Particle Rebound and Phase State in Amazonia

Abstract:

The effect of relative humidity (RH) on the extent of semisolidity of particulate matter during GoAmazon 2014 was investigated through the use of particle rebound (or lack thereof) during impaction. Particle rebound measurements from the same region during the 2013 dry and wet season are also presented. The physics governing particle rebound has been previously modeled and can be attributed to the surface and material properties. Recent studies suggest that a particle phase transition from liquid to semisolid or solid phase states can alter chemical reaction pathways by shifting from absorption to adsorption mechanisms. The phase state of secondary organic material is regulated by the local relative humidity (RH), the recent history of RH in the case of hysteresis, and chemical composition, among other factors. Across the range of atmospheric RH, hygroscopic water uptake can occur and transitions from higher to lower viscosity phases are possible.

By varying the particulate matter water content and observing particle rebound as a function of RH, the phase state of the organic material under investigation can be determined. Custom made impactors were employed to study the effects of RH (up to 95%) on the particle phase. Results inferred from the particle rebound measurements indicate that under ambient conditions (RH > 70%) particles in Amazonia can be considered in a liquid phase state. However, during certain time periods, a small fraction (10 - 20 %) of particles are found to rebound during the highest RH conditions, an indication of a semi-solid or solid phase state. The possible chemical composition of this fraction of particles is investigated and correlated with additional measurements from the GoAmazon 2014 campaign.