Author: Peng, Xiang
Title: Reactive bromine and chlorine chemistry in the polluted region
Advisors: Wang, Tao (CEE)
Degree: Ph.D.
Year: 2021
Award: FCE Awards for Outstanding PhD Theses (2021/22)
PolyU PhD Thesis Award - Outstanding Award (2022)
Subject: Chlorine -- Environmental aspects
Bromine -- Environmental aspects
Air -- Pollution
Environmental chemistry
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxxvi, 245 pages : color illustrations
Language: English
Abstract: Halogen atoms (Cl and Br), which can be produced from the photolysis of reactive halogen species (RHS), are highly reactive and can influence the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Most of the earlier halogen studies focused on the stratosphere and the polar region. In the recent decade, there has been growing interest in Cl radicals' effect on atmospheric chemistry over polluted areas, and these studies focused on ClNO2. Thus, litter is known about the abundance, sources, and roles of reactive bromine and other reactive chlorine (apart from ClNO2) compounds in the polluted troposphere. This study presents a detailed study of reactive bromine and chlorine chemistry in the polluted region. Five field measurements of RHS (including Br2, Cl2, BrCl, and HOBr) by using a quadrupole chemical ionization mass spectrometer (Q-CIMS) were conducted at different locations in China, including a rural site (Wangdu), an urban site (Beijing), a mountain station (Mt. Tai), a suburban site (Nanjing), and a coastal site (Hong Kong). In addition to the in-depth field data analysis, the laboratory studies were undertaken to further investigate the unknown molecular chlorine (Cl2) sources, and a photochemical box model that includes up-to-date Cl and Br chemistry was also used to calculate the level of Br and Cl radicals, to analyze the budget of Br and Cl radicals, and to estimate their impact on volatile organic compounds (VOCs) oxidation, ROx budget, and ozone production. During a winter field study in the North China Plain (NCP) during 9-31 December of 2017, unprecedented levels (averaging to 60 parts per trillion) of bromine chloride (BrCl) were observed at a highly polluted continental polluted site (Wangdu of Hebei Province). This is the first observation of elevated levels of BrCl at the ground level outside the polar region. Widespread coal burning in rural households and photochemical reactions are responsible for the observed BrCl levels and other reactive bromine gases (Brx), including Br2 and HOBr. The photolysis of BrCl was a substantial source (about 55%) of both bromine (Br) and chlorine (Cl) atoms. Through the model simulation, the halogen atoms increased the abundance of 'conventional' tropospheric radicals (OH, HO2, and RO2) by 26-73% and the net ozone production by 55%, and enhanced oxidation of hydrocarbon by nearly a factor of two. This reveals the significant role of reactive halogen in winter atmospheric chemistry and deterioration of air quality in continental regions where uncontrolled coal combustion is prevalent. Subsequently, during another two field campaigns in the NCP within the periods of typically domestic coal burning for heating, elevated levels of reactive bromine gases, coinciding with the coal burning tracer (SO2), were observed in Beijing during 6-31 January of 2018 and at the summit of Mt. Tai from 7 March-4 April of 2018. The concentrations of Brx (= BrCl + HOBr + 2√óBr2) in Beijing (up to 120 pptv) and Mt. Tai (up to 200 pptv) were lower than that observed in the Wangdu campaign (up to 700 pptv), as there were fewer uncontrolled coal burning sources in Beijing and Mt. Tai was far away from the emission source. BrCl was the dominant source of both bromine (Br) and chlorine (Cl) atoms in Beijing and Mt Tai through the radical budget analysis. In addition, reactive bromine species, Brx, were observed below the detection limit after the domestic heating periods in Nanjing during 11-26 April of 2018 and in Hong Kong during 31 August-9 October of 2018. These results strongly suggest that a significant fraction of tropospheric bromine and chlorine atoms may arise from widespread coal burning activities, which is not yet recognized by the research community.
