Spatiotemporally Resolved pH Measurement in Aerosol Microdroplets Undergoing Chloride Depletion: An Application of In Situ Raman Microspectrometry

Abstract

Acidity is a defining property of atmospheric aerosols that profoundly affects environmental systems, human health, and climate. However, directly measuring the pH of aerosol microdroplets remains a challenge, especially when the microdroplets' composition is nonhomogeneous or dynamically evolving or both. As a result, a pH measurement technique with high spatiotemporal resolution is needed. Here, we report a spatiotemporally resolved pH measurement technique in microdroplets using spontaneous Raman spectroscopy. Our target sample was the microdroplets comprising sodium chloride and oxalic acid�laboratory surrogates of sea spray aerosols and watersoluble organic compounds, respectively. Our measurements show that the chloride depletion from the microdroplets caused a continuous increase in pH by ∼0.5 units in 2 hours. Meanwhile, the surface propensity of chloride anions triggers a stable pH gradient inside a single droplet, with the pH at the droplet surface lower than that at the core by ∼ 0.4 units. The uncertainties arising from the Raman detection limit (±0.08 pH units) and from the nonideal solution conditions (−0.06 pH units) are constrained. Our findings indicate that spontaneous Raman spectroscopy is a simple yet robust technique for precise pH measurement in aerosols with high spatiotemporal resolution.

Rapid Sulfate Formation via Mn2+-Catalyzed SO2 Oxidation on the Surface of NaCl Microdroplets

“…The experimental details are similar to those reported in our previous studies. 25 In brief, a mixture of NaCl (1 M) and Mn 2 Cl (1.43 × 10 −5 −0.01 M) solutions was atomized and deposited onto a polytetrafluoroethylene film, which was placed at the center of the sample cell. The Mn 2+ concentration can be controlled using the ratio between NaCl and MnCl 2 in droplets.…”

Direct Observation of Particle-To-Particle Variability in Ambient Aerosol pH Using a Novel Analytical Approach Based on Surface-Enhanced Raman Spectroscopy

“…Such evaporation could potentially alter their pH levels. 33,38 To minimize the potential of volatile ion evaporation and to ensure accurate pH measurements, both the assessment of hygroscopic behavior and the pH measurements were conducted within 30 min after collection.…”

“…Specifically, in situ RMS and AOT methods determine the pH of individual aerosol particles by calibrating Raman peak areas of acids and their conjugate bases to molar concentrations in various model systems. [31][32][33]35,36 These approaches have provided insights into how aerosol particle pH or ionic strength varies in response to changes in RH, chemical compositions, time, and interactions with gaseous compounds across different idealized model systems. [31][32][33]35,36 Additionally, there is a unique approach to estimate aerosol pH based on Raman peaks related to the water content within the particles.…”

“…[31][32][33]35,36 Additionally, there is a unique approach to estimate aerosol pH based on Raman peaks related to the water content within the particles. 33 SERS has recently emerged as a cutting-edge technique for assessing the pH of individual aerosol particles. 37−39 This method measures the ratio between protonated and deprotonated peaks of pH indicator molecules, such as 4mercaptobenzoic acid (4-MBA) and 4-mercaptopyridine (4-MPY), which are embedded within the aerosol particles.…”

Influence of Application Method on Disinfectant Byproduct Formation during Indoor Bleach Cleaning: A Case Study on Phenol Chlorination

“…CO 2 will partition to airborne droplets and lower the pH through dissociation to carbonic acid, as with bulk solution; however, this would be expected to occur more rapidly within droplets due to a larger surface area to volume ratio. Microdroplets have also been observed to exhibit pH gradients with pH decreasing towards the edge of the droplet, potentially providing a lower pH environment. A variety of reactions have also been observed to accelerate to some extent within microdroplets, which may affect the relative reaction rates for different chlorination steps. ,, Overall, unique aspects of reactions occurring within microdroplets provide plausible explanations for our results.…”