Quantitative description of surface tension for nanodroplets containing ammonium sulfate and its application to the particle hygroscopicity prediction

Surface tension is critically important in the hygroscopicity prediction of nano-aerosols, but experimental measurements of suspended nanodroplet surface tension are currently unattainable. In this study, effect of size and solute concentration on the surface tension of aqueous ammonium sulfate (AS) droplet smaller than 10 nm is investigated based on molecular dynamics simulations. A surface tension formula is proposed and applied in the theoretical model for particle hygroscopicity, and the prediction is compared with models using traditional methods of surface tension description. Results show that with the increase in AS concentration, surface tension decreases first and then increases, and the correlation between surface tension and droplet size changes from positive to negative. Compared to the surface tension model considering only the AS concentration effect, the surface tension formula including both AS concentration and size corrections enables the particle hygroscopicity model to obtain a smaller hygroscopic growth factor, which is closer to the experimental results. The improvement effect of the new formula on the model accuracy increases with the decreasing particle size. Additionally, the new surface tension formula leads to an increase in the critical supersaturation for particles smaller than 5 nm but a decrease for larger particles. The present study highlights the importance of the molecular dynamics investigation for nanodroplets comprising of complex ingredients.