On the role of particle mixing state in heterogeneous and multiphase reaction kinetics

Description

The rate at which trace gases are accommodated at the gas-particle interface is a complex function of single-particle chemical composition, morphology, and physical phase state. To date, the vast majority of atmospheric observations and models designed to interpret these measurements focus on the ensemble average of aerosol particle mass, assuming that each individual particle has the same chemical identity as the average state (internally mixed) as opposed to treating individual particles as having unique chemical composition (externally mixed). Here, we assess the impact of particle mixing state on heterogeneous and multiphase reaction kinetics using the reactive uptake of N2O5 and HOCl on sea-spray aerosol as a model system. Simultaneous observations of single-particle chemical composition made using single particle aerosol mass spectrometry and scanning transmission X-ray microscopy, combined with in situ measurements of N2O5 and HOCl reactive uptake, are used to constrain model parameterizations under both internal and external mixing assumptions. We describe results obtained from ambient studies conducted at the Scripps Institution of Oceanography (SIO) Pier in La Jolla, California, and laboratory studies conducted using a newly developed air-sea interaction facility at the SIO hydraulics laboratory. These studies permit assessment of the dependence of the reactive uptake for N2O5 and HOCl on the presence of organic films and the mixing state of organic compounds within nascent sea-spray aerosol.