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Title: Reactive nitrogen oxides (HONO, N₂O₅ and ClNO₂) in different atmospheric environment in China : concentrations, formation and the impact on atmospheric oxidation capacity
Authors: Yun, Hui
Degree: Ph.D.
Issue Date: 2018
Abstract: At Mong Kok, the average daytime HONO mixing ratio was 3.91 ppbv, which was much higher than that at night. OBM simulations constrained by the observed HONO showed that the maximum concentrations of OH, HO₂, and RO₂ reached 4.65×10⁶, 4.40×10⁶, and 1.83×10⁶ molecules cmˉ³, which were 7.9, 5.0, and 7.5 times, respectively, the results in the case without HONO constrained. Photolysis of HONO contributed to 86.5% of the total primary radical production rates and led to efficient NO₂ and O₃ production under the condition of weak regional transport of O₃. The formation of HNO₃ contributed to 98.4% of the total radical termination rates. The results suggest that HONO could significantly increase the atmospheric oxidation capacity (AOC) in a street canyon and enhance the secondary formation of HNO₃ and HCHO, which can damage outdoor building materials and pose health risks to pedestrians. At Heshan, high HONO concentration of 1.1 ppbv around noon time was observed together with high HONO/NO₂ ratio of 6.5%. HONO photolysis was the dominant primary radical source, and accounted for 43.7%-64.1% of the total production rates of radicals. HONO also governed the in-situ production of O₃ in this environment. The daytime sources of HONO seemed complex which was probably related to NO₂ heterogeneous reaction with water, photo-enhanced reactions on surfaces and photolysis of adsorbed HNO3/nitrate. Considering the importance of HONO in both the street canyon environment and the regional polluted environment, more studies about the HONO sources are suggested. In the previous study of TMS campaign, high ClNO₂ of 4.7 ppbv was observed in an aged air mass from PRD, which caused 16% of O₃ enhancement at the ozone peak and Cl chemsitry contributed to 41% of the produced O₃ in the following day. In this study, as the aging of "young" air masses captured just after sunset, the predicted ClNO₂ could increase up to 6 ppbv at sunrise, indicating more frequent ClNO₂ peaks in the downwind open sea. A significant portion of the NOx in the air masses (70% ± 10%) was removed during the nighttime via N₂O₅ chemistry. At Wangdu, morning peaks of ClNO₂ up to 550 pptv and 2070 pptv were observed in the average case and the highest case, respectively. Cl reactions with VOCs contributed to 3% and 9% of the average daytime ROx budget, and contributed to 10% and 30% in the morning (8:00-8:30). Integrated ozone concentrations also increased 3.3% (4 ppbv) and 13.3% (10 ppbv) for the average case and the highest case, respectively. At Heshan, the maximum N₂O₅ and ClNO₂ concentrations reached up to 3.4 ppbv and 8.3 ppbv (1 min time resolution) in a severe pollution process. The amount of aerosol nitrate formed during the nighttime was 39.7 to 77.3 μg mˉ³ during the severe haze event, which was comparable to the nitrate formed through OH+NO₂ during the daytime. Morning peaks of 3.5 ppbv ClNO₂ led to 7% increase of in-situ O₃ production. This thesis indicates the importance of N₂O₅ chemistry to aerosol nitrate formation at near-ground level, and to atmospheric oxidation capacity (AOC) in the upper boundary layer, and the significant contribution of HONO to AOC in street canyons and regional pollution episodes in China.
Reactive nitrogen oxides play an important role in the formation of both PM₂.₅ and O₃. HONO photolysis is an important source of OH radical which initializes photochemical reactions and leads to the production of O₃ and other secondary products during the daytime. The heterogeneous uptake of N2O5 on aerosol surfaces at night is an important source of aerosol nitrate and simultaneously releases ClNO₂, which is a dominant source of Cl radical in the early morning via photolysis. Cl radical reacts with volatile organic compounds (VOCs) much faster than OH radical. Therefore, reactive nitrogen chemistry has significant impact on atmospheric oxidation capacity (AOC). There is still limited research about the N₂O₅ and ClNO₂ chemistry in China. For HONO, the daytime missing source has long been a concern for scientists, and its level and impact in severe pollution conditions, like in a street canyon microenvironment and in a typical pollution episode, are still seldomly reported. For this thesis, datasets observed by our group from four separate field campaigns were analyzed. HONO was measured in a street canyon at Mong Kok in urban Hong Kong and at a semi-rural site at Heshan in the western Pearl River Delta (PRD). N₂O₅ and ClNO₂ were observed at a high-elevation site (Mt. Tai Mo Shan, 957 m a.s.l.) in Hong Kong, at a rural site at Wangdu in North China Plain (NCP), and at Heshan in PRD. An observation-based box model (OBM) built on the Master Chemical Mechanism (MCM v3.3.1) and an updated Cl chemistry module was developed to investigate the contribution of HONO and ClNO₂ to the formation of radicals and secondary products (e.g. HNO₃, OVOCs and O₃). An iterative box model was also employed to investigate the contribution of N₂O₅ chemistry to NOx loss and ClNO₂ accumulation over the entire night in the TMS campaign. At Heshan, the formation processes of N₂O₅ and ClNO₂ and the impact on nitrate formation were also investigated. The two sites for HONO can represent a highly-polluted urban environment (Mong Kok) and a regional polluted environment (Heshan), respectively. And the sites for N₂O₅/ClNO₂ can represent the environments in the upper boundary layer (TMS) and in rural areas near ground surface (Wangdu and Heshan). This thesis focuses on the characteristics and impacts of reactive nitrogen chemistry in different ambient environments in China.
Subjects: Hong Kong Polytechnic University -- Dissertations
Nitrogen oxides -- Environmental aspects -- China
Nitrogen oxides -- Environmental aspects -- China -- Hong Kong
Atmospheric chemistry
Pages: xx, 190 pages : color illustrations
Appears in Collections:Thesis

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