Importance of Particle Morphology on the Heterogeneous Oxidation of Organic Aerosol

Abstract:

It is well-recognized that heterogeneous oxidation results in the chemical and physical transformation of organic aerosol during its atmospheric lifetime; however, the importance of this process as a source of volatile oxidized organics and as an aerosol sink is debated with some studies showing a significant loss of mass (and the production of volatile organics) and others indicating that only a minor change occurs on atmospherically relevant time scales. Here, we present results from both laboratory experiments and a simple model showing that the ability for heterogeneous oxidation to act as a sink of (initially) reduced organic aerosol depends on the detailed structure of the particle. For instance, the organic compounds within a well-mixed particle will experience only a few oxidative lifetimes over the particle’s atmospheric lifetime of several days. In the case of reduced organic species (e.g., primary organic aerosol), these early-generation oxidation products will generally be of low enough volatility to remain in the particle phase. As a result, heterogeneous oxidation is an inefficient loss process of organic aerosol, a finding which is consistent with observations from many aerosol flow tube studies. In contrast, an organic coating on a core can undergo numerous oxidative lifetimes in the same timeframe. This can result in dramatic changes to the degree of oxidation state of the organic and even the loss of organic aerosol mass. These results indicate that particle structure and morphology must be considered when assessing the impact of heterogeneous oxidation on organic aerosol chemical composition and lifetime.