Probing pH of Particle in HCO3-/CO32- System through Optical Tweezers Coupled with Raman Spectroscopy

Acidity stands as a pivotal physicochemical parameter influencing aerosol particles, impacting their morphology and environmental interactions, such as phase separation and the formation of secondary organic aerosols. However, directly measuring particles pH remains a challenge, necessitating urgent exploration. This study utilizes optical tweezers to investigate and compare methods for measuring pH of particle. Initial steps involved preparing a solution with sodium carbonate and sodium bicarbonate system, followed by measurement of ion concentration calibration curves for HCO3- and CO32-. Droplets were then generated using an atomizer and prepared solution. A single-beam Gaussian optical tweezers captured individual particles and obtained their Raman spectra in conjunction with a Raman spectrometer. Four methods—Henderson-Hasselbalch equation, Debye-Hückel theory, specific ion interaction theory, and thermodynamic model—were then applied to calculate pH values based on HCO3-/CO32- conjugate acid/base pairs and ion concentration calibration curves. The experimental results demonstrated small error in each calculated pH value. Additionally, the study revealed a rapid decomposition process of HCO3- in droplets, possibly attributed to the high specific surface area of small droplets or the absence of CO2 in the optical tweezers chamber. The study also monitored the evolution of pH values in sodium bicarbonate particles over time. Furthermore, the study investigated difference in pH values calculated by the four calculation methods under different ion strengths and pH values. The study also measured the pH value of sodium carbonate particles in relation to relative humidity. Overall, the experimental outcomes were reasonable and validated the capability of optical tweezers in probing the pH of atmospheric particles, offering insights into the applicable conditions of different methods and directions for refining thermodynamic models.