10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Aerosol Droplets Exhibit Stable pH Gradient
Haoran Wei, Qishen Huang, Linsey Marr, PETER VIKESLAND, Virginia Tech
Abstract Number: 1287 Working Group: Aerosol Chemistry
Abstract Suspended aqueous aerosol droplets (<50 µm) are micro-reactors for many important atmospheric reactions. In droplets, and other aquatic environments, pH is arguably the key parameter dictating aqueous chemistry. The dynamic air/water interface of a droplet has the potential to significantly alter droplet pH from bulk water values. However, it is challenging to measure the pH inside single droplets due to their inaccessibility when using conventional pH meters. In this study, phosphate buffer droplets containing ≈75 nm gold nanoparticle (AuNP) based pH nanoprobes were collected onto a superhydrophobic substrate and scanned by laser confocal Raman microscopy. The Raman signal of the pH reporter, 4-mercaptobenzoic acid (4-MBA), was enhanced substantially through surface-enhanced Raman scattering (SERS) enabled by the AuNP clusters. By using these nanometer-sized pH probes the pH inside individual aerosol droplets was for the first time acquired. We show that the pH in a droplet is higher than that of bulk water by ≈3.3 pH units, which we attribute to the accumulation of protons at the air/water interface. The existence of this unexpected pH shift was corroborated by the observation that the catalytic reaction that only occur under basic conditions (i.e., dimercaptoazobenzene (DMAB) formation from 4-aminothiophenol (4-ATP)) occur within droplets, but not bulk solution. Results of three-dimensional scan also showed that the pH decreased from the droplet centroid to air/water interface. As the molar ratio of ammonia sulfate to phosphate buffer increased, the pH gradient decreased and finally was reversed indicating that the chemical compositions inside the droplet significantly affect its pH gradient.