In this project, Drs. Yong Cai, Guangliang Liu, and Kevin O'Shea at the Florida International University are funded by the Environmental Chemical Sciences Program of the NSF Division of Chemistry to study the reactions of mercury (Hg) that is associated with particles in aquatic environments. These particulate Hg species (pHg) are very common in natural waters, and they are often thought of as unreactive and not very mobile. However, they do react under the influence of sunlight (photochemical reactions) and need to be considered for an accurate description of Hg cycling, especially the air-water exchange of Hg. This project aims to provide a basic understanding of these photochemical reactions and explore the role of pHg in the air-water Hg exchange. The project has a broad range of benefits to research, education, and outreach at a Hispanic Serving Institution. It provides training for PhD and undergraduate students. Underrepresented minority students gain research experience. The curriculum development of the unique Environmental Chemistry-focused PhD track program is being enhanced. The project also promotes public awareness on Hg contamination and health risks. Since the aquatic particles are semiconducting and photosensitizing in nature, the investigators focus on the photochemical reactions of pHg involving electron-hole pairs and reactive oxygen species. They define the reaction pathways of pHg phototransformations and quantify the kinetics of photochemical reduction of oxidized Hg species and the oxidation of elemental pHg. They also study the mechanisms related to the pHg photoreactions. Experiments using model iron, titanium, and manganese oxide particles of different band gap energies and field-collected particles are combined with quantum chemistry calculations. The bandgap energies analysis uses spectrometric measurements. Particle chemical composition and optical properties are determined using inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy (SEM). The analysis of elemental and oxidized Hg in the dissolved and particulate phases is based on an isotope dilution, purge and trap, thermal desorption, and ICP-MS method. The research is expected to clarify the involvement of pHg species in Hg photochemical transformation and the role in air-water Hg exchange.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.