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<p>Three instruments that use different techniques to measure gaseous formaldehyde (<span class="inline-formula">HCHO</span>) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. One instrument (AL4021, Aero-Laser GmbH) detects <span class="inline-formula">HCHO</span> using the wet-chemical Hantzsch reaction (for efficient gas-phase stripping), chemical conversion and fluorescence measurement. An internal <span class="inline-formula">HCHO</span> 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 <span class="inline-formula">s</span> and a limit of detection (3<span class="inline-formula"><i>σ</i></span>) of 0.3 <span class="inline-formula">ppbv</span>. In addition, a new commercial instrument that makes use of cavity ring-down spectroscopy (CRDS) determined the concentrations of <span class="inline-formula">HCHO</span>, water vapour, and methane (G2307, Picarro, Inc.). Its limit of detection (3<span class="inline-formula"><i>σ</i></span>) is specified as 0.3 <span class="inline-formula">ppbv</span> for an integration time of 300 <span class="inline-formula">s</span>, and its accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 <span class="inline-formula">ppbv</span>). A custom-built high-resolution laser differential optical absorption spectroscopy (DOAS) instrument provided <span class="inline-formula">HCHO</span> measurements with a limit of detection (3<span class="inline-formula"><i>σ</i></span>) of 0.9 <span class="inline-formula">ppbv</span> 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 <span class="inline-formula">HCHO</span> concentrations were up to 8 <span class="inline-formula">ppbv</span>. 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 <span class="inline-formula">HCHO</span> mixing ratios in the range of 0.5–1.5 <span class="inline-formula">ppbv</span>. The baseline of the CRDS instrument showed a linear dependence on the water vapour mixing ratio with a slope of (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">11.20</mn><mo>±</mo><mn mathvariant="normal">1.60</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="b52a375c6283a43945b58ec274df1e6d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-4239-2021-ie00001.svg" width="70pt" height="10pt" src="amt-14-4239-2021-ie00001.png"/></svg:svg></span></span>) <span class="inline-formula">ppbv</span> %<span class="inline-formula">⁻¹</span> below and (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.72</mn><mo>±</mo><mn mathvariant="normal">0.08</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="0cf150a646c24c970e453b50cc913828"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-4239-2021-ie00002.svg" width="64pt" height="10pt" src="amt-14-4239-2021-ie00002.png"/></svg:svg></span></span>) <span class="inline-formula">ppbv</span> %<span class="inline-formula">⁻¹</span> 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 <span class="inline-formula">ppbv</span>) 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 <span class="inline-formula"><i>R</i>²&gt;0.96</span>). The new small CRDS instrument measures <span class="inline-formula">HCHO</span> 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.</p>