In addition to high reactive bromine species, unexpected high levels of daytime molecular chlorine (Cl2) were observed both at three continental polluted sites (Wangdu, Beijing, Mt Tai) and in a coastal area (Hong Kong). Especially during a photochemical season field study, the highest level of daytime Cl2 (up to 1 ppbv) ever reported was observed during onshore wind flow in a coastal area in Hong Kong during 31 August­9 October of 2018. In this study, the average diurnal Cl2 mixing ratios showed a maximum around noon at ~400 pptv in polluted air with trajectories original from the continental region, while the Cl2 level was very low (around 10 pptv) in clean air with open ocean back trajectories, indicating there was significant unknown photochemistry involving the chloride activation process in the polluted air masses. Given the short lifetime of Cl2 (~7 minutes at noon), the observation indicated the presence of a large daytime unknown production rate of Cl2 (up to 2.7 pptv/s). The production rate of Cl radical that was released from the photolysis of Cl2 was even larger than the primary OH radical source from the photolysis of ozone and HONO in the polluted air masses, highlighting the significant role of daytime Cl2. Field evidence reveals that photolysis of nitrate containing particles may lead to large chloride activation and Cl2 production at our site. Upon photolysis, the Cl2 increased the oxidation rate of atmospheric VOCs by 24-132%, ROx by 4-27%, and daytime O3 integrated production by 17%. As current air quality models lack the source/mechanism to account for such high daytime levels of Cl2, the role of Cl2 in atmospheric chemistry and air pollution has been severely under-estimated in these models. To pin down the photochemical mechanism for the observed Cl2 production in the Hong Kong campaign, a series of dynamic chamber experiments were undertaken by illuminating nitrate and chloride containing solutions and ambient aerosols in the presence of gaseous O3 with a high-pressure xenon lamp. Significant Cl2 production was observed when acidic solutions (pH <3.33) containing both chloride and nitrate were illuminated, while no detectable amount of Cl2 was observed if we added O3 in our chamber. The yields of Cl2 were strongly dependent on the acidity in solution (pH in the range of 1.89-3.33) and the irradiation wavelength <360nm, but it was suppressed with the use of hydroxyl radical (OH) scavenger and nitrite. These results suggest that the oxidation of chloride is facilitated by a photochemical mechanism of the aqueous OH that is initiated by nitrate photolysis. Similar bromide chemistry should also occur to generate Br2 or BrCl. Indeed, both Br2 and BrCl were observed, as expected, in our laboratory system. To further confirm the Cl2 formation under ambient aerosols, four aerosols samples collected (for 24-hour duration each) at the same measurement site in Hong Kong during 11-13 October of 2020 were irradiated in the dynamic chamber. Different levels of Cl2 production were observed along with HONO. Additionally, the generation of chlorine depended on the irradiation wavelength < 360nm, which further proved that particulate nitrate's photolysis played an essential role in the Cl2 generation. Thus, field and laboratory results reveal that photolysis of particulate nitrate under acidic conditions can activate chloride and could account for 13%-68% of the observed daytime Cl2 in the 2018 Hong Kong campaign. Given the broad presence of chloride, nitrate, and acidic aerosols, we propose that the photolysis of particulate nitrate is a significant daytime chlorine source globally in the polluted region and thus has profound impacts on tropospheric oxidation chemistry and production of ozone and secondary aerosols. This study also reveals a new pathway of the nitrogen cycle in boosting atmospheric oxidation. During the laboratory studies of the molecular chlorine activation, nitryl chloride, ClNO2, has also been observed when liquid solutions containing both chloride and nitrate were illuminated by a high-pressure xenon lamp. Furthermore, the production of ClNO2 was increased with the nitrate concentrations, acidity (pH in the range of 1.89-6.87), and the surface area. This laboratory evidence suggests that nitrate photolysis could be a new ClNO2 activation mechanism during the daytime in addition to the previously known heterogeneous reaction of N2O5 during nighttime. Recently, several field measurements reported the early morning or daytime peak of ClNO2 when N2O5 remained low or was below the detection limit, which was discrepant from the previously observed typical diurnal pattern. Thus, this study may explain the daytime ClNO2 observed in field measurement, providing a new daytime source of Cl radicals. Overall, this study investigates the abundance, sources, and impact of reactive bromine and chlorine chemistry in the polluted region in China. This study suggested that widespread coal burning activity is an important anthropogenic source for reactive bromine species. Furthermore, HOBr, as a reactive bromine species, can activate particulate chloride into BrCl, which can contribute to large bromine and chlorine atoms in the polluted region. The new chloride activation mechanisms for Cl2 and ClNO2 from the nitrate photolysis, suggesting a significant daytime chlorine source globally which can increase the atmospheric oxidation capacity. It is of great interest to further investigate the kinetics and detailed mechanisms of these reactive chlorine activation processes in the polluted region. This study indicates that reactive bromine and chlorine may have larger roles in the chemistry and air quality of the polluted environment than previously thought.
Rights: All rights reserved
Access: open access

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