During atmospheric transport aerosol particles can undergo chemical transformations by multiphase chemical reactions with atmospheric oxidants. These gas-to-particle reactions, also called heterogeneous reactions, can significantly alter atmospheric chemistry as observed in the case of the Antarctic ozone hole. Multiphase chemical reactions can change the physical and chemical properties of the particles with subsequent implications of the aerosol particles’ role in air quality, cloud formation, and climate. Furthermore, it has been shown that these gas-to-particle interactions also contribute to the formation of new particles such as secondary organic aerosols.
Left hand side demonstrates the interplay between organic aerosol, aerosol phase state, multiphase chemical oxidation reactions leading to “chemical aging”, and its effects on cloud formation. The amorphous nature of organic aerosol is indicated by the phases solid, semi-solid, and liquid. This plot is taken from Shiraiwa et al. (2011).
Our group focuses on the multiphase chemical reactions between organic aerosol particles, which can be in an amorphous phase state (i.e., a gradual change from liquid to solid phases), and atmospheric oxidants such as such as O3, NO2, N2O5, NO3, and OH. These heterogeneous reactions constitute complex interactions between gaseous and condensed phase species where various processes can proceed sequentially or in parallel as depicted below.
The right hand side shows a sketch of the mechanisms involved in multiphase chemical reactions. First a gas molecule diffuses to the aerosol particle, then reaction on the surface can occur or it is taken up, i.e. dissolved, and eventually reacts within the particle. These various processes can also occur in parallel. The decoupling of these processes is also shown: the resistor model in which the individual processes are treated as resistors as in an electrical circuit. This allows the separation of the underlying differential equations under certain assumptions. (adapted from Finlayson-Pitts & Pitts, 2000).
In our laboratory we measure the multiphase chemical kinetics by monitoring the reactive uptake of the gaseous oxidant to the condensed-phase matter. This allows us to determine the reactive uptake coefficient and second order rate coefficients. Experimentally this is achieved by online monitoring the loss of gaseous reactants by means of a chemical ionization mass spectrometer.
Left hand side shows a typical experimental setup to study multiphase chemical kinetics using our home-built chemical ionization mass spectrometer (CIMS). Different techniques are employed to generate gaseous oxidants which enter the flow reactor. CIMS is attached to the flow reactor to monitor the changes in oxidant concentration. Taken from Li et al. (2020).
Besides applying experimental studies to advance our understanding on gas-to-particle interactions, we conduct modeling studies that represent in-detail multiphase chemical kinetics based on the Pöschl-Rudich-Ammann (PRA) kinetic framework for aerosols. Applying a Langmuir-Hinshelwood reaction mechanism, the reversible co-adsorption of O3, NO2, and H2O on soot coated with Benzo[a]pyrene (BaP) for an urban plume scenario was simulated over a period of five days (Springmann et al., 2009). Furthermore, we coupled the PRA kinetic framework to PartMC-MOSAIC (Particle Monte Carlo model, coupled to the MOdel for Simulating Aerosol Interactions and Chemistry, in collaboration with Nicole Riemer) which allows to monitor compositional changes of individual soot particles with a realistic size distribution and a continuous range of chemical ages (Kaiser et al., 2011).