Abstract
Photoacoustic spectroscopy is widely used to measure the light absorption of aerosols. However, the impact of key factors such as the effect of relative humidity and mass exchange on photoacoustic measurements are still poorly understood. We assess such measurement biases and their physical origin by analysing the photoacoustic signal of single tetraethylene glycol (TEG) particles at varying relative humidities. Our results show a decrease in the photoacoustic signal at elevated relative humidities for small particles (0.8-1.5 μm), while for larger sizes (2.2-3.2 μm) the trend is reversed. We model the photoacoustic signal to interpret the observed behaviour in terms of mass and heat flux contribution. The single particle photoacoustic signal analysis presented in this paper additionally allows for the retrieval of the mass accommodation coefficient. Fitting our experimental data to the theoretical model reveals values of αM ≈ 0.02-0.005 for water on TEG in the temperature range 295-309 K.
Highlights
Atmospheric aerosols play a major role in the Earth’s radiative budget, yet questions regarding their exact contributions remain unanswered
The dependence of the PA–Relative humidity (RH) trend upon particle sizes is a newly discovered phenomena which has to be taken into account to accurately interpret the absorption measurements of atmospheric aerosols measured by photoacoustic spectroscopy
These results could explain the discrepancies in the literature over the exact effect of relative humidity on the photoacoustic signal
Summary
Atmospheric aerosols play a major role in the Earth’s radiative budget, yet questions regarding their exact contributions remain unanswered. The corresponding acoustic signal (sound) can be measured by a microphone or a piezoelectric tuning fork.[7,14] In addition to the heat transfer, the absorbed energy can be dissipated via evaporation of semi-volatile species, such as water (Fig. 1) This mass flux from the particle contributes to the photoacoustic signal, its conversion efficiency. Retrieval of mass accommodation coefficients for such heterogeneous systems is of huge importance for atmospheric sciences as the kinetics of water condensation on particles plays a key role in defining activated aerosol concentration in the atmosphere.[55,60,61,62,63] This work is the first. Through the comparison of the experimental and theoretical data we retrieve the value of the mass accommodation coefficient for our system
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