The Effects of Aerosol Acidity on the Morphology and Ice Nucleation of Aerosol Particles

Restricted (Penn State Only)
Author:
Losey, Delanie Jean
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 10, 2017
Committee Members:
  • Miriam Arak Freedman, Dissertation Advisor
  • Miriam Arak Freedman, Committee Chair
  • Christine Dolan Keating, Committee Member
  • Paul S Cremer, Committee Member
  • Jose Dolores Fuentes, Outside Member
Keywords:
  • aerosol particles
  • ice nucleation
  • atmospheric chemistry
  • liquid-liquid phase separation
Abstract:
Aerosol particle morphology can influence the water uptake, heterogeneous chemistry, and optical properties of the particle. This morphology is known to be dependent on the composition of the particle as well as relative humidity. Recent studies have pointed out the importance of pH in aerosol particles. The pH of systems being investigated during phase transition studies have not investigated the role of pH, which could lead to the deprotonation of the organic component and would affect phase transitions. To investigate the influence of high pH and therefore deprotonation on the phase transitions of 3-methylglutaric acid:ammonium sulfate, sodium hydroxide was used to make solutions with pHs below the pKa1, between the pKa1 and pKa2, and above pKa2 of 3-methylglutaric acid. Using optical microscopy and an environmental chamber, the separation relative humidity (SRH), mixing relative humidity (MRH), efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) were recorded for each system. As the pH of the system was increased, the SRH decreased. This was attributed to the increased solubility of the organic component in water when it was deprotonated. The ERH also changed to higher values with added sodium hydroxide. The MRH and DRH values, however, remained constant over all pH. A previously unobserved hysteresis was found between SRH and MRH and the atmospheric implications are discussed. The influence of low pH was also explored in a similar manner, but through addition of concentrated sulfuric acid. Stoichiometric amounts of sulfuric acid was added to six different organic:ammonium sulfate systems to change the salt identity to letovicite, (NH4)3H(SO4)2, and ammonium bisulfate, NH4HSO4. An experiment at low pH was also conducted for each system. In every case, the addition of sulfuric acid led to an overall decrease in SRH, as expected with changing salt identity. Furthermore, at the lowest pH studied the SRH of four of the six systems were so low that phase separation would not occur in atmospherically relevant conditions. Also, phase separation could occur with no inorganic salt present at all. The ERH and DRH for each system also were affected. These results could affect mass transfer and water uptake for systems at low pH, and can be further explored by investigating the role of pH in particle viscosity and submicron aerosol morphology. Fly ash can undergo aging in the atmosphere through interactions with sulfuric acid and water. These reactions can lead to physical and chemical changes caused by reaction products or chemical leaching. These changes could influence the amount of soluble material on the particle as well as the ability of the particle to nucleate ice. Both of these affect the fly ash particle’s ability to serve as a cloud nucleus. The extent of these changes is expected to be linked to the chemical composition of the fly ash so three fly ash types were investigated. The effect of water- and acid-treatment were assessed using X-ray diffraction, attenuated total reflectance infrared spectroscopy, transmission electron microscopy with selected area electron diffraction and energy dispersive spectroscopy, inductively coupled plasma-atomic emission spectroscopy, Brunauer-Emmett-Teller surface area analysis, and immersion freezing. The results show the presence of soluble material on fly ash and indicate that sulfuric-acid treatment has major physical and chemical effects on fly ash. These effects are dependent on composition of the fly ash. Acid-treatment results in gypsum being created and a variety of metals to be leached, and these changes did affect the immersion ice nucleation activity of the samples. Further studies of these effects on deposition mode freezing are expected for the future.