and ozone concentrations were present in these experiments. Results demonstrate the need to correct the baseline in the measurements of the Hantzsch instrument to compensate for drifting background signals. Corrections were equivalent to HCHO mixing ratios in the range of 0.5 to 1.5 ppbv. The baseline of the CRDS instrument showed a linear dependence on the water-vapour mixing ratio with different slopes of (−11.20 ± 1.60) ppbv %−1 and (−0.72 ± 0.08) ppbv %−1 above and below 0.2 % water vapour mixing ratio, respectively. In addition, the intercept of these linear relationships drifted with time within the specification of the instrument (1.5 ppbv), but appeared to be equal for all water mixing ratios. Regular zero measurements are required to account for the changes in the instrument zero. After correcting for the baselines of measurements by the Hantzsch and the CRDS instruments, a linear regression analysis of measurements from all three instruments in experiments with ambient air results in a good agreement with slopes between 0.93 and 1.07 with negligible intercepts (linear correlation coefficients R2 > 0.96). The new, small-sized CRDS instrument measures HCHO with a good precision and is accurate, if the instrument zero is taken into account. Therefore, it can provide accurate and calibration-free measurements like the DOAS instrument with a slightly reduced precision compared to the Hantzsch instrument.