A diagonal volatility basis set to assess the condensation of organic vapors onto particles

Abstract

We present a "diagonal" Volatility Basis Set (dVBS) comparing gas-phase concentrations of oxygenated organic molecules (OOM) to their condensed-phase mass fractions. This permits closure of vapor concentrations with particle composition constrained by particle growth rates, allowing the contributions of quasi non-volatile condensation, equilibrium partitioning, and reactive uptake to be separated. The dVBS accommodates both equilibrium and dynamical (growth) conditions. Growth implies an association between gas and particle concentrations governed by a "condensation line" that is set by the particle growth rate, which fixes the total (excess) concentration of condensible vapors. The condensation line defines an infeasible region of high particle mass fraction and low gas concentration; under steady-state growth conditions, compounds cannot appear in this infeasible region without being formed by condensed-phase chemistry. We test the dVBS with observations from the CLOUD experiment at CERN using data from a FIGAERO I⁻ Chemical Ionization Mass Spectrometer measuring vapors directly and particle composition via temperature programmed desorption from a filter. A dVBS analysis finds that data from an α-pinene + O₃ run at 243 K are consistent with volatility driven condensation forming the large majority of particle mass, with no compounds clearly within the infeasible region.