Surfactant effect on the hygroscopicity of aerosol particles at relative humidity ranging from 80% to 99.5%: Internally mixed adipic acid-ammonium sulfate particles

Surfactant, which is ubiquitous in the atmosphere, has an important effect on the hygroscopicty of nanoscale atmospheric aerosol particles. However, the knowledge on the high relative humidity (RH) section (90-100%) is insufficient due to the limitation of traditional measuring instruments. In this study, the hygroscopicity of aerosol particles consisted of adipic acid (AA) and ammonium sulfate (AS) at RH = 80-99.5% was studied using the experimental and the theoretical method. The growth factor (GF) values at different RH were measured by the high humidity tandem differential mobility analyzer (HHTDMA). With some assumptions, a new model (Model 3 (UNIFAC)) accounting for the reduction of surface tension, the effect of the bulk-surface partitioning of AA and the non-ideal of solution on the particle hygroscopicity was established to describe the experimental results. An overall better performance is shown after comparing with six traditional models. Moreover, the partitioning of AA and the non-ideal of solution were investigated in details by comparing the outcomes of corresponding models. The comparisons between Model 2 (UNIFAC) and Model 3 (UNIFAC) and between Model 3 (Ideal) and Model 3 (UNIFAC) indicate that both the partitioning of AA and the non-ideal properties of solution in particles result in the decrease of GF. In addition, the change of water activity affected by these two dynamic processes acts as a more important factor than surface tension in the decrease of GF. Moreover, RH, the mass ratio and the initial diameter have significant impact on the AA partitioning, the non-ideal properties of solution and their influences on the hygroscopicity of particles.

Automated summary

In this study, the hygroscopicity of aerosol particles composed of adipic acid and ammonium sulfate at 80-99.5% relative humidity was investigated using experimental and theoretical methods. A new model (Model 3 (UNIFAC)) was developed to account for the reduction of surface tension and the effects of bulk-surface partitioning of adipic acid on particle hygroscopicity. Results showed that the change in water activity had a greater impact on particle hygroscopicity than surface tension.