Aging of atmospheric aerosols and the role of iron in catalyzing brown carbon formation

Abstract

Extensive research has been done on the processes that lead to the formation of secondary organic aerosol (SOA) including the atmospheric oxidation of volatile organic compounds (VOCs) from biogenic and anthropogenic sources, gas–particle partitioning, and multiphase/heterogeneous reactions. Also, a number of chemical and photochemical aging processes of primary aerosols and SOA were reported to lead to the formation of “brown carbon (BrC)”, a term that refers to light absorbing soluble and insoluble components. However, the role of transition metals such as iron in these processes is not well understood. This review summarized the current state of knowledge on iron chemistry that lead to BrC formation. Dark iron chemistry with phenolic and aliphatic organic precursors is shown to be responsible for the efficient formation of soluble and insoluble BrC, including organonitrogen compounds, under a wide range of atmospheric aerosol physical states and chemical compositions. These efficient processes are not completely suppressed in the presence of competing ligands or light. The atmospheric impact of SOA and BrC from these pathways is discussed in the context of aerosols' direct and indirect effects on the climate. Additional laboratory, field, and modeling studies are needed to better understand the contributions of these potentially important metal-catalyzed pathways to SOA and BrC formation and the overall aerosol chemistry.