A Review of Laboratory Studies on the Heterogeneous Chemistry of NO2: Mechanisms and Uptake Kinetics.

NO₂ is a significant primary atmospheric pollutant that plays a key role in atmospheric chemistry. It serves as a crucial precursor to photochemical smog, acid rain, and secondary particulate matter and is instrumental in determining the atmospheric oxidation capacity. In this review, we focus on the heterogeneous chemistry of NO₂, which has been demonstrated to significantly influence the sources and sinks of various nitrogen-containing species through field measurements and model simulations. We provide a comprehensive summary of laboratory studies investigating the reaction mechanisms and uptake kinetics of NO₂ in heterogeneous reactions. NO₂ can undergo disproportionation reactions on atmospheric particles. For instance, it may hydrolyze on wetted surfaces to form HONO and HNO₃, produce nitrate and NO on mineral dust, or generate nitrate and NOCl on sea salt. Additionally, NO₂ can be reduced to HONO on soot and Fe-bearing minerals or photocatalytically reduced to HONO and NO on photosensitive components. Furthermore, NO₂ can be photo-oxidized to nitrate or N₂O₅ on illuminated TiO₂. In addition, the synergistic effect of the heterogeneous reactions between NO₂ and SO₂ is discussed. The uptake coefficients of NO₂ on typical particles and the factors influencing these coefficients are also summarized. Finally, based on the current insufficient understanding of heterogeneous reactions of NO₂, we propose prospects for future research.