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.