Modelling aerosol chamber experiments with kinetic gas-to-particle partitioning of organic molecules under humid conditions

The partitioning of organic vapors between the gas and the condensed phase is a crucial process influencing the formation of organic aerosol (OA). Investigating the key factors affecting  partitioning is quite challenging because of the disparate range and variability of atmospheric conditions (temperature, relative humidity, precursors etc.). Therefore, the combination of chamber experiments and chemical box models allows for investigating a specific OA formation via gas-aerosol partitioning under controlled experimental conditions. On this basis, we have developed a novel multiphase chemical box model, the so-called MESSy DWARF, to simulate chamber experiments. MESSy DWARF is part of the MESSy (Modular Earth Submodel System) modeling framework. This allows the box model to utilize the full range of process parameterizations  available in MESSy, which originally have been designed for global atmospheric chemistry simulations. 

Here, we present a MESSy DWARF application to simulate a chamber experiment for studying gas-to-particle partitioning of organic molecules under humid conditions. The kinetic partitioning scheme is based on the Schwartz mass transfer coefficient and is governed by the liquid water content (LWC) and water solubility. Losses of organic vapors to the walls have been included. To assess the kinetic partitioning of the model, we selected an experiment of alpha-pinene photooxidation with the presence of ammonium sulfate seeds at 50 % relative humidity. This experiment was conducted in SAPHIR-STAR, an indoor continuous glass tank chamber at Forschungszentrum Jülich. The model simulations emphasize the significance of LWC for organic aerosol concentration. Thus, the model has been expanded to include capabilities for estimating LWC from aerosol number counts and inorganic mass concentrations and the wet radius. LWC is calculated as the difference between the average wet and dry volumes of the particles. The volume of dry particles is estimated by use of either densities or grow factors of solute components.

The analysis of the model results indicates a correlation between NO-levels and water solubility of alpha-pinene oxidation products. As the level of NO decreases, the reaction pathways of the organic peroxy radicals shift towards the production of species bearing carboxyl and hydro(pero)xyl functional groups. The enhanced production of water-soluble products is consistent with the observed increase in organic mass at low NO. The preliminary success in simulating the multiphase chamber experiment indicates the potential for further model applications.