Variation in pH of Model Secondary Organic Aerosol\nduring Liquid–Liquid Phase Separation

Abstract

The majority of atmospheric aerosols\nconsist of both organic and\ninorganic components. At intermediate relative humidity (RH), atmospheric\naerosol can undergo liquid–liquid phase separation (LLPS) in\nwhich the organic and inorganic fractions segregate from each other.\nWe have extended the study of LLPS to the effect that phase separation\nhas on the pH of the overall aerosols and the pH of the individual\nphases. Using confocal microscopy and pH sensitive dyes, the pH of\ninternally mixed model aerosols consisting of polyethylene glycol\n400 and ammonium sulfate as well as the pH of the organic fraction\nduring LLPS have been directly measured. During LLPS, the pH of the\norganic fraction was observed to increase to 4.2 ± 0.2 from 3.8\n± 0.1 under high RH when the aerosol was internally mixed. In\naddition, the high spatial resolution of the confocal microscope allowed\nus to characterize the composition of each of the phases, and we have\nobserved that during LLPS the organic shell still contains large quantities\nof water and should be characterized as an aqueous organic-rich phase\nrather than simply an organic phase.