Atmospheric Measurements at the Foot and the Summit of Mt. Tai – Part I: HONO Formation and Its Role in the Oxidizing Capacity of the Upper Boundary Layer
Chaoyang XueCan YeJörg KleffmannChenglong ZhangValéry CatoireFengxia BaoAbdelwahid MelloukiLikun XueJianmin ChenKeding LuYong ZhaoHengde LiuZhaoxin GuoYujing Mu
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Abstract. A comprehensive field campaign, with measurements of HONO and related parameters, was conducted in summer 2018 at the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). At the summit station, high HONO mixing ratios were observed during this campaign (mean ± 1σ: 133 ± 106 pptv, maximum: 880 pptv), with a diurnal noontime peak (mean ± 1σ: 133 ± 72 pptv at 12:30 local time). Constraints on the kinetics of aerosol-derived HONO sources (NO2 uptake on the aerosol surface and particulate nitrate photolysis) were performed and discussed, which enables a better understanding of the interaction of HONO and aerosols, especially in the polluted North China Plain. Various shreds of evidence of air mass transport from the ground to the summit levels were provided. Furthermore, daytime HONO formation from different paths and its role in radical production were quantified and discussed. We found that the homogeneous reaction NO + OH could only explain 8.0 % of the daytime HONO formation, resulting in strong unknown sources (Pun). Campaigned-averaged Pun was about 290 ± 280 pptv h−1 with a maximum of about 1800 pptv h−1. Aerosol-derived HONO formation mechanisms were not the major sources of Pun. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO + OH), depending on the used chemical kinetics. Coupled with sensitivity tests on the used kinetics, the NO2 uptake on the aerosol surface and particulate nitrate photolysis contributed 1.5–19 % and 0.6–9.6 % of the observed Pun, respectively. Based on synchronous measurements at the foot and the summit stations, a bunch of field evidence was proposed to support that the remaining majority (70–98 %) of Pun was dominated by the rapid vertical transport from the ground to the summit levels and heterogeneous formation on the ground surfaces during the transport. HONO photolysis at the summit level initialized daytime photochemistry and represented an essential HOx (OH + HO2) source in the daytime, with a contribution of 26 %, more than one-third of that of O3. We provided evidence that ground-derived HONO played a significant role in the oxidizing capacity of the upper boundary layer through the enhanced vertical air mass exchange driven by mountain winds. The follow-up impacts should be considered in the regional chemistry-transport models.Keywords:
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