The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band

Abstract

Abstract. Photoacoustic\nspectroscopy is a sensitive in situ technique for measuring the absorption\ncoefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS)\ninstruments require accurate calibration by comparing the measured\nphotoacoustic response with a known level of absorption for a calibrant.\nOzone is a common calibrant of PAS instruments, yet recent work by Bluvshtein\net al.\xa0(2017) has cast uncertainty on the validity of ozone as a calibrant at\na wavelength of 405\u2009nm. Moreover,\nFischer and Smith\xa0(2018) demonstrate\nthat a low O2 mass fraction in the bath gas can bias the measured PAS\ncalibration coefficient to lower values for wavelengths in the range\n532–780\u2009nm. In this contribution, we present PAS sensitivity measurements\nat wavelengths of 405, 514 and 658\u2009nm using ozone-based calibrations with\nvariation in the relative concentrations of O2 and N2 bath\ngases. We find excellent agreement with the results of Fischer and Smith at\nthe 658\u2009nm wavelength. However, the PAS sensitivity decreases significantly\nas the bath gas composition tends to pure oxygen for wavelengths of 405 and\n514\u2009nm, which cannot be rationalised using arguments presented in previous\nstudies. To address this, we develop a model to describe the variation in PAS\nsensitivity with both wavelength and bath gas composition that considers\nChappuis band photodynamics and recognises that the photoexcitation of\n O3 leads rapidly to the photodissociation products O(3P) \nand O2 (X, v\u2009>\u20090 ). We show that the rates of two\nprocesses are required to model the PAS sensitivity correctly. The first\nprocess involves the formation of vibrationally excited\n O3 ( X ) through the reaction of the nascent\n O(3P) with bath gas O2 . The second process involves the\nquenching of vibrational energy from the nascent O2 (X,\n v\u2009>\u20090 ) to translational modes of the bath gas. Both of these\nprocesses proceed at different rates in collisions with N2 or\n O2 bath gas species. Importantly, we show that the PAS sensitivity is\noptimised for our PAS instruments when the ozone-based calibration is\nperformed in a bath gas with a similar composition to ambient air and\nconclude that our methods for measuring aerosol absorption using an\nozone-calibrated PAS are accurate and without detectable bias. We emphasise\nthat the dependence of PAS sensitivity on bath gas composition is\nwavelength-dependent, and we recommend strongly that researchers characterise\nthe optimal bath gas composition for their particular instrument.