Atmospheric aerosols play a key role in weather formation and prediction, making the understanding of their formation mechanisms highly important. Despite this, there is a fundamental lack in understanding of the processes that govern their formation and growth because it is extremely challenging to measure the surface tension of single atmospheric aerosol droplets. In response, we have developed a technique to probe the surface properties of single aerosols as well as monitor their surface tension during exposure to various organic molecules. The technique relies on thermally induced capillary waves on a liquid interface which can be observed with the quasi-elastic laser scattering (QELS) technique. Capillary wave spontaneous resonance has previously been demonstrated on confined liquid surfaces by both microchannel and circular aperture. The observed characteristic peaks correspond to the capillary resonance and appear in the QELS power spectrum. Surface tension is then calculated using the Lamb equation, which correlates surface tension to the spatial characteristics of the aerosol and the observed resonance peak maxima. In this work, aerosols were generated by a pump atomizer and were immobilized in the path of a focused laser. Humid air was flown over the droplet to maintain size and shape and QELS spectra collection was initiated. Next, an organic compound such as limonene was introduced to the air stream, interacting directly with the confined droplets. QELS spectra were monitored until equilibrium was reached, and then the trend in the surface tension of the droplets was calculated. The results show that the presence of a hydrophobic substance causes a clear suppression of surface tension. In the case for limonene, surface tension changed from 72.8 to 70.2 mN/m after 10 minutes of exposure, then recovered to 73.0 mN/m in 5 minutes after removing limonene. The understanding of the role that organic molecules play in the surface tension of aerosols is expected to lead to enhanced knowledge on atmospheric phenomena.

Surface tension monitoring of single aerosol and sessile droplets exposed to hydrophobic organic compounds using the quasi-elastic laser scattering technique

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