Amplified role of potential HONO sources in O<sub>3</sub> formation in North China Plain during autumn haze aggravating processes

Abstract

Abstract. Co-occurrences of high concentrations of PM2.5 and ozone (O3) have been frequently observed in haze-aggravating processes in the North China Plain (NCP) over the past few years. Higher O3 concentrations on hazy days were hypothesized to be related to nitrous acid (HONO), but the key sources of HONO enhancing O3 during haze-aggravating processes remain unclear. We added six potential HONO sources, i.e., four ground-based (traffic, soil, and indoor emissions, and the NO2 heterogeneous reaction on ground surface (Hetground)) sources, and two aerosol-related (the NO2 heterogeneous reaction on aerosol surfaces (Hetaerosol) and nitrate photolysis (Photnitrate)) sources into the WRF-Chem model and designed 23 simulation scenarios to explore the unclear key sources. The results indicate that ground-based HONO sources producing HONO enhancements showed a rapid decrease with height, while the NO + OH reaction and aerosol-related HONO sources decreased slowly with height. Photnitrate contributions to HONO concentrations were enhanced with aggravated pollution levels. The enhancement of HONO due to Photnitrate on hazy days was about 10 times greater than on clean days and Photnitrate dominated daytime HONO sources (∼ 30 %–70 % when the ratio of the photolysis frequency of nitrate (Jnitrate) to gas nitric acid (JHNO3) equals 30) at higher layers (>800 m). Compared with that on clean days, the Photnitrate contribution to the enhanced daily maximum 8 h averaged (DMA8) O3 was increased by over 1 magnitude during the haze-aggravating process. Photnitrate contributed only ∼ 5 % of the surface HONO in the daytime with a Jnitrate/JHNO3 ratio of 30 but contributed ∼ 30 %–50 % of the enhanced O3 near the surface in NCP on hazy days. Surface O3 was dominated by volatile organic compound-sensitive chemistry, while O3 at higher altitudes (>800 m) was dominated by NOx-sensitive chemistry. Photnitrate had a limited impact on nitrate concentrations (<15 %) even with a Jnitrate/JHNO3 ratio of 120. These results suggest the potential but significant impact of Photnitrate on O3 formation, and that more comprehensive studies on Photnitrate in the atmosphere are still needed.

Spatiotemporal variations of ozone exposure and its risks to vegetation and human health in Cyprus: an analysis across a gradient of altitudes

“…Although VOCs have not been included in the present study, they play a crucial role in regulating O 3 in the atmosphere, and the ratio NO x to VOC is more important than NO x concentrations to regulate O 3 (Akimoto and Tanimoto 2022 ; Wang et al 2022 ; Zhang et al 2022 ). An important point that needs attention is the VOC emissions from vegetation, since a considerable part of the VOC is contributed from vegetation, with plant species showing a wide variability in the size and composition of emissions, even among species of the same family (Richards et al 2013 ; Sicard et al 2018 ; Fu 2022 ; Masui et al 2022 ).…”

Toxicological Effects of Secondary Air Pollutants

“…Secondary pollutants are the main hazardous substances in photochemical smog, and their formation heavily relies on gasphase reactions. The transformation of nitrogenous species plays a central role because the photolysis of NO2 is the main source of ozone in the troposphere and the high chemical activity of reactive nitrogen [70][71][72] . The ozone reaction with NO2 can generate nitrate radical(NO3) and N2O5, which can further generate nitric acid and nitrate.…”

“…The ozone reaction with NO2 can generate nitrate radical(NO3) and N2O5, which can further generate nitric acid and nitrate. Conversely, nitric acid and nitrate photolysis can produce NO, NO2, HONO, and so on [72][73][74] . In addition, NO and NO2 can react to form HONO, which is an important OH radical precursor.…”

Effects of SO2 on NH4NO3 Photolysis: The Role of Reducibility and Acidic Products

“…Nitrate, one of the major constituents of PM 2.5 , , has surpassed sulfate to become the main component of secondary inorganic aerosol in China. , The formation of nitrate includes oxidation and neutralization processes. First, gaseous HNO 3 is formed from nitrogen oxides (NOx = NO + NO 2 ) through gas-phase oxidation by OH radicals and O 3 − and heterogeneous hydrolysis of N 2 O 5 . , Then, HNO 3 is neutralized by NH 3 to form NH 4 NO 3 . , Absorbed nitrate is also formed through the transformation of nitrogen oxides absorbed on the reactive surface of particles. − On the other hand, nitrate is not only a main sink of NOx but also can be converted into NOx and HONO by photochemistry (–), , which is called renoxification. , The NOx and HONO released into the atmosphere by nitrate photolysis can further promote the formation of O 3 and OH radicals. − The photolysis of nitrate may make important contributions to the atmospheric oxidation capacity. − NO 3 − + h ν → false[ NO 3 − false] * false[ NO 3 − false] * → NO 2 + normalO − NO 2 + h ν → NO + normalO ( P 3…”