Modeling aerosol mixing state in chamber coagulation experiments using the stochastic particle-resolved model PartMC

Description

The stochastic particle-resolved aerosol model PartMC explicitly tracks the size and composition of individual particles as they undergo transformations by coagulation and condensation in the atmosphere. We recently adapted PartMC to represent the aerosol evolution in aerosol chambers, with the intention to use the model as a tool to interpret and guide chamber experiments in the future. The new model development included chamber-specific processes according to Naumann (2003), such as wall loss due to particle diffusion and sedimentation, and dilution effects due to sampling, as well as a treatment of fractal particles to account for the morphology of agglomerates and its impact on particle dynamics. We present our model’s capability of simulating the evolution of particle size distributions and mixing state. We used two sets of size distribution data, the first from a single-species ammonium sulfate coagulation experiment conducted at the University of Illinois, and the second from a soot-ammonium sulfate mixing experiment performed at Aerodyne Research, Inc. Using an objective fitting optimization approach we determined the best-estimate values for the wall loss parameters based on minimizing the L2-norm of the model errors of number and mass distributions. The single-species dataset revealed that obtaining the best fit required taking into account the non-spherical structure of the particles as confirmed by scanning electron microscope imaging of the filters. The simulation result from the mixing experiment showed that about half of the initially externally-mixed soot and ammonium sulfate particles became internally mixed after two hours after the start of the experiment.