Hygroscopicity of mixed inorganic/surfactant ultrafine aerosol particles.

摘要

Ultrafine organic aerosol particles UFP (defined as particles with aerodynamic diameters 100nm) are ubiquitous in the atmosphere. They are characterized by having an enhanced Kelvin effect and a greater surface energy contribution to the overall energy. The combination of their small size and chemical composition can drastically alter their hygroscopic behavior compared to fine particles. While a significant fraction of these particles were found to comprise organic surfactants, their hygroscopic effect is not well characterized.; Phase transitions in binary systems are usually described assuming ideal behavior. However, mixtures of inorganic salts and surface active agents may or may not be ideal, and therefore experimental measurements presented here are needed in order to assess the deviations from the ideal behavior. The main goal of this work is to understand the effects of surface active agents on hygroscopicity of binary system of inorganic/surfactants in ultrafine particles and the relationship between the structure and hygroscopicity in such systems.; Hygroscopic growth measurements were preformed on internally mixed particles (7-20nm) of Sodium Chloride and anionic surfactants Dioctyl Sodium Sulfosuccinate (AOT) and Sodium Dodecyl Sulfate (SDS) at different weight fractions. The deliquescence relative humidities were measured using a novel Tandem Nano Differential Mobility Analyzer (Tandem-NDMA). Hygroscopicity of mixtures of NaCl and AOT were reported in terms of hygroscopic growth factors. The growth factors of internally mixed 17.0 nm aerosol particles of NaCl and AOT or SDS surfactants were investigated at different total conditioning residence times. Initial results for 25% w/w AOT with NaCl and 25% w/w SDS with NaCl showed a growth factor depression as well as a shift in the deliquescence relative humidity to lower values compared to the pure NaCl particles. Such a behavior was theoretically attributed to differences between the rates of spreading of each surfactant at the particle interface compared to rate of surface area increase of the deliquescing droplets. The experimental results presented here suggest that a small amount of surfactant is sufficient to cause a significant change in the water uptake by the inorganic soluble core.; The rate of deliquescence of ultrafine ambient aerosol particles can substantially alter both its potential role as cloud condensation Nuclei (CCN) and its deposition rate in human respiratory system. There is one important concern in investigating hygroscopicity of ultrafine particles (UFP) as to whether or not particles have actually attained final equilibrium during the measurement time. In this work, the effects of mass transfer and size on deliquescence of aerosol particles were also addressed. It was shown that unless the deliquescence relative humidity is at equilibrium, hygroscopicity measurements can be potentially compromised by mass transfer effects.; Chapter 5 of this dissertation describes how organic ultrafine particles can be generated indoors by ionization air purifiers in the presence of certain VOCs. It was shown that the operation of an ozone-generating air-purifier in a closed indoor environment results in an increase in the steady-state ozone concentration that is directly proportional to the air-purifier's ozone emission rate. Chapter 6 addresses the kinetic analysis of competition between aerosol particle removal and generation by ionization air purifiers. It was shown that air purifiers may increase the respirable PM mass concentration instead of reducing it as advertised by the manufacturer.