Calibration of Photoacoustic Spectrometers at Reduced Pressure Using Aerosols or Ozone-Laden Gas

The scattering and absorption of light by atmospheric aerosols are constrained poorly in climate models. In particular, there is large uncertainty in aerosol light absorption arising from a lack of accurate measurements for absorbing aerosols. Photoacoustic spectroscopy (PAS) is the technique of choice for contact-free light absorption measurements by aerosol particles. In PAS instruments, the light intensity of a laser source is modulated periodically at typical frequencies in the range 1 – 2 kHz and the light absorbing species of interest absorbs energy from this modulated light. The absorbed energy is subsequently transferred to translational degrees-of-freedom of the surrounding bath gas through collisional relaxation and generates an acoustic pressure wave that is detected by a sensitive microphone. The recorded amplitude of the microphone response is related directly to the sample absorption coefficient, while the phase shift of the microphone response with respect to the laser power modulation provides information on the timescale for energy transfer to the bath gas.

Recent years have seen PAS instruments deployed in the field on aircraft measurement platforms. These airborne studies facilitate spatially-resolved measurements of aerosol light absorption, including with variation in altitude. The accuracy of the resulting aerosol absorption measurements depends chiefly on the calibration of the PAS microphone response. Moreover, this calibration for microphone response varies with pressure, with an increased sample pressure dampening the microphone membrane motion to a greater extent. This pressure-dependent microphone sensitivity is particularly pertinent to measurements from aircraft platforms that sample at varying pressures typically over the range 400 – 1000 mbar. Largely, field instruments have used ozone-laden gas to calibrate PAS instruments operating at visible wavelengths, and repeated this calibration for several values of absolute pressure.

In this contribution, we report photoacoustic amplitude and phase shift measurements which demonstrate ozone-laden gas is a poor calibrant of PAS instruments operating at visible wavelengths and at pressures reduced from those at ambient conditions (~1000 mbar). The nascent photodissociation products following photoexcitation of O₃ do not liberate their energy to the surrounding bath gas on a fast timescale compared to the photoacoustic modulation frequency regardless of the bath gas composition. Instead, we show that the PAS instrument can be calibrated at ambient pressure and then a miniature speaker can be used to excite an acoustic response for calibrating the pressure sensitivity in the microphone response. In this way, we show that we accurately measure aerosol absorption at reduced pressure for sub-micrometre diameter aerosols consisting of dyed polystyrene latex spheres or nigrosin dye. These results will be of utmost interest to those measuring aerosol absorption using PAS from airborne platforms or those calibrating PAS instruments for ground based or laboratory measurements.