Collaborative Research: Applicability Limits of Aqueous pKa Values for Bulk and Surface Nanoparticle Processes

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This award is supported by the Chemistry Division's Environmental Chemical Sciences Program. The collaborative study involves Professors Kelley Barsanti and Bryan Wong at the University of California, Riverside, and Prof. James Smith at the University of California, Irvine. Together with their graduate and undergraduate students, they investigate the acidity of atmospheric aerosols (very fine droplets in the atmosphere). Several key processes that cause small particles to form and grow in the atmosphere are controlled by the acidity of the surface and interior of aerosol particles. The formation and growth of these small particles in the atmosphere have implications for their effects on health as well as their potential to directly and indirectly impact climate. Atmospheric aerosols are particles that are often smaller than 100 nanometers in diameter. Typical descriptions of acidity fail to adequately represent the acid-base chemistry in these particles because they are so small. This project seeks to understand the limits of the applicability of an acidity measurements (here, acid dissociation constants (pKa)) in describing the behavior of acids and bases in sub-100 nanometer particles. As part of a long-standing summer internship program for undergraduates, the researchers give real-time demonstrations using the AVOGADRO program, which provides an easy-to-use graphical user-interface to create acid-base molecular systems and perform preliminary, low-level simulations relevant to this project. As such, the project provides real-time demonstrations may have a substantial impact in increasing awareness of computationally-guided experiments in underrepresented minority populations, potentially leading to more minority students seeking careers in the environmental chemical sciences.The goal of this research is to determine the limits of applicability for bulk-phase aqueous acid and base dissociation constants (pKa and pKb), particularly as regards acid-base chemistry in and on sub-100 nm particles. In this size range, the direct composition and indirect hygroscopicity measurements performed at the CLOUD chamber show that nanoparticles formed by sulfuric acid in the presence of excess based(amines and ammonia) are more acidic than predicted using thermodynamic models. Further, the acidity appears to vary with particle size from ~10 nm (most acidic) to 50 nm (more neutralized). Knowledge of the acidity of sub-100 nm particles is important for understanding relatively simple acid-base chemistry, but also for accurately predicting acid-catalyzed processes on particle surfaces and in the bulk phase. The specific objectives of this project are as follows: to use molecular modeling to predict pKa values for inorganic and organic acids and bases as a function of particle composition and size. This activity combines techniques such a computational quantum chemistry and molecular dynamics to model particles with diameters of up to 5 nm and to perform laboratory experiments to evaluate the predicted pKa values as a function of composition and size. This is done using a temperature- and humidity-controlled reaction chamber and chemical ionization mass spectrometers to determine precursor and nanoparticle composition. The researchers will also develop and apply a parameterization applicable for multiscale modeling of relevant processes taking place in and on atmospheric aerosol particles. This latter research aim may lead to improvements in the predictive power of models. These models address the mechanisms and species responsible for the formation of new particles in the atmosphere, and the effects of atmospheric nanoparticles on human health and climate.