A systematic review of reactive nitrogen simulations with chemical transport models in China

As primary emissions have been declining in recent decades in China, secondary pollution, like ozone (O₃) and secondary aerosols are main pollutants deteriorating air quality. The chemistry of reactive nitrogen (Nr) plays a crucial role in the chemical transformation of secondary aerosols and O₃, and its representation in three-dimensional (3-D) regional/global chemical transport models is vital for quantitatively understanding Nr's role in air pollution as well as future emission control. This study reviews the parameterizations of the Nr modeling in chemical transport models (CTMs) and their environmental implications over the past decade in China. A number of Nr species are present in the atmosphere. Given the significant impact of Nr on air quality, we have identified four research topics within the broader field of Nr research. These Nr topics include the multiphase chemistry of inorganic nitrates (which involves NO, NO₂, NH₃, HNO₃, NO₃ radicals, N₂O₅, ClNO₂ and nitrate), organic nitrates (which involves NO, NO₃, RONO₂ and HNO₃), HONO (which involves NO, NO₂, HONO, and nitrate), and reduced‑nitrogen (which involves NH₃ and amines). Regional CTM modeling of inorganic nitrates mainly focuses on homogeneous NO₂ oxidation and heterogeneous N₂O₅ hydrolysis, which contribute to over 80% of inorganic nitrate formation in China. During hazy days, the contribution of heterogeneous uptake of N₂O₅ can be amplified. By contrast, organic nitrates modeling, including their gas-phase reactions, gas-particle conversion, and hydrolysis, is poorly explored in China, despite their importance for secondary organic aerosol (SOA) formation. HONO sources and their impacts on atmospheric oxidizing capacity (AOC) are an emerging research topic. Parameterizations of major HONO sources were presented, including the homogeneous pathway, direct emissions, heterogeneous uptake of NO₂ on solid surfaces, and nitrate photolysis. The heterogeneous uptake of NO₂ is the predominant pathway of HONO formation. HONO affects AOC by regulating the OH budget, particularly during the early morning when O₃ photochemistry is not efficient. For NH₃ and amines, current CTMs fail to accurately represent their sources in urban regions due to the lack of non-agricultural emissions. Although amines do not significantly influence chemical cycles, they show strong potential in inducing new particle formation. Overall, further work is needed to improve Nr modeling in China by developing parameterizations suitable for the Chinese atmospheric conditions, establishing accurate sector-based emission inventories for NH₃ and amines, and realizing effective modeling of organic nitrates in the future.