Quantification of the pH of Aerosol Droplets via Nanoprobe Based Sensing

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PI Name: VikeslandProposal Number: 1705653Atmospheric aerosol droplets are well recognized for their capacity to affect both local-scale as well as global air quality. To date, efforts to measure the acidity (or basicity) of aerosols at the individual droplet level have been largely unsuccessful. This effort supports research conducted to quantify droplet acidity (or basicity) using nanotechnology and laser-enabled spectroscopy. The expected results will provide an improved understanding of aerosol droplet chemistry and will thus enhance societal appreciation of the complexity of aerosol interactions. Both undergraduate and graduate students will receive training in advanced spectroscopic methods, and an extensive K-12 outreach effort will be initiated. The primary research goal of this effort is to quantify the pH of micron-scale aerosol droplets and evaluate how it affects droplet chemistry. pH is a defining characteristic of all natural waters; however, the pH of individual aerosolized droplets has not been well established through direct measurements. In this effort recently developed novel nanoprobes will be used to probe pH inside individual micron-scale droplets as well as to investigate the influence of reactive gases on droplet pH. The PIs will take advantage of the spatial sensitivity enabled by gold nanoparticle (AuNP) based probes that are entrained within the confined environment of a 15-40 ìm diameter droplet. By utilizing a nanoscale optical technique--surface enhanced Raman spectroscopy (SERS)--the PIs expect to spectroscopically detect differences in pH in both 2-D and 3-D with ~300 nm resolution. The following three research objectives will be met: Objective 1: Determine the spatial distribution of pH inside micron-scale droplets. Objective 2: Determine how pH changes with relative humidity (RH) as an aerosol is subjected to cycles of efflorescence and deliquescence. Objective 3: Quantify the effect of reactive gases (CO2, SO2, NH3) on droplet pH. Previous studies have established that gaseous pollutants undergo transformations inside aerosol droplets. Many such reactions are pH sensitive, and if the preliminary pH results are correct, these reactions could be either accelerated or decelerated relative to bulk phase expectations. Such an outcome would have a major impact on our understanding of aerosol interactions across many ecosystems. The diverse individuals engaged with this project will contribute to the scientific workforce, but more importantly they will interact with K-12 and other groups via outreach activities organized by the Virginia Tech Sustainable Nanotechnology (VTSuN) program and the NSF-funded NanoEarth Center.