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

<p>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&thinsp;nm. Moreover, Fischer and Smith (2018) demonstrate that a low <span class="inline-formula">O₂</span> mass fraction in the bath gas can bias the measured PAS calibration coefficient to lower values for wavelengths in the range 532–780&thinsp;nm. In this contribution, we present PAS sensitivity measurements at wavelengths of 405, 514 and 658&thinsp;nm using ozone-based calibrations with variation in the relative concentrations of <span class="inline-formula">O₂</span> and <span class="inline-formula">N₂</span> bath gases. We find excellent agreement with the results of Fischer and Smith at the 658&thinsp;nm wavelength. However, the PAS sensitivity decreases significantly as the bath gas composition tends to pure oxygen for wavelengths of 405 and 514&thinsp;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 <span class="inline-formula">O₃</span> leads rapidly to the photodissociation products <span class="inline-formula">O(³P)</span> and <span class="inline-formula">O₂</span>(X, <span class="inline-formula"><i>v</i> <i>&gt;</i> 0</span>). 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 <span class="inline-formula">O₃</span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mover accent="true"><mi mathvariant="normal">X</mi><mo mathvariant="normal">̃</mo></mover></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="10pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="5408a645858085a368178457bac35054"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-2371-2019-ie00001.svg" width="10pt" height="13pt" src="amt-12-2371-2019-ie00001.png"/></svg:svg></span></span>) through the reaction of the nascent <span class="inline-formula">O(³P)</span> with bath gas <span class="inline-formula">O₂</span>. The second process involves the quenching of vibrational energy from the nascent <span class="inline-formula">O₂</span>(X, <span class="inline-formula"><i>v</i> <i>&gt;</i> 0</span>) to translational modes of the bath gas. Both of these processes proceed at different rates in collisions with <span class="inline-formula">N₂</span> or <span class="inline-formula">O₂</span> 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.</p>