COLLEGE OF SCIENCES

SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES

EAS Ph.D. Defense

Hongyu Guo

July 17, 2017

10:00 AM

Earth and Atmospheric Sciences

Ford Environmental Science & Technology (ES&T)311 Ferst Drive, ES&TAtlanta, GA 30332-0340Web: eas.gatech.edu

ES&T Room L1114

Title: Investigating the pH of atmospheric fine particles and implications for atmospheric chemistry

Committee members: Dr. Rodney Weber, Dr. Greg Huey, Dr. AthanasiosNenes, Dr. Ted Russell, and Dr. Sally Ng

Abstract: Investigating the pH of atmospheric fine particles and implications for atmospheric chemistry Particle acidity is a critical but poorly understood quantity that affects many aerosol processes and properties, including aerosol composition and toxicity. In this study, particle pH and water (which affects pH) are predicted using a thermodynamic model and measurements of RH, T, and inorganic gas and particle species. The method was first developed during the SOAS field campaign conducted in the SE US in summer (pH = 0.94 ±0.59), and then extended to aircraft observations in the NE US in winter (WINTER study; pH = 0.77 ±0.96) and ground observations in the coastal SW US in early summer (CalNexstudy; PM1pH = 1.9 ±0.5 and PM2.5pH = 2.7 ±0.3). All studies have consistently found highly acidic PM1with pH generally below 3. The results are supported by reproducing particle water and gas-particle partitioning of inorganic NH4+, NO3-, and Cl-. Nonvolatile cations may increase pH with particle size above 1µm depending on mixing state but have little effect on PM1pH.Ion balance or molar ratio, are not accurate pH proxy and highly sensitive to observational uncertainties. Impacts of low particle pH were investigated, including the effects on aerosol nitrate trends and the role of acidity in heterogeneous chemistry. We found that PM2.5remained highly acidic despite a ~70% sulfate reduction in the southeastern US in the last 15 years, due to buffering by semivolatileNH3; that the bias in molar ratio predictions in past studies is linearly correlated to nonvolatile cations but not organics, challenging the organic film postulation; that recently proposed rapid SO2oxidation by NO2during China haze events may not be a significant source of sulfate due to relatively low pH (~4); and lastly that pH is also not highly sensitive to NH3, a 10-fold increase in NH3only increases pH by one unit in various locations and seasons, which has implications for use of NH3controls to reduce PM2.5concentrations.