Nitrous acid (HONO) plays a vital role in atmospheric chemistry due to the fact that its photolysis is a major source (Michoud et al., 2014; Kleffmann et al., 2005) of hydroxyl radical (OH) which determines the atmospheric oxidative capacity and plays crucial role in tropospheric chemistry in processes such as ozone formation, the degradation of volatile organic compounds and secondary aerosol formation (Cheng et al., 2016; Wang et al., 2016). Hence, the source study of nitrous acid (HONO) is of crucial importance for the understanding of the tropospheric chemistry, for chemistry and climate modeling, and for developing effective pollution control strategies (Lu et al., 2018).
The North China Plain (NCP) is troubled by persistent complex air pollution with high loadings of both photochemical pollutants and particulate pollution (Zheng et al., 2015; Ran et al., 2011); the simultaneous mitigation of these two types of pollution has encountered trouble due to the nonlinear dependence of ozone on NOx (Xing et al., 2018). Unknown daytime sources of HONO have gained attention over the past few years (Michoud et al., 2014; Liu et al., 2014; Su et al., 2011), and results from a recent study indicate that an additional missing source is required to explain more than 50 % of the observed HONO concentration during daytime in western China (Huang et al., 2017). Results from several recent studies have demonstrated that intense heterogeneous conversion of NO2 to HONO on particle surfaces might be a significant source of HONO (Cui et al., 2018; Liu et al., 2014).
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Two main HONO heterogeneous production pathways involving aerosol water and NO2 have been proposed. In light of the drastic decrease in solar radiation during severe haze events and rich ammonia conditions on the NCP, the first pathway hypothesized that NO2 (g) dissolved in aerosol water at aerosol pH >5.5 rapidly formed HONO while oxidizing HSO3- (aq) to sulfate. The stoichiometry of this mechanism is as follows (Cheng et al., 2016; Wang et al., 2016):
Based on this mechanism, good agreement between modeled and observed sulfate formation rates were achieved. However, the assumption that the pH of ambient aerosols can reach beyond 5.5 is debatable. Results from several recent studies indicate that the pH values of ambient aerosols fall in the range of 3-5 in most cases (Ding et al., 2019; Liu et al., 2017a; Song et al., 2018). Therefore, it was proposed that HONO and NO2- were produced during the hydrolysis process of NO2, releasing OH radicals upon photolysis, which indirectly oxidize SO2 to sulfate (Li et al., 2018b):
The results of Yabushita et al. (2009) suggest that anions (such as Cl−, Br− and I−) greatly enhance the hydrolysis of NO2 on water, and that the NO2 uptake coefficients of Reaction (R2) can be enhanced several orders of magnitude by increasing the electrolyte concentration. The ambient aerosol particles in the boundary layer are in the aqueous phase under high RH (Liu et al., 2017b), and the aerosol or fog water is not pure with different dissolved anions (Wu et al., 2018; Lu et al., 2010). Therefore, HONO and nitrate formed through this mechanism should be independent of aerosol acidity, and should be primarily affected by the aerosol surface area density (SA), aerosol liquid water content and NO2 concentration (Li et al., 2018b). Moreover, recent theoretical simulations have proposed a HONO formation mechanism involving NO2 and water and have identified that NH3 can promote the hydrolysis of NO2 (Li et al., 2018a; Reaction R2 in this paper). Despite this, no direct evidence from field observations was available to support their findings.
Although the proposed HONO formation mechanisms are all heterogeneous reactions of NO2, the details regarding how SO2, pH and NH3 are involved in heterogeneous formation are still under debate (Li et al., 2018b). A clear mechanism is also still missing in current models to explain both the daytime concentration of observed HONO and the secondary inorganic aerosol formation. Measurements of HONO are rare and simultaneous observations of HONO and aerosol physical and chemical characteristics are lacking to thoroughly analyze or directly support the aerosol heterogeneous HONO formation mechanisms involving NO2. In this paper, we present simultaneous measurements of HONO, sulfate and nitrate as well as other precursor gases, oxidants and meteorological parameters during both fog and haze episodes under high ambient RH. Fog water pH is usually greater than 5.5 in eastern China (Safai et al., 2008; Lu et al., 2010), although calculations in this work and previous studies collectively indicate a moderately acidic condition (4< pH <5) for fine particles in northern China winter haze. The observational results reveal that NH3 is the key factor that promotes the hydrolysis of NO2, resulting in explosive formation of HONO, nitrate and sulfate.
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