Date of Award
Doctor of Philosophy (PhD)
Atmospheric aerosols impact health outcomes, visibility, and the energy balance of the earth. The atmosphere contains a variety of compounds, and the volatility (phase change enthalpy and vapor pressure) of each compound determines its partitioning between the gas phase and the particle phase. The hygroscopicity (an aerosol’s affinity for water) of an atmospheric aerosol particle is determined by the many compounds present in the particle, and thus, the volatility impacts hygroscopicity. Changes in hygroscopicity alter the fraction of the aerosol deposited in the lungs and the fraction of the aerosol activated into cloud droplets. Thus, understanding the volatility and hygroscopicity of atmospheric aerosols is important to fully understanding their impacts on health, visibility, and the earth’s energy balance. Tandem Differential Mobility Analyzers (TDMAs) measure volatility and hygroscopicity by first selecting particles by size, then performing an experiment on the selected particles, and finally measuring the particles’ final diameter. For volatility, the experiment evaporates a portion of the aerosol particles, and for hygroscopicity, the experiment condenses water on the particles. Unfortunately, the TDMA does not select a single particle size; it selects a set of particle sizes (multi-charged particles). Each member of the set can behave differently creating complicated experimental responses. Traditional inversion routines assume the sampled population is singly charged. This assumption is not always correct. Methods to measure singly and multiply charged particles exist but require additional equipment.We built a new volatility and hygroscopicity TDMA (VH-TDMA), which reproduced the size distributions, hygroscopicities, and volatilities of pure atmospheric aerosol particles without using the design recommended by literature. In hygroscopicity, literature recommends temperature control of the second Differential Mobility Analyzer (DMA) to establish an accurate knowledge of internal DMA temperature. However, temperature control ensures the second DMA will not operate at ambient conditions. We were able to forgo temperature control of the second DMA by directly measuring the temperature inside. In volatility, short ovens are recommended to maximize transmission of particles. However, short ovens increase the bias between the set point temperature and the measured vapor pressure. Thus, we instituted a 15.25 m long oven increasing accuracy between the measured vapor pressure and the set point temperature. To understand complicated VH-TDMA responses, we created a model (TAO) that reproduced both hygroscopic and volatility responses from first principals. Using TAO and responses from the VH-TDMA, we show that Condensation Particle Counter (CPC) responses often separate into two peaks during volatility experiments. CPC responses during hygroscopicity experiments are often biased by a changing inlet size distribution. In response to these observations, we created a new inversion routine for hygroscopicity, which removes the observed biases; and we developed a new analysis method to study particle surface energy by capitalizing on the two-peak response from volatility measurements.We applied the VH-TDMA to the study of Primary Organic Aerosol (POA) emitted from the flaming combustion of grass, to the study of Secondary Organic Aerosol (SOA) produced by the oxidation of toluene, and to the study of mass fraction remaining curves produced from the evaporation of azelaic acid. Previous studies of grass burning POA assert that the bimodal hygroscopic distribution is caused by the external mix of disparate particles. We find the aerosol is an internal mixture, not external, and can be modeled as a simple volumetric combination of smoldering-like and flaming-like aerosol. Additionally, we found that the emitted POA will evaporate 3.75 times faster at 20% relative humidity compared to dry conditions. These observations should influence the predicted activation of the aerosol particles into cloud droplets. In the toluene experiments, we related the observed aerosol hygroscopicity to the oxygen-to-carbon ratio. The hygroscopicity of the SOA decreased with an increase in toluene concentration, and the hygroscopicity increased with an increase in oxidation. Last, we used the VH-TDMA and TAO together to create mass fraction remaining plots from evaporated azelaic acid. These plots were compared to mass fraction remaining plots measured by the Aerosol Mass Spectrometer (AMS) and show that multi-charged particles are a significant contributor to the sigmoidal shape of mass fraction remaining plots.This work demonstrates the significant role multi-charged particles can play in VH-TDMA responses. In hygroscopicity, multi-charged particles enabled false trends in high growth (ammonium sulfate and greater) aerosols. In volatility, the CPC response becomes bimodal due to the presence of multi-charged particles, and we used the bimodal response to study the surface energy. The sigmoidal shape of mass fraction remaining curves from a DMA-AMS system are significantly influenced by the presence of multi-charged particles. During the study of grass burning POA, multi-charged particles were used to assess morphology. In nearly every experiment above, multi-charged particles played a significant role in the experimental outcome. These examples show the many ways multi-charged particles can both impair conclusions and answer questions about atmospheric aerosols.
Brent J. Williams
Pratim Biswas, Christopher D. Cappa, Rajan K. Chakrabarty, Markus D. Petters,