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.
Highlights
Nitrous acid (HONO) is an important source of the hydroxyl radical (OH) through its photolysis (Reaction R1), and contributes ∼ 20 %–80 % of primary OH production (Alicke et al, 2002; Hendrick et al, 2014; Kim et al, 2014).hypothesized to be related to nitrous acid (HONO) + hv → nitric oxide (NO) + OH. (R1) 40 years have passed since the first detection of HONO in the atmosphere (Perner and Platt, 1979), the sources of HONO and the dynamic parameters of HONO formation mechanisms are still not well understood (Ge et al, 2021)
The simulated wind direction (WD) bias within 45◦ accounted for ∼ 56 %, and the bias within 90◦ accounted for ∼ 80 %, suggesting that the simulated WD captured the main observed WD
Three direct emission sources, the improved NO2 heterogeneous reactions on aerosol and ground surfaces, and particulate nitrate photolysis in the atmosphere were included in the WRF-Chem model to explore the key HONO sources producing O3 enhancements during typical autumn haze-aggravating processes with co-occurrence of high PM2.5 and O3 in North China Plain (NCP)
Summary
Nitrous acid (HONO) is an important source of the hydroxyl radical (OH) through its photolysis (Reaction R1), and contributes ∼ 20 %–80 % of primary OH production (Alicke et al, 2002; Hendrick et al, 2014; Kim et al, 2014).HONO + hv → NO + OH. (R1) 40 years have passed since the first detection of HONO in the atmosphere (Perner and Platt, 1979), the sources of HONO (especially daytime) and the dynamic parameters of HONO formation mechanisms are still not well understood (Ge et al, 2021). HONO sources can be generally classified into three categories, i.e., direct emissions and homogeneous and heterogeneous reactions. The reaction of nitric oxide (NO) with OH (Pagsberg et al, 1997; Stuhl and Niki, 1972) is usually thought to be the dominant homogeneous reaction and is significant during daytime, but may be neglected at night due to low OH concentrations, other minor homogeneous HONO sources including nucleation of NO2, H2O, and NH3 (Zhang and Tao, 2010), via the photolysis of orthonitrophenols (Bejan et al, 2006; Chen et al, 2021; Lee et al, 2016), via the electronically excited NO2 and H2O (Crowley and Carl, 1997; Dillon and Crowley, 2018; Li et al, 2008) and via HO2 qH2O + NO2 reaction (Li et al, 2015, 2014; Ye et al, 2015). 2021) and nitrate photolysis (Photnitrate) (Romer et al, 2018; Ye et al, 2016a, b; Zhou et al, 2003), and are usually considered the main contributors to HONO concentrations in the atmosphere
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