10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Can Ozone Be Used as a Calibrant for Photoacoustic Spectrophotometers?
AL FISCHER, Geoffrey Smith, University of Georgia
Abstract Number: 905 Working Group: Instrumentation
Abstract Photoacoustic spectroscopy (PAS) has become a popular technique for measuring absorption of light by atmospheric aerosols in both the laboratory and field. It has low detection limits, measures suspended aerosols, and is insensitive to scattering, although it often requires rigorous calibration. Typically, the PAS instrument is either (a) filled with a gas of known concentration (N) and absorption cross-section (σ) such that the absorption in the cell (α) can be calculated according α = Nσ or (b) the absorption is measured independently with a technique such as cavity ringdown spectroscopy. Then, the PAS signal can be regressed upon the known absorption to determine a calibration slope that reflects the sensitivity constant of the cell and microphone. For a calibrant to be viable for PAS, it must only undergo thermal relaxation upon exposure to light and must not (for example) undergo photodissociation at the wavelength used for calibration.
Ozone has historically been used for calibrating PAS's due to its well-known UV-visible absorption spectrum and the ease with which it can be generated. However, it is well-known to photodissociate at visible and near-IR wavelengths via the O3 + hν → O2 + O(3P) pathway. Further, Bluvshtein et al (2017) cast doubt upon the accuracy of calibrations performed with ozone, although they saw no evidence of photodissociation and provided no explanation for the discrepancy between ozone- and particle-based calibrations. Contradicting this, Davies et al (2018) validated the accuracy of ozone calibrations and showed no problems with accuracy. The work presented here is meant to add to this discussion by presenting direct evidence of photodissociation inside the PAS cell and showing a mismatch of roughly 50% between calibration slopes obtained with nitrogen dioxide or nigrosin particles and ozone. Further, the dependence of the ozone signal on oxygen concentration will be discussed with data revealing that 5% added O2 yields a roughly 75% increase in the PAS signal for a given concentration of ozone. These results suggest that ozone calibration may be viable but only when used with caution.
[1] Bluvshtein, N., Flores, J. M., He, Q., Segre, E., Segev, L., Hong, N., Donohue, A., Hilfiker, J. N., and Rudich, Y.: Calibration of a multi-pass photoacoustic spectrometer cell using light-absorbing aerosols, Atmos. Meas. Tech., 10, 1203-1213, https://doi.org/10.5194/amt-10-1203-2017, 2017. [2] Davies, N. W., Cotterell, M. I., Fox, C., Szpek, K., Haywood, J. M., and Langridge, J. M.: On the accuracy of aerosol photoacoustic spectrometer calibrations using absorption by ozone, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-434, in review, 2018.