Phase State of Secondary Organic Material (SOM)

Description

The effect of relative humidity (RH) on the phase state of particles composed of secondary organic material (SOM) was studied using a three-arm impaction apparatus. Particle rebound measurements were obtained from 5 to 95% RH. The experimental results were modeled taking into account multiple rebound, the incoming kinetic energy, and the adhesion energy arising from van der Waals and capillary forces. The modeled results agree well with the experimental results for the critical velocity at which rebound first occurred. The agreement is explained by the incoming kinetic energy Ek,0 being the same order of magnitude as the adhesion energy Ea and therefore dominating the energy-loss mechanisms during impaction. The model does not succeed, however, in explaining the experimentally observed decrease in rebound at elevated RH for particles of large incoming kinetic energy Ek,0. Experimental results indicate as RH increases particles have a decreasing tendency to rebound. At sufficiently high RH, rebound ceases. The model, however, predicts large rebound (0.9) in all cases. This discrepancy can be explained by particle softening and liquefaction (i.e., a liquid phase state) resulting in additional energy loss mechanisms during impaction, such as particle flattening and plastic deformation. Major differences between the modeled results and experimental results can therefore be attributed to changes in the particle phase state. The RH threshold for transition to a liquid state differs for each SOM, ranging from >60% RH for isoprene SOM to >80% for α-pinene and toluene SOMs. At intermediate RH values the SOM materials smoothly transition from solid to liquid behavior. At >30% RH all SOMs can be considered solid. The interaction of the various SOM with water is investigated using empirical formulations for the hygroscopic growth factors of each material. A linear correlation is obtained between rebound fraction and growth factor of the various materials. Above a threshold hygroscopic growth factor all particles appeared to be liquid independent of the chemical composition. Thus indicating that the effect of RH on the particle phase state is dependent on the specific interaction of water with each SOM, with associated implications for the RH dependence of SOM viscosities and related adhesion parameters.