The Role of Aerosol Acidity and Buffering Capacity in the Reactive Uptake and Chemical Feedbacks of Gaseous N2O5-Organic Aerosol Systems
GRAHAM THORNHILL, Luke Monroe, Jack Hall, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 571
Working Group: Aerosol Chemistry
Abstract
The reactive uptake of trace gases onto aerosols and the resulting changes in aerosol properties such as composition, morphology, and further reactive uptake are heavily affected by aerosol acidity. Aerosol pH, as well as the kinetics of reactive uptake and changes in droplet properties, are difficult to measure continuously and directly due to wide aerosol size distributions, tiny volumes, compositional complexity, and the need for non-ideal thermodynamic calculations. As such, the effect of acidity on aerosol reactive uptake and chemical feedbacks during prolonged multiphase chemistry is not well understood. Using aerosol optical tweezers with its cavity-enhanced Raman spectroscopy and whispering gallery modes to determine chemical information, and calculate aerosol size and refractive index, respectively, we have both developed a method of estimating droplet buffering capacity in inorganic sodium bisulfate droplets as well as observed chemical feedbacks in the uptake of gaseous N2O5 in the presence of terpene oxidation products. Through tracking aerosol pH and acid molality in sodium bisulfate aerosol in equilibrium with water vapor we were able to estimate the droplet’s effective pKa to be about -0.28, a stark contrast from the system’s bulk pKa of 1.99. In other experiments, droplets in the presence of N2O5(g) and ????-pinene or limonene formed a new organic phase consisting of organonitrates. The morphology change resulting in this new phase occurs via the amount of organic acid added and the acidification by N2O5 hydrolysis or HONO oxidation to HNO3. Raman modes from the products formed by N2O5/NO3• oxidation of the new organic phase was also retrieved. These experiments enable measurements of aerosol pH and buffer capacity during ongoing multiphase chemistry involving the N2O5-organic aerosol system, characterization of the reaction products from these systems, and determining the controls and feedbacks present in this complex kinetic system.