Mass transfer effects on the hygroscopic growth of ammonium sulfate particles with a water-insoluble coating

Organic coatings have been commonly found in atmospheric particles and can affect the hygroscopicity of atmospheric particles. In this paper, we examine the effect of a water-insoluble coating on the hygroscopic growth and phase transformation properties of ammonium sulfate ((NH4)2SO4) particles using an electrodynamic balance. Octanoic acid was utilized as a model compound for water-insoluble coating. (NH4)2SO4 particles coated with octanoic acid (9 and 34 wt% octanoic acid) were subjected to relative humidity (RH) cycling to effect two cycles of deliquescence and crystallization. The coated particles exhibited very similar hygroscopicity in two cycles. Octanoic acid was likely present on the surface of solid (NH4)2SO4 cores and (NH4)2SO4 solution droplets throughout the RH cycling. Mass transfer effects were found in the deliquescence and evaporation of the heavily coated (34 wt%) particles. The mass transfer of water molecules to and through octanoic acid coating led to a significant delay in the deliquescence and crystallization of the coated particles, an increase in the observed deliquescence RH and a decrease in observed crystallization RH as compared to that of uncoated (NH4)2SO4 particles. A water accommodation coefficient in the order of 10−3 can explain these observations. When sufficient time was given to ensure equilibrium, the coating had no effect in altering the equilibrium phase transitions and hygroscopic growth of the (NH4)2SO4 particles. It is important to allow sufficient time for equilibration in hygroscopic measurements of aerosol particles coated with organic coatings.