Abstract
The effect of relative humidity (RH) on the physical state of particulate matter during the GoAmazon 2014/15 campaign was investigated through the use of particle rebound (or lack thereof) during impaction. The physics governing particle rebound have been previously modeled and can be attributed to the surface and material properties. The physical state of secondary organic material is regulated by several factors, including the local RH, the recent RH history in the case of hysteresis, and chemical composition. 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 physical state. Results inferred from the particle rebound measurements indicate that, under ambient conditions (RH >80%), particulate matter in Amazonia can be considered in a liquid phase state. However, during certain time periods, a fraction (10 to 30%) of particulate matter is found to rebound during the highest RH conditions, which indicates the presence of hydrophobic particles.more » The source of the hydrophobic particulates appears to be anthropogenic, from either biomass burning or the Manaus plume.« less
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