On the accuracy of aerosol photoacoustic spectrometer calibrations using absorption by ozone

Abstract

Abstract. In recent years, photoacoustic spectroscopy has emerged as an invaluable tool for the accurate measurement of light absorption by atmospheric aerosol. Photoacoustic instruments require calibration, which can be achieved by measuring the photoacoustic signal generated by known quantities of gaseous ozone. Recent work has questioned the validity of this approach at short visible wavelengths (404 nm), indicating systematic calibration errors of the order of a factor of 2. We revisit this result and test the validity of the ozone calibration method using a suite of multipass photoacoustic cells operating at wavelengths 405, 514 and 658 nm. Using aerosolised nigrosin with mobility-selected diameters in the range 250–425 nm, we demonstrate excellent agreement between measured and modelled ensemble absorption cross sections at all wavelengths, thus demonstrating the validity of the ozone-based calibration method for aerosol photoacoustic spectroscopy at visible wavelengths.

Technical Note: Can ozone be used to calibrate aerosol photoacoustic spectrometers?

“…Here, we expand on this work by systematically investigating the dependence of the ozone sensitivity on O 2 concentration and performing kinetic modeling suggesting that N 2 as a bath gas results in efficient deactivation of O 3 * . Interestingly, our results are not sufficient to explain the differences between the findings of Bluvshtein et al (2017) and Davies et al (2018) as each study used 15 a bath gas composition of 10% O 2 and 90% N 2 . We find that ozone is a suitable calibrant for PAS in a bath gas of 100% O 2 but that its use at lower O 2 concentrations requires careful comparison to other calibrants, such as NO 2 or nigrosin particles, and will incur increased uncertainties associated with the necessary correction.…”

“…Despite the fact that this photodissociation pathway is well established, ozone has been used to calibrate aerosol PAS instruments with a dearth of discussion on the impact of photodissociation until very recently. Significantly, Davies et al (2018) find good agreement 10 between an ozone calibration and one performed with nigrosin particles, while Bluvshtein et al (2017) measured an ozone calibration half that of the one obtained with nigrosin particles with no obvious explanation for the disparity. Here, we expand on this work by systematically investigating the dependence of the ozone sensitivity on O 2 concentration and performing kinetic modeling suggesting that N 2 as a bath gas results in efficient deactivation of O 3 * .…”

“…We chose to take an alternate approach to calibrating with ozone compared to prior studies (Bluvshtein et al, 2017;Davies et al, 2018). Instead of using the flow directly out of an ozone generator, we trapped ozone on a silica gel trap prior to analysis.…”

“…Photoacoustic spectroscopy (PAS) has become a popular technique for measuring absorption of light by atmospheric aerosols for PAS (Lack et al, 2006(Lack et al, , 2012, recent works exploring its validity have come to contradictory conclusions: Bluvshtein et al (2017) saw a discrepancy between ozone calibrations and particle-based calibrations, while Davies et al (2018) found this not to be the case. Concurrent to these publications, we have been exploring the use of ozone as a PAS calibrant for multipass, multi-wavelength, aerosol photoacoustic spectrometers; our observations are presented here to add to the discussion on the topic.…”

“…They then used an independently measured refractive index and Mie theory to calculate the known absorption for each sample and found generally good agreement between their sensitivity factors; however, when they performed a calibration with ozone, they found a much lower sensitivity factor (by roughly 50%). Alternatively, Davies et al (2018) found their measured nigrosin absorption cross sections agreed well with Mie theory when they calibrated their PAS with ozone prior to nigrosin measurements.…”