Molecular Insights into Gas-Particle Partitioning and Viscosity of Atmospheric Brown Carbon
Qiaorong Xie, Nealan Gerrebos, Diego Calderon-Arrieta, Isaac Morton, Emily Halpern, Chunlin Li, Janice Zeng, Yinon Rudich, Allan K. Bertram, ALEXANDER LASKIN, Purdue University
Abstract Number: 335
Working Group: Carbonaceous Aerosols
Abstract
Biomass burning organic aerosol (BBOA), containing numerous brown carbon (BrC) compounds, exerts a notable influence on atmospheric chemistry and radiative forcing of climate. The influence of evaporation processes on optical properties, volatility, and viscosity of BBOA under different environmental conditions remains poorly understood, leading to uncertainties in predicting the atmospheric and climate effects of BrC. This study presents a molecular characterization of laboratory BBOA proxies analyzed at various stages of evaporation, labeled as POx (where x represents the initial to remaining volume ratio after evaporation). The molecular characterization results are then applied for a bottom-up assessment of the volatilities and viscosities of the analyzed mixtures. The individual compound saturation mass concentration values measured using temperature programmed desorption − direct analysis in real time – high resolution mass spectrometry are used to construct volatility basic set (VBS) distributions. These distributions quantify the extent of gas-particle partitioning, revealing more compounds transition from the particle phase to the gas phase with atmospheric dilution. Conversely, atmospheric cooling enhances condensation of the gas-phase compounds into the particle phase, underscoring their sensitivity to both temperature and atmospheric dilution. Furthermore, the VBS-based assessment of gas-particle partitioning is leveraged to estimate the viscosities of BBOA represented by POx. These estimates align closely with viscosity values obtained through poke-flow experiments, thus validating the estimation approach. As the degree of evaporation increases, the viscosities of POx mixtures exhibit a substantial increase, resulting in a substantial slowdown in particle-phase diffusion. This new understanding emphasizes the critical role of evaporation processes and environmental conditions in transforming the volatility and viscosity of BBOA particles. These factors should be adequately integrated into atmospheric transport models for improved accuracy in predicting BBOA aging transformations.