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Secondary air pollution, especially ozone (O3) and secondary aerosols, are emerging air quality challenges confronting China. Nitrous acid (HONO), as the predominant source of hydroxyl radicals (OH), are acknowledged to be essential for secondary pollution. However, HONO concentrations are usually underestimated by current air quality models due to the inadequate representations of its sources. In the present study, we revised the Weather Research and Forecasting & Chemistry (WRF‐Chem) model by incorporating additional HONO sources, including primary emissions, photo‐/dark oxidation of NOx, heterogeneous uptake of NO2 on surfaces, and nitrate photolysis. By combining in‐situ measurements in the Yangtze River Delta (YRD) region, we found the improved model show much better performance on HONO simulation and is capable of reproducing observed high concentrations. The source‐oriented method is employed to quantitatively understand the relative importance of various processes, which showed that heterogeneous NO2 uptake on the ground surface was the major contributor to HONO formation in urban areas. Comparatively, photo‐oxidation of NOx is a main contributor in rural areas. The introduction of multiple sources of HONO led to an apparent increase in OH and hydroperoxyl (HO2) radicals. The promoted HO2 levels further increased diurnal O3 concentration by 4.5–12.9 ppb, while secondary inorganic and organic concentrations were also increased by 14%–32% during a typical secondary pollution event. The improved description of HONO emission and formation in the model substantially narrowed the gaps between simulations and observations, highlighting the great importance in understanding and numerical representations of HONO in secondary pollution study. Plain Language Summary: Nitrous acid (HONO) has a vigorous potential to produce hydroxyl radicals, contributing to tropospheric oxidation reactions. However, current air quality models lack detailed representation of HONO‐related processes. We revised a widely‐used air quality model based on recently reported mechanisms. The improved model can reasonably simulate actual HONO concentrations measured in the Yangtze River Delta region, one of the most urbanized city clusters in China. It is identified that the heterogeneous conversion of nitrogen dioxide on the ground surface and photo‐oxidation of nitrogen oxide may play an important role in HONO formation. The modeling result is indicative of the pronounced enhancement of HONO on secondary air pollutants, including ozone and secondary aerosols. This study highlights the critical impact of HONO on deteriorating secondary air pollution in city clusters. Key Points: Incorporating missing sources of HONO in the air quality model helps to better represent the observed magnitude and temporal variations of HONOHeterogeneous uptake of NO2 on the ground surface and photo‐oxidation of NOx are major contributors to HONO formationHONO can significantly deteriorate ozone and secondary aerosol pollution by enhancing atmospheric oxidizing capacity [ABSTRACT FROM AUTHOR]

As primary emissions have been declining in recent decades in China, secondary pollution, like ozone (O 3) 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 O 3 , 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, NO 2 , NH 3 , HNO 3 , NO 3 radicals, N 2 O 5 , ClNO 2 and nitrate), organic nitrates (which involves NO, NO 3 , RONO 2 and HNO 3), HONO (which involves NO, NO 2 , HONO, and nitrate), and reduced‑nitrogen (which involves NH 3 and amines). Regional CTM modeling of inorganic nitrates mainly focuses on homogeneous NO 2 oxidation and heterogeneous N 2 O 5 hydrolysis, which contribute to over 80% of inorganic nitrate formation in China. During hazy days, the contribution of heterogeneous uptake of N 2 O 5 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 NO 2 on solid surfaces, and nitrate photolysis. The heterogeneous uptake of NO 2 is the predominant pathway of HONO formation. HONO affects AOC by regulating the OH budget, particularly during the early morning when O 3 photochemistry is not efficient. For NH 3 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 NH 3 and amines, and realizing effective modeling of organic nitrates in the future. • Reactive nitrogen plays a vital role in the formation of secondary air pollution. • Parameterizations of reactive nitrogen in chemical transport models are summarized. • Numerous studies have been conducted in China to model inorganic nitrates and HONO. • Organic nitrates are rarely simulated due to their high computational complexity. • More attention has been paid to amines due to their importance in particle nucleation. [ABSTRACT FROM AUTHOR]