Abstract
Abstract. The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous NO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether SO2 directly takes part in the HONO production process and what roles NH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with 4< pH <6) and haze episodes under high relative humidity (pH ≈4) – that NH3 was the key factor that promoted the hydrolysis of NO2, leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions. The results also suggest that SO2 plays a minor or insignificant role in HONO formation during fog and haze events, but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms. Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation, further promoting HONO production as a positive feedback. For future photochemical and aerosol pollution abatement, it is crucial to introduce effective NH3 emission control measures, as NH3-promoted NO2 hydrolysis is a large daytime HONO source, releasing large amounts of OH radicals upon photolysis, which will contribute largely to both atmospheric photochemistry and secondary aerosol formation.
Highlights
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)
To further investigate the acceleration effect of NH3 on the hydrolysis of NO2, we examined the correlations between the NO2∗-to-HONO (HONO/NO2∗ ratio), the NO∗2-to-NO−3 (NO−3 /NO∗2 ratio) conversion efficiencies and the NH3 concentration during the entire field campaign (Fig. 8)
Explosive HONO growth was observed for the first time on the North China Plain (NCP) during fog and haze episodes with high-RH conditions, only occurring with evident increases in NH3, indicating that NH3 is the key factor promoting the hydrolysis of NO2, resulting in rapid HONO and nitrate formation
Summary
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). 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 HSO−3 (aq) to sulfate. The stoichiometry of this mechanism is as follows (Cheng et al, 2016; Wang et al, 2016): 2NO2(aq) + HSO−3 (aq) + H2O(l) → 2H+ + HSO−4 (aq) + 2NO−2 (aq). 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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