# Comparison of formaldehyde measurements by Hantzsch, CRDS and DOAS in the SAPHIR chamber

Three instruments that use different techniques to measure gaseous formaldehyde (inline-formulaHCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. One instrument (AL4021, Aero-Laser GmbH) detects inline-formulaHCHO using the wet-chemical Hantzsch reaction (for efficient gas-phase stripping), chemical conversion and fluorescence measurement. An internal inline-formulaHCHO permeation source allows for daily calibrations. This instrument was characterized by sulfuric acid titration (overall accuracy 8.6 %) and yields measurements with a time resolution of 90 inline-formulas and a limit of detection (3inline-formulaσ) of 0.3 inline-formulappbv. In addition, a new commercial instrument that makes use of cavity ring-down spectroscopy (CRDS) determined the concentrations of inline-formulaHCHO, water vapour, and methane (G2307, Picarro, Inc.). Its limit of detection (3inline-formulaσ) is specified as 0.3 inline-formulappbv for an integration time of 300 inline-formulas, and its accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 inline-formulappbv). A custom-built high-resolution laser differential optical absorption spectroscopy (DOAS) instrument provided inline-formulaHCHO measurements with a limit of detection (3inline-formulaσ) of 0.9 inline-formulappbv and an accuracy of 7 %​​​​​​​ using an optical multiple reflection cell. The measurements were conducted from June to December 2019 in experiments in which either ambient air flowed through the chamber or the photochemical degradation of organic compounds in synthetic air was investigated. Measured inline-formulaHCHO concentrations were up to 8 inline-formulappbv. Various mixtures of organic compounds, water vapour, nitrogen oxides and ozone were present in these experiments. Results demonstrate the need to correct the baseline in measurements performed by the Hantzsch instrument to compensate for drifting background signals. Corrections were equivalent to inline-formulaHCHO mixing ratios in the range of 0.5–1.5 inline-formulappbv. The baseline of the CRDS instrument showed a linear dependence on the water vapour mixing ratio with a slope of (inline-formula $M19inlinescrollmathml-normal 11.20±normal 1.60$ 70pt10ptsvg-formulamathimgb52a375c6283a43945b58ec274df1e6d amt-14-4239-2021-ie00001.svg70pt10ptamt-14-4239-2021-ie00001.png ) inline-formulappbv %inline-formula−1 below and (inline-formula $M22inlinescrollmathml-normal 0.72±normal 0.08$ 64pt10ptsvg-formulamathimg0cf150a646c24c970e453b50cc913828 amt-14-4239-2021-ie00002.svg64pt10ptamt-14-4239-2021-ie00002.png ) inline-formulappbv %inline-formula−1 above a water vapour mixing ratio of 0.2 %. In addition, the intercepts of these linear relationships drifted within the specification of the instrument (1.5 inline-formulappbv) over time but appeared to be equal for all water mixing ratios. Regular zero measurements are needed to account for the changes in the instrument zero. After correcting for the baselines of measurements by the Hantzsch and the CRDS instruments, linear regression analysis of measurements from all three instruments in experiments with ambient air indicated good agreement, with slopes of between 0.98 and 1.08 and negligible intercepts (linear correlation coefficients inline-formulaR2>0.96). The new small CRDS instrument measures inline-formulaHCHO with good precision and is accurate if the instrument zero is taken into account. Therefore, it can provide measurements with similar accuracy to the DOAS instrument but with slightly reduced precision compared to the Hantzsch instrument.

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Glowania, Marvin / Rohrer, Franz / Dorn, Hans-Peter / et al: Comparison of formaldehyde measurements by Hantzsch, CRDS and DOAS in the SAPHIR chamber. 2021. Copernicus Publications.

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