(5) Furthermore, various models have been used to study HONO formation and its impact on air quality in China. For instance, CTMs and box models may employ a higher vertical resolution and a reasonable mixing layer height, respectively, to improve model performance in predicting near-ground HONO ( Text S1). Moreover, the vertical distribution of HONO should be considered in assessing the performance of those models. The validity of those parametrizations largely relies on laboratory kinetic studies or emission factors of the additional HONO sources for both box models and CTMs.
Along with the properties of the model (chemical scheme, inventory, meteorology, etc.), the parametrizations of additional HONO sources are of vital importance for those models. Moreover, the evolution of construction and coating materials should be considered.Ītmospheric models, including box models and chemistry-transport models (CTMs), generally contain one HONO source, NO+OH, by default. (4) Laboratory experiments are also needed to study the heterogeneous HONO formation on various indoor surfaces. Measurements of HONO, as well as related parameters (NO x, OH, actinic flux, relative humidity, etc.), should be conducted in real-life indoor environments to clearly understand the characteristics and formation of indoor HONO. Indoor NO 2 reacts rapidly on indoor surfaces, and this reaction can be light-enhanced to produce HONO. Reported average levels of indoor HONO are generally higher than ambient conditions and can reach 40 ppbv with the presence of indoor combustion processes, which can directly emit HONO and its precursors (e.g., NO 2). Besides, HONO itself acts as an important indoor pollutant that threatens public health. Similar to ambient conditions, indoor HONO can exacerbate indoor AOC and the formation of secondary pollutants. Indoor HONO chemistry has rarely been studied in China. This is of particular interest in China considering that vast agricultural fields are regularly treated with high application rates of chemical nitrogen fertilizers, constituting an important HONO source. Furthermore, compared to budget analysis based on ambient HONO measurements, flux measurements hold the advantage of providing direct information about ground-derived HONO sources (e.g., soil emissions and heterogeneous NO 2 reactions).
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Moreover, there are still no aircraft measurements throughout the boundary layer or even reaching the free troposphere, where the formation and impact of HONO are still poorly understood, especially over polluted regions. Understanding the interaction between HONO chemistry and aerosols should be one important chapter of the “haze chemistry” referring to the complicated atmospheric chemistry in polluted regions.
While it is confirmed that HONO can accelerate aerosol formation, whether aerosol plays an important role in HONO formation remains highly controversial. This is of particular interest concerning the abundance of aerosols in polluted regions such as the North China Plain (NCP). These measurements help to distinguish potential HONO sources such as ground- and aerosol-derived sources. Measurements of vertical profile and flux are important to study HONO formation, but few are reported in China.
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