Theoretical modeling of the size-dependent influence of surface tension on the absorptive partitioning of semi-volatile organic compounds

The influence of surface tension on the formation of secondary organic aerosol (SOA) is investigated in this study using a size-dependent absorptive partitioning model. A theoretical framework is offered to estimate the surface tension of multi-component aerosols consisting of organic compounds and water. The size-dependent influence of surface tension on the absorptive partitioning of semi-volatile organic compounds is examined via numerical simulations of systems of representative pre-existing aerosol (PA) components and semi-volatile organic compounds that have been observed to constitute SOA. Results indicate that if nonpolar organic species constitute a significant fraction of the PA, the Kelvin effect on SOA formation may be negligible. However, if PA is dominated by polar organic compounds, the Kelvin effect on SOA formation is significant when the PA initial diameter is smaller than approximately 200 nm. If the PA is an aqueous aerosol, the Kelvin effect on SOA formation is most important. A simplified computational scheme for estimation of the Kelvin effect is developed in this study and feasibly could be coupled into three-dimensional air quality models that simulate SOA formation. Available observations also suggest that future modeling and analysis of SOA formation may need to consider the Kelvin effect. Concrete testing of the purely theoretical model presented here requires carefully designed observations that examine the phase distribution of secondary organic compounds between the gas phase and aerosol particles small enough to be affected by surface tension.