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

Abstract

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 (Mg 2+ ) and ferric ion (Fe 3+ ) 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 Mg 2+ (6.02 × 10 −10 mol s −1 ) and Fe 3+ (2.07 × 10 −11 mol s −1 ). The markedly decreased fluorescence lifetime induced by Mg 2+ and the change in the valence of Fe 3+ revealed that Mg 2+ and Fe 3+ significantly affect the concentration of nitrate decomposition products by distinct photochemical reactivity with photogenerated electrons. Mg 2+ promotes NO X production by accelerating charge transfer, while Fe 3+ hinders nitrate decomposition by engaging in a redox cyclic reaction with Fe 2+ to consume photogenerated carriers continuously. Furthermore, when Fe 3+ coexists with other metal ions (e.g., Mg 2+ , Ca 2+ , Na + , and K + ) and surpasses a proportion of approximately 12%, the photochemical reactivity of Fe 3+ 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 Fe 3+ 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.