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.