This book systematically describes the instrument setup for the measurement of nitrate radical (NO₃) and dinitrogen pentoxide (N₂O₅), as well as the mixing ratio, chemical behaviors, and atmospheric impacts of NO₃ and N₂O₅ in Beijing, China. It also discusses the instrument design and data analysis method in detail. Based on several field measurements of NO₃ and N₂O₅ in Beijing, it shows the variation in concentration and the budget of NO₃ and N2O5. The N₂O₅ heterogeneous uptake coefficient was determined using various methods, and the relationship between the N₂O₅ uptake coefficient and the particle properties was demonstrated, as well as the impact of NO₃–N₂O₅ chemistry to the atmospheric oxidation and the formation of particulate nitrate. These results increase our understanding of nighttime chemistry and provide insights into the role of NO₃–N₂O₅ chemistry in other polluted regions.