Abstract
Abstract. Three instruments that use different techniques to measure gaseous formaldehyde (HCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. One instrument (AL4021, Aero-Laser GmbH) detects HCHO using the wet-chemical Hantzsch reaction (for efficient gas-phase stripping), chemical conversion and fluorescence measurement. An internal HCHO 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 s and a limit of detection (3σ) of 0.3 ppbv. In addition, a new commercial instrument that makes use of cavity ring-down spectroscopy (CRDS) determined the concentrations of HCHO, water vapour, and methane (G2307, Picarro, Inc.). Its limit of detection (3σ) is specified as 0.3 ppbv for an integration time of 300 s, and its 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 (3σ) of 0.9 ppbv 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 HCHO concentrations were up to 8 ppbv. 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 HCHO mixing ratios in the range of 0.5–1.5 ppbv. The baseline of the CRDS instrument showed a linear dependence on the water vapour mixing ratio with a slope of (-11.20±1.60) ppbv %−1 below and (-0.72±0.08) ppbv %−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 ppbv) 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 R2>0.96). The new small CRDS instrument measures HCHO 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.
Highlights
Formaldehyde (HCHO) is a pollutant that is present in ambient air and indoors
The largest contributions to HCHO production come from the oxidation of methane (CH4) (∼ 970 Tg yr−1) and the oxidation of nonmethane volatile organic compounds (NMVOCs) such as terpenes and other hydrocarbons (∼ 250 Tg yr−1) from biogenic sources (Anderson et al, 2017; Zhang et al, 2019)
More than 100 d of measurements were performed by instruments that detected formaldehyde in photochemical experiments in the SAPHIR chamber in 2019
Summary
Formaldehyde (HCHO) is a pollutant that is present in ambient air and indoors It affects human health by irritating the respiratory system and by being carcinogenic (Gupta et al, 1982; Casset et al, 2005; World Health Organization, 2010; Fortems-Cheiney et al, 2012; Liu et al, 2015; Salthammer, 2019; Qin et al, 2020). It is formed in the atmosphere as a product of the oxidation of volatile organic compounds (VOCs) and in combustion processes, includ-. HCHO has a typical chemical lifetime in the troposphere of a few hours during the day and up to 2 d during the night, when photolysis and the reaction with OH do not play a big role (Lowe and Schmidt, 1983; Seinfeld and Pandis, 2016)
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