Abstract. Three instruments using different techniques measuring gaseous formaldehyde (HCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. 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.

中文翻译：

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在SAPHIR箱中通过Hantzsch，CRDS和DOAS测量甲醛的比较

摘要。在ForschungszentrumJülich的大气模拟室SAPHIR中的实验中，比较了使用不同技术测量气态甲醛（HCHO）浓度的三种仪器。一种仪器通过使用湿化学Hantzsch反应检测HCHO，以进行有效的气相汽提，化学转化和荧光测量（AL4021，Aero Laser GmbH）。内部渗透HCHO源可用于日常校准。其特征在于硫酸滴定法（总准确度为8.5％）。测量的时间分辨率为90 s，检测极限（3σ）为0.3 ppbv。此外，使用腔衰荡光谱仪（CRDS）的新型商用仪器可测定HCHO，水和甲烷的浓度（G2307，Picarro Inc.）。检测极限（3σ）指定为0。3 ppbv的积分时间为300 s，其精度受到零信号漂移的限制（制造商规格为1.5 ppbv）。定制的高分辨率激光差分光学吸收光谱（DOAS）仪器使用光学多重反射池为HCHO测量提供的检测极限（3σ）为0.9 ppbv，准确度为6％。测量是从2019年6月至2019年12月在实验中进行的，其中要么使环境空气流过腔室，要么研究合成空气中有机化合物的光化学降解。测得的HCHO浓度高达8 ppbv。这些实验中存在有机化合物，水蒸气，氮氧化物和臭氧浓度的各种混合物。结果表明需要校正Hantzsch仪器测量中的基线以补偿漂移的背景信号。校正值等于0.5到1.5 ppbv范围内的HCHO混合比。CRDS仪器的基线显示出对水汽混合比的线性依赖性，斜率（-11.20±1.60）ppbv％高于和低于0.2％水蒸气混合比^-1和（-0.72±0.08）ppbv％^-1。另外，这些线性关系的截距在仪器的规格范围内（1.5 ppbv）随时间漂移，但对于所有水混合比而言似乎相等。需要定期进行零位测量以解决仪器零位的变化。在校正了Hantzsch和CRDS仪器的测量基准之后，对所有三种仪器在环境空气实验中进行的测量值进行线性回归分析，得出斜率在0.93和1.07之间且截距可忽略不计（线性相关系数R ^{2为良好）} > 0.96）。如果将零位考虑在内，那么新型的小型CRDS仪器可以高精度，准确地测量HCHO。因此，与Hantzsch仪器相比，它可以像DOAS仪器一样提供准确且无需校准的测量，而精度却略有降低。