Dissecting the Photochemical Reactivity of Metal Ions during Atmospheric Nitrate Transformations on Photoactive Mineral Dust

Dissecting the photochemical reactivity of metal ions is a significant contribution to understanding secondary pollutant formation, as they have a role to be reckoned with atmospheric chemistry. However, their photochemical reactivity has received limited attention within the active nitrogen cycle, particularly at the gas–solid interface. In this study, we delve into the contribution of magnesium ion (Mg²⁺) and ferric ion (Fe³⁺) to nitrate decomposition on the surface of photoactive mineral dust. Under simulated sunlight irradiation, the observed NO_X production rate differs by an order of magnitude in the presence of Mg²⁺ (6.02 × 10^–10 mol s^–1) and Fe³⁺ (2.07 × 10^–11 mol s^–1). The markedly decreased fluorescence lifetime induced by Mg²⁺ and the change in the valence of Fe³⁺ revealed that Mg²⁺ and Fe³⁺ significantly affect the concentration of nitrate decomposition products by distinct photochemical reactivity with photogenerated electrons. Mg²⁺ promotes NO_X production by accelerating charge transfer, while Fe³⁺ hinders nitrate decomposition by engaging in a redox cyclic reaction with Fe²⁺ to consume photogenerated carriers continuously. Furthermore, when Fe³⁺ coexists with other metal ions (e.g., Mg²⁺, Ca²⁺, Na⁺, and K⁺) and surpasses a proportion of approximately 12%, the photochemical reactivity of Fe³⁺ tends to be dominant in depleting photogenerated electrons and suppressing nitrate decomposition. Conversely, below this threshold, the released NO_X concentration increases sharply as the proportion of Fe³⁺ decreases. This research offers valuable insights into the role of metal ions in nitrate transformation and the generation of reactive nitrogen species, contributing to a deep understanding of atmospheric photochemical reactions.