title: Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity.
abstract: Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO4(2-)) and νs(HSO4(-)), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of νs(SO4(2-)) and νs(HSO4(-)) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.
authors.affiliation: Department of Chemistry, Purdue University , West Lafayette, Indiana 47907, United States.
authors.forename: Joel D
authors.initials: JD
authors.lastname: Rindelaub
authors.affiliation: Department of Chemistry, University of Michigan , Ann Arbor, Michigan 48109, United States.
authors.forename: Rebecca L
authors.initials: RL
authors.lastname: Craig
authors.affiliation: Department of Mechanical Engineering, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Lucy
authors.initials: L
authors.lastname: Nandy
authors.affiliation: Department of Chemistry, University of Michigan , Ann Arbor, Michigan 48109, United States.
authors.forename: Amy L
authors.initials: AL
authors.lastname: Bondy
authors.affiliation: Department of Mechanical Engineering, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Cari S
authors.initials: CS
authors.lastname: Dutcher
authors.affiliation: Department of Chemistry, Purdue University , West Lafayette, Indiana 47907, United States.|Department of Earth, Atmospheric, and Planetary Sciences, Purdue University , West Lafayette, Indiana 47907, United States.|Purdue Climate Change Research Center , West Lafayette, Indiana 47907, United States.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Shepson
authors.affiliation: Department of Chemistry, University of Michigan , Ann Arbor, Michigan 48109, United States.|Department of Environmental Health Sciences, University of Michigan , Ann Arbor, Michigan 48109, United States.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Ault