Photochemical reaction between triclosan and nitrous acid in the atmospheric aqueous environment

Nitrous acid (HONO) is an important tropospheric pollutant and a major source of hydroxyl radical in the atmospheric gas phase. However, studies on the role of HONO in atmospheric aqueous phase chemistry processes are relatively few. The present work investigated the photochemical reaction of HONO with triclosan (TCS), which is an emerging contaminant, using a combination of laser flash photolysis spectrometry and gas chromatography mass spectrometry. With these techniques, the reaction pathway of HONO with TCS was proposed by directly monitoring the transient species and detecting the stable products. ·OH was generated from the photodissociation of the HONO aqueous solution and attacked TCS molecules on different sites to produce the TCS-OH adducts with a second-order rate constant of 1.11 × 109 L mol−1 s−1. The ·OH added a C atom adjacent to the ether bond in the aromatic ring of TCS and self-decayed when the ether bond broke. The intermediates generated from the addition of ·OH to the benzene ring of the TCS molecular structure were immediately nitrated by HONO, which played a key role in the formation process of nitrocompounds. An atmospheric model suggests that the aqueous oxidation of TCS by ·OH is a major reaction at high liquid water concentrations, and the photolysis of TCS dominates under low-humidity conditions.