Molecular-Scale Investigation on the Formation of Brown Carbon Aerosol via Iron-Phenolic Compound Reactions in the Dark

In this study, we systematically revealedhow chemicallydiverse BrC products were formed by Fe-induced oxidation of phenoliccompounds at the molecular level using ultrahigh-resolution mass spectrometryand chemical reaction network analysis. Brown carbon (BrC) is one of the most mysterious aerosolcomponentsresponsible for global warming and air pollution. Iron (Fe)-inducedcatalytic oxidation of ubiquitous phenolic compounds has been consideredas a potential pathway for BrC formation in the dark. However, thereaction mechanism and product composition are still poorly understood.Herein, 13 phenolic precursors were employed to react with Fe underenvironmentally relevant conditions. Using Fourier transform ion cyclotronresonance mass spectrometry, a total of 764 unique molecular formulaswere identified, and over 85% of them can be found in atmosphericaerosols. In particular, products derived from precursors with catechol-,guaiacol-, and syringol-like-based structures can be distinguishedby their optical and molecular characteristics, indicating the structure-dependentformation of BrC from phenolic precursors. Multiple pieces of evidenceindicate that under acidic conditions, the contribution of eitherautoxidation or oxygen-induced free radical oxidation to BrC formationis extremely limited. Ligand-to-Fe charge transfer and subsequentphenoxy radical coupling reactions were the main mechanism for theformation of polymerization products with high molecular diversity,and the efficiency of BrC generation was linearly correlated withthe ionization potential of phenolic precursors. The present studyuncovered how chemically diverse BrC products were formed by the Fe-phenoliccompound reactions at the molecular level and also provide a new paradigmfor the study of the atmospheric aerosol formation mechanism.

Automated summary

In this study, the formation of chemically diverse brown carbon (BrC) products through the oxidation of phenolic compounds by iron (Fe) catalysts was systematically investigated. The results provide insights into the reaction mechanism and product composition of BrC, which is important for understanding global warming and air pollution.