Gas-Particle Interactions of Organic Aerosol

Abstract

Atmospheric organic aerosols play significant roles in climate, air quality, and human health. Quantitative understanding and predicting the gas-particle interactions of organic aerosols and their role in particle formation are, however, challenging. This thesis presents results from laboratory experiments, model simulations, and ambient measurements of gas-particle interaction of atmospheric-relevant organic molecules and the role of this interaction in the growth and evolution of aerosol particles. The gas-to-particle interaction of organic molecules significantly alters the distribution of reactive atmospheric gases. The uptake of unary and binary mixtures of glyoxal and pinanediol by neutral and acidic sulfate particles was investigated. Results shown that the uptake to acidic aerosol particles greatly increased for a binary mixture of glyoxal and pinanediol compared to the unary counterparts. Possible mechanisms of acid-catalyzed cross-reactions between the carbonyl and hydroxyl functionalities were proposed to explain the synergistic uptake. These synergistic uptake reactions can significantly influence the gas-particle partitioning of organic molecules. The gas-particle interaction of the organic molecules also greatly affects the growth of particles. The RH dependence condensational growth of α-pinene ozonolysis was studied. The particle growth coefficients, representing a combination of the thermodynamic driving force and the kinetic resistance to mass transfer, increased from 0.35 to 2.3 nm2 s-1 from 0% RH to 75% RH. The chemical composition did not depend on RH. The Model for Simulating Aerosol Interactions and Chemistry was applied to reproduce the observed size- and RH-dependent particle growth by optimizing the diffusivities Db within the particles of the condensing molecules. The Db values increased from 5 α 1  10 16 at 0% RH to 2 α 1  10 12 cm-2 s-1 at 75% RH for mass accommodation coefficients α of 0.1 to 1.0, highlights the importance of particle viscosity on the growth of the aerosol particles. This particle viscosity is sensitive not only to RH but also temperature. The work presented in this thesis measured temperature-dependent viscosity using an atomic force microscope (AFM). The resonant frequency response of the AFM cantilever in the atmospheric relevant organic materials was recorded at a range of temperature. Dioctyl phthalate and sucrose were chosen as prototypical organic materials. The viscosity was retrieved from the resonant frequency by a hydrodynamic model and validated by previously reported viscosities. Apply fundamental processes from laboratory results to the actual atmospheric situation is challenging. The challenge lies in the complexity of the interactions and simultaneous changes of the numerous atmospheric parameters. A machine-learning algorithm was developed to predict particle concentrations of primary organic aerosol (POA) and secondary organic aerosol (SOA) and assess their quantitative relationships with parameters of interest while isolating cofounding parameters. Urban and rural sites in Hong Kong were chosen as a representative of real-world conditions. The algorithm explained more than 80% of the observed OA in the two sites and able to explain the dependence of OA concentrations on different atmospheric conditions, such as the changes in NOx and O3. The approach can also be applied to other ambient datasets in capturing the complex relationships. For future application, additional information on the VOCs, SVOCs concentrations would be helpful to elucidate the gas-particle interaction of organic aerosol. Additional studies in different geographical regions would be useful to improve model generalization for OA pollution control.