Organic Component Vapour Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers

Chen Cai, David Stewart, Jonathan Philip Reid, Yun-Hong Zhang, Peter Ohm, Cari S. Dutcher, and Simon Leslie Clegg

Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semi-volatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapour pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for an homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterisation of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <± 2.5 % to discriminate between different thermodynamic treatments.

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