Understanding the Hygroscopicity of Organosulfates through Density Functional (DFT) Calculations and Molecular Dynamics (MD) Simulations

SHEUNG KI CHAN, Tianren Zhu, Ying-Lung Steve Tse, Man Nin Chan, The Chinese University of Hong Kong

     Abstract Number: 261
     Working Group: Aerosol Physics

Abstract
Organosulfates (OS), also known as organic sulfate esters are the most abundant class of organosulfur compounds, which contribute up to 30% of total aerosol mass, showing their ubiquity in atmospheric aerosols. Recent research has extensively studied the formation and reaction pathways of OS and revealed that some OS exhibit non-crystallizing behavior during dehydration. Meanwhile, their physiochemical properties, for instance, hygroscopicity are yet to be comprehensively understood. As aerosol particles absorb moisture from surroundings, they undergo alterations in size, physical state, and composition, consequently impacting their physicochemical properties such as acidity, viscosity, and optical properties, which in turn influence air quality, cloud formation, and atmospheric reactions.

This study focuses on exploring the hygroscopic nature of short-chain OS aerosols through Density Functional Theory (DFT) calculations and Molecular Dynamics (MD) simulations. Using a computational approach, Raman spectra were generated for both concentrated and dilute OS aerosols to study the molecular motions of OS molecules. Stretching of sulfate ester group, ν_s(SO₃) is found at around 1065 cm^-1 while the stretching of O–H bond in water molecules, including ν_s(–OH) and ν_a(–OH) occurs at a range from approximately 3250 cm^-1 to 4000 cm^-1. Furthermore, the intermolecular forces between the sulfate ester group and water molecules were examined to understand the hygroscopic behavior of OS at a molecular level.