Research Theme 2, Chemical Reactions at Complex Interfaces, is focused on advancing our understanding of the critical dependence of chemical reactions and mechanisms on the molecular scale properties of complex environmental surfaces. It has been hypothesized that the rates of of heterogeneous and multiphase reactions occurring at complex environmental interfaces (e.g., sea spray aerosol) are tied to the molecular-scale physical and chemical properties of the interface. RT2 leverages extensive investigation of the composition and morphology of environmental interfaces uncovered in CAICE to explore a host of chemical and biological processes occurring at atmospheric interfaces.
Reactions at complex atmospheric interfaces
Recent work has focused on the role of surface-bound organic material in the suppression (or enhancement) of the reactive uptake of oxidized nitrogen compounds (e.g., HNO3 and N2O5). These studies have focused on population averaged kinetic measurements that yield quantitative insight, at the expense of single particle information. Results from these studies have demonstrated that organic compounds present in sea spray aerosol do not impede the accomodation of trace gases such as N2O5 (at high relative humidity), but can significantly alter product yields (e.g., the formation of ClNO2). In a related approach, single particle measurements of chloride and nitrate have been investigated to explore the particle-to-particle variability in acid-displacement reactions (e.g., HNO3) demonstrating that the extent of chloride displacement can be correlated with the particulate organic substance.
A second line of research in this area focuses on the biochemical production of volatile organic compounds in surface waters from coupled biological and chemical processes. These studies explore the catabolism of organic macromolecules found in surface waters as unique production pathways for the formation of small molecules such as alkyl nitrates and glyoxal. These enzymatic and bacteria-mediated pathways serve as unique conduits for the production of a wide suite of atmospherically relevant gas phase molecules.
Detailed Studies of the Elementary Reaction Steps
Gaining an understanding of heterogeneous reactions from first principles starts with experiments that can probe elementary reaction steps. These experiments, carried out by the Nathanson Research Group, use a molecular beam of reagent molecules directed at a liquid microjet of aerosol mimic material, analyzing the gas phase products with a mass spectrometer. These studies will help clarify the mechanistic details of gas-particle heterogeneous reactions, and will facilitate a direct connection to computational chemistry studies.
Computational Studies of Reactions at Interfaces
From a computational approach, the Paesani Research Group will simulate the reactive scattering of gas molecules on the surface of a bulk aqueous phase that will be experimentally characterized by the Nathanson Research Group. The modeling will use a direct dynamics reactive scattering approach that, coupled with adaptive quantum mechanical/molecular mechanics, will allow consistent, quantum mechanical treatment of the reactive process. Prof. Paesani and his group have recently implemented this technique, termed DDRS-adQM/MM, in the AMBER molecular dynamics package.