The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band
The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band
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DOI：
10.5194/amt-12-2371-2019
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发表时间：
2019-04-15
影响因子：
3.8
通讯作者：
Langridge, Justin M.
中科院分区：
文献类型：
--
作者：
Cotterell, Michael I.;Orr-Ewing, Andrew J.;Langridge, Justin M.
光声光谱法是一种灵敏的测量气体和气溶胶样品吸收系数的原位技术。光声光谱仪（PAS）仪器需要通过比较测量的光声响应与已知的校准剂吸收水平来精确校准。臭氧是PAS仪器的常用校准剂，但Bluvshtein等人（2017）最近的工作对臭氧作为405nm波长校准剂的有效性提出了不确定性。此外，Fischer和Smith（2018）证明，在532-780 nm波长范围内，浴气中较低的O-2质量分数会使测量的PAS校准系数偏置到较低的值。在这篇论文中，我们介绍了在405,514和658 nm波长下，使用臭氧校准的O-2和N-2浴气相对浓度变化的PAS灵敏度测量。我们发现在658nm波长上与Fischer和Smith的结果非常吻合。然而，由于在405和514 nm波长的沐浴气成分倾向于纯氧，PAS灵敏度显著降低，这不能用先前研究中提出的论点来合理化。为了解决这个问题，我们开发了一个模型来描述PAS灵敏度随波长和浴气成分的变化，该模型考虑了Chappuis波段光动力学，并认识到O-3的光激发会迅速导致光解产物O（P-3）和O-2（X, v > 0）。我们表明，两个过程的速率需要正确地模拟PAS灵敏度。第一个过程是通过新生的O（P-3）与浴气O-2的反应形成振动激发的O3(X)。第二个过程涉及从新生的O-2（X, v > 0）到浴气的平移模式的振动能量的淬火。这两个过程在与N-2或O-2浴气的碰撞中以不同的速率进行。重要的是，我们表明，当基于臭氧的校准在与环境空气成分相似的浴气中进行时，我们的PAS仪器的PAS灵敏度得到了优化，并得出结论，我们使用臭氧校准的PAS测量气溶胶吸收的方法是准确的，没有可检测到的偏差。我们强调，PAS灵敏度对浴气成分的依赖是波长相关的，我们强烈建议研究人员对其特定仪器的最佳浴气成分进行表征。
Photoacoustic spectroscopy is a sensitive in situ technique for measuring the absorption coefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS) instruments require accurate calibration by comparing the measured photoacoustic response with a known level of absorption for a calibrant. Ozone is a common calibrant of PAS instruments, yet recent work by Bluvshtein et al. (2017) has cast uncertainty on the validity of ozone as a calibrant at a wavelength of 405 nm. Moreover, Fischer and Smith (2018) demonstrate that a low O-2 mass fraction in the bath gas can bias the measured PAS calibration coefficient to lower values for wavelengths in the range 532-780 nm. In this contribution, we present PAS sensitivity measurements at wavelengths of 405, 514 and 658 nm using ozone-based calibrations with variation in the relative concentrations of O-2 and N-2 bath gases. We find excellent agreement with the results of Fischer and Smith at the 658 nm wavelength. However, the PAS sensitivity decreases significantly as the bath gas composition tends to pure oxygen for wavelengths of 405 and 514 nm, which cannot be rationalised using arguments presented in previous studies. To address this, we develop a model to describe the variation in PAS sensitivity with both wavelength and bath gas composition that considers Chappuis band photodynamics and recognises that the photoexcitation of O-3 leads rapidly to the photodissociation products O(P-3) and O-2(X, v > 0). We show that the rates of two processes are required to model the PAS sensitivity correctly. The first process involves the formation of vibrationally excited O3((X) over tilde) through the reaction of the nascent O(P-3) with bath gas O-2. The second process involves the quenching of vibrational energy from the nascent O-2(X, v > 0) to translational modes of the bath gas. Both of these processes proceed at different rates in collisions with N-2 or O-2 bath gas species. Importantly, we show that the PAS sensitivity is optimised for our PAS instruments when the ozone-based calibration is performed in a bath gas with a similar composition to ambient air and conclude that our methods for measuring aerosol absorption using an ozone-calibrated PAS are accurate and without detectable bias. We emphasise that the dependence of PAS sensitivity on bath gas composition is wavelength-dependent, and we recommend strongly that researchers characterise the optimal bath gas composition for their particular instrument.