In Situ Surface Tension Measurement of Deliquesced Aerosol Particles

The surface tension of aerosol particles can potentially affect cloud droplet activation. Hence, direct measurement of the surface tensions of deliquesced aerosol particles is essential but is challenging. Here, we report in situ surface tension measurements based on a novel method that couples a linear quadrupole electrodynamic balance (EDB) with quasi-elastic light scattering (QELS). The EDB-QELS is validated using surface tension measurements of atmospherically relevant inorganic and organic droplets. The surface tension results reasonably agree with the reference values in the range of similar to 50-90 mN m(-1). We find a significant size dependence for sodium chloride droplets containing surface-active species (sodium dodecyl sulfate) in the size range of similar to 5-18 mu m. The surface tension increases from similar to 55 to 80 mN m(-1) with decreased size. Relative humidity (RH)-dependent surface tensions of mixed ammonium sulfate (AS) and polyethylene glycol droplets reveal the onset of liquid-liquid phase separation. Droplets containing water-soluble matter extracted from ambient aerosol samples and 2.3-2.9 M AS exhibit a similar to 30% reduction in surface tension in the presence of similar to 50 mmol-C L-1 water-soluble organic carbon, compared to pure water (similar to 72 mN m(-1)). The approach can offer size-resolved and RH-dependent surface tension measurements of deliquesced aerosol particles.

Automated summary

In this study, a novel method combining a linear quadrupole electrodynamic balance (EDB) with quasi-elastic light scattering (QELS) was used to measure the surface tension of deliquesced aerosol particles in situ. The results showed that the surface tension measurements were in the range of 50-90 mN/m and exhibited size dependence and relative humidity dependence.