Volatility Parametrization of Low-Volatile Components of Ambient Organic Aerosols Based on Molecular Formulas

Volatility Basis Set Distributions and Viscosity of Organic Aerosol Mixtures: Insights from Chemical Characterization Using Temperature-Programmed Desorption-Direct Analysis in Real-Time High-Resolution Mass Spectrometry

Quantitative assessment of gas-particle partitioning of individual components within complex atmospheric organic aerosol (OA) mixtures is critical for predicting and comprehending the formation and evolution of OA particles in the atmosphere. This investigation leverages previously documented data obtained through a temperature-programmed desorption-direct analysis in real-time, high-resolution mass spectrometry (TPD-DART-HRMS) platform. This methodology facilitates the bottom-up construction of volatility basis set (VBS) distributions for constituents found in three biogenic secondary organic aerosol (SOA) mixtures produced through the ozonolysis of α-pinene, limonene, and ocimene. The apparent enthalpies (ΔH*, kJ mol-1) and saturation mass concentrations (CT*, μg·m-3) of individual SOA components, determined as a function of temperature (T, K), facilitated an assessment of changes in VBS distributions and gas-particle partitioning with respect to T and atmospheric total organic mass loadings (tOM, μg·m-3). The VBS distributions reveal distinct differences in volatilities among monomers, dimers, and trimers, categorized into separate volatility bins. At the ambient temperature of T = 298 K, only monomers efficiently partition between gas and particle phases across a broad range of atmospherically relevant tOM values of 1-100 μg·m-3. Partitioning of dimers and trimers becomes notable only at T > 360 K and T > 420 K, respectively. The viscosity of SOA mixtures is assessed using a bottom-up calculation approach, incorporating the input of elemental formulas, ΔH*, CT*, and particle-phase mass fractions of the SOA components. Through this approach, we are able to accurately estimate the variations in SOA viscosity that result from the evaporation of its components. These variations are, in turn, influenced by atmospherically relevant changes in tOM and T. Comparison of the calculated SOA viscosity and diffusivity values with literature reported experimental results shows close agreement, thereby validating the employed calculation approach. These findings underscore the significant potential for TPD-DART-HRMS measurements in enabling the untargeted analysis of organic molecules within OA mixtures. This approach facilitates quantitative assessment of their gas-particle partitioning and allows for the estimation of their viscosity and condensed-phase diffusion, thereby contributing valuable insights to atmospheric models.

Secondary organic aerosols from oxidation of 1-methylnaphthalene: Yield, composition, and volatility

Alkyl-PAHs (APAHs) have been identified worldwide, which could rapidly react with chlorine and OH radicals in the atmosphere. In this study, a comprehensive investigation is conducted for SOA generated by a representative alkyl-naphthalene (1-methyl naphthalene, 1-MN) initiated by Cl, including yield, chemical composition, and volatility of SOA. To better understand 1-MN atmospheric oxidation, reaction mechanisms of 1MN with Cl atoms and OH radicals are proposed and compared under different nitrogen oxides (NOx) conditions. The SOA yields are comparable for Cl-initiated and OH-initiated reactions under high NOx conditions but increased in Cl-initiated reactions under low NOx conditions. The compounds with ten carbons are more abundant in Cl-initiated SOA, while compounds with nine carbons have higher intensity, suggesting that Cl caused ring-retained and alkyl-lost products and OH produces ring-broken and alkyl-retained compounds. The volatility of SOA is remarkably low, and SOA formed from Cl oxidation is slightly higher than that from OH oxidation. These results reveal that 1MN-derived SOA with OH and Cl radicals would have different physical-chemical properties and may play an important role in air quality and health effects.