Molecular Insights into Atmospheric Brown Carbon: Optical Properties, Gas-Particle Partitioning, Viscosity Assessment.

Biomass burning organic aerosol (BBOA) is a major source of ‘brown carbon’ in the atmosphere, and the chemical composition and properties of brown carbon within these aerosols can vary depending on factors such as the type of biomass burned, combustion conditions, and atmospheric ageing. Molecular characterization of BBOA using advanced methods of high-resolution mass spectrometry provides essential insights into the composition, properties, and behavior of atmospheric brown carbon, contributing to a more comprehensive understanding of its environmental impacts. These studies have enabled the identification and quantification of specific components of BBOA, including strongly absorbing chromophores, such as phenolic, quinone and nitroaromatic compounds, N-heteroatom compounds, polycyclic aromatic hydrocarbons, etc. Through comprehensive understanding the chemical composition of brown carbon, we can now assess its sources and atmospheric transformations. Recent investigations have also broadened their scope to explore the partitioning of brown carbon species between the particle and gas phases. These measurements yield valuable data on particle-to-gas transition enthalpies and apparent volatilities of individual BBOA species, crucial for constructing volatility basis sets (VBS). The resulting VBS distributions enable an assessment of equilibrium gas-particle partitioning across various atmospheric conditions of organic mass loadings, temperatures, and pressures. under conditions relevant to Earth’s atmosphere, encompassing a broad range of total organic mass loadings and temperatures. Furthermore, novel parameterization models leverage chemical characterization and volatility datasets to evaluate the viscoelastic properties of BBOA. This comprehensive molecular understanding of BBOA chemistry is essential for predicting their ability to undergo chemical reactions, partition between gas and particle phases, and impact atmospheric environment and related processes, such as radiative forcing of climate and cloud formation. This presentation will provide an overview of recent advancements in this field and outline future directions for continued research.