Atmospheric Chemistry Program Seminar

Investigating the pH of atmospheric fine particles and implications for atmospheric chemistry by Hongyu Guo, Postdoctoral Researcher, Jimenez lab, CU Boulder
"Particle acidity is a critical but poorly understood quantity that affects many aerosol processes and properties. In this talk, I will introduce a popular pH prediction method in recent years since pH detection technique is limited. 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 (fine particle pH = 0.94 ± 0.59), and then extended to aircraft observations in the NE US in winter (WINTER study; pH = 0.77 ± 0.96). The results are validated by reproducing particle water and gas-particle partitioning of NH4 + and NO3 - (sensitive to pH). I will show why commonly used pH proxy, ion balance or molar ratio, doesn’t necessarily represent pH. Some impacts of low particle pH were investigated, including the effects on aerosol nitrate trends and the role of acidity in heterogeneous chemistry. Despite a ~70% sulfate reduction in the southeastern US in the last 15 years, the fine particles remained highly acidic due to buffering by semivolatile  NH3. Importantly, pH is not highly sensitive to NH3, a 10-fold increase in NH3 only increases pH by one unit in various locations and seasons, which has implications for use of NH3 controls to reduce PM 2.5 concentrations."
 
*Note: this is a short seminar, expected to end early*