Effect of common inorganic anions on iron-catalyzed secondary brown carbon formation from guaiacol

Abstract

Brown carbon (BrC) is the important component of aerosol with strong UV–visible absorbance. However, the formation of BrC is still elusive. Inorganic anions, e.g., Cl−, NO3− and SO42−, exist ubiquitously in the atmosphere, while their effects on the formation of BrC are poorly understood. In this study, we have systematically investigated the effects of pH (1, 2 and 3), inorganic anion (Cl−, NO3− and SO42−) and ionic strength (0.1, 0.5 and 1.0 M) on BrC generation process by measuring the optical, aggregation and product properties. Our results clearly show that the three factors strongly affect the BrC formation by influencing the oxidation activity and the complexation capability of different Fe(III) species. Marcus theory was used in this research to calculate the oxidation activity of different Fe(III) species. Among all the species of Fe(III), FeOH2+ is the most reactive form in the BrC formation reaction. Furthermore, the aggregation process of BrC was also studied, which is affected by different anions due to their different concentration and hydrability, and SO42− exhibits the highest efficiency to induce the aggregation of BrC. This study will deepen our understanding about the natural formation of BrC under environmentally relevant conditions, and be beneficial for controlling the production of atmospheric particulates and the subsequent health effects.