Aerosol Acidity Sensing via Polymer Degradation

Abstract

The acidity of atmospheric aerosols is a critical property that affects the chemistry and composition of the atmosphere. Many key multiphase chemical reactions are pH-dependent, impacting processes like secondary organic aerosol formation, and need to be understood at a single particle level due to differences in particle-to-particle composition that impact both climate and health. However, the analytical challenge of measuring aerosol acidity in individual particles has limited pH measurements for fine (<2.5 μm) and coarse (2.5–10 μm) particles. This has led to a reliance on indirect methods or thermodynamic modeling, which focus on average, not individual, particle pH. Thus, new approaches are needed to probe single particle pH. In this study, a novel method for pH measurement was explored using degradation of a pH-sensitive polymer, poly­(ε-caprolactone), to determine the acidity of individual submicron particles. Submicron particles of known pH (0 or 6) were deposited on a polymer film (21–25 nm thick) and allowed to react. Particles were then rinsed off, and the degradation of the polymer was characterized using atomic force microscopy and Raman microspectroscopy. After degradation, holes in the PCL films exposed to pH 0 were observed, and the loss of the carbonyl stretch was monitored at 1723 cm–1. As particle size decreased, polymer degradation increased, indicating an increase in aerosol acidity at smaller particle diameters. This study describes a new approach to determine individual particle acidity and is a step toward addressing a key measurement gap related to our understanding of atmospheric aerosol impacts on climate and health.

Liquid–Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

“…50 Currently, no direct measures of the pH of ambient aerosol particles exist, though several groups are working to develop new probes. [51][52][53][54][55][56][57] Aerosol particles are predicted to be highly acidic (average pH = 0 -4) based on aerosol and gas phase composition combined with thermodynamic models (e.g. ISORROPIA, E-AIM).…”

Aerosol Acidity: Novel Measurements and Implications for Atmospheric Chemistry

“…Particle-to-particle variability of chemical composition further complicates pH measurements, and particle-to-particle variability in pH is poorly understood . If each particle is thought of as a tiny atmospheric beaker with no walls, a particle from the ocean will be very different than from a car or from a forest fire .…”

“…Particles analyzed by Raman were typically impacted on small, quartz microscope slides placed in the MPS or mini-MOUDI. Further details are available in our prior publications. ,,− …”

“…This photothermal effect is detected by the AFM tip, and a spectrum is obtained by processing the change in deflection. Our group was the first to apply AFM-PTIR to atmospheric aerosols in Bondy et al We have continued to use AFM-PTIR, , as have several other research groups, , to study a range of environmental questions ranging from Arctic aerosol composition to indoor surface chemistry to pH. In Olson et al, we recently were the first to probe aerosol particles with optical PTIR (O-PTIR) with simultaneous Raman microspectroscopy.…”

“…Overcoming the current lack of measurements for atmospheric aerosol pH has been a focus of my research group for the past 5 years, which we have approached from a number of different experimental perspectives. We developed a spectroscopic acid–conjugate base method using vibrational spectroscopy of individual particles, , a colorimetric method using pH-sensitive indicators to measure size-resolved bulk pH, and a method measuring polymer degradation from individual particles with atomic force microscopy (AFM) . The development of the different methods, the scientific insights that have been gained regarding aerosol acidity, and the bigger picture implications of aerosol acidity are explored below.…”