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.
Highlights
50 In the past two decades, atmospheric nitrous acid (HONO) has attracted numerous laboratory experiments and field campaigns because of its significant contribution to the atmospheric concentration 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 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
Summary
50 In the past two decades, atmospheric nitrous acid (HONO) has attracted numerous laboratory experiments and field campaigns because of its significant contribution to the atmospheric concentration 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 A recent laboratory flow tube study (Wang et al, 2021) revealed that the EF was lower than 1 in 85 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. Atmospheric measurements at the foot (~150 m a.s.l.) and the summit (~1534 m a.s.l.) of Mt. Tai 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 100 aerosol-derived sources as the ground sources might be less effective; 3) the oxidizing capacity of the upper boundary layer and its contributors. To compare pollutants between the foot and the summit levels during the same period (Section 3.2.2), measurements (only hourly CO, NO2, PM2.5, PM10, O3, and SO2 were available) from the monitoring station (~200 m east to the foot station) were used
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