Abstract
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 (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 evidence of air mass transport from the ground to the summit level was 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 at the summit station. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO + OH), depending on the chemical kinetic parameters used. Coupled with sensitivity tests on the kinetic parameters used, 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 station, an amount of field evidence was proposed to support the finding that the remaining majority (70 %–98 %) of Pun was dominated by the rapid vertical transport from the ground to the summit level and heterogeneous formation on the mountain surfaces during transport. HONO photolysis at the summit level initialized daytime photochemistry and still represented an essential OH source in the daytime, with a contribution of about one-quarter 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 regional chemistry transport models.
Highlights
In the past 2 decades, atmospheric nitrous acid (HONO) has attracted numerous laboratory experiments and field campaigns because of its significant contribution to the production of hydroxyl radicals (OH) and the incomplete understanding of its sources (Kleffmann, 2007)
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
Coupled with sensitivity tests on the kinetic parameters used, 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
Summary
In the past 2 decades, atmospheric nitrous acid (HONO) has attracted numerous laboratory experiments and field campaigns because of its significant contribution to the production of hydroxyl radicals (OH) and the incomplete understanding of its sources (Kleffmann, 2007). A recent chamber study (Ge et al, 2019) found a high dark NO2 uptake coefficient (2.0 × 10−5 to 1.7 × 10−4) on NaCl particles under high RH (90 %), NH3 (50–2000 ppbv), and SO2 (600 ppbv) conditions If such a high NO2 coefficient on the aerosol surface was applied in nighttime HONO budget analysis, the dominant role of NO2 uptake on the ground surface in nighttime HONO formation, which was already generally accepted, might be challenged (Kleffmann, 2007; Kurtenbach et al, 2001; Stutz et al, 2002; Xue et al, 2020). A recent laboratory flow tube study (Wang et al, 2021) revealed that the EF was lower than 1 in the aqueous phase Another flow tube study (Laufs and Kleffmann, 2016) reported a slow HONO formation from secondary heterogeneous reactions of NO2 produced during HNO3 photolysis. Comprehensive measurements allow us to understand more about (1) the transport of ground-formed HONO and its role in the upper boundary layer, (2) HONO formation from the aerosolderived sources as the ground-derived sources might be less effective compared to measurements near to ground surface, and (3) the oxidizing capacity of the upper boundary layer and its contributors
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