Air Pollution's Chlorine And Bromine: A Toxic Mix

Air pollution is a pressing issue that affects many regions worldwide, and among the various pollutants, halogen atoms, including chlorine and bromine, play a significant role in degrading air quality. Halogens are a group of chemical elements that encompass fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). While natural sources, particularly the sea, contribute to the presence of these elements in the atmosphere, anthropogenic activities, such as coal combustion and industrial emissions, have led to excessive levels of chlorine and bromine in the air, causing a range of environmental and health issues. This topic explores the sources, chemical behaviours, and impacts of chlorine and bromine pollution, aiming to shed light on their role in air pollution and potential strategies for mitigation.

Coal burning in China

China is the world's largest manufacturer and user of solar panels and wind turbines. It is also a leader in producing energy from hydroelectric dams and is building more nuclear power plants than any other country. However, China also burns more coal than the rest of the world combined. Coal burning in China has accelerated mining and the construction of coal-fired power plants, driving up the country's emissions of energy-related greenhouse gases.

A study by Xiang Peng et al. observed unprecedented levels (averaging 60 parts per trillion) of bromine chloride (BrCl) at a mid-latitude site in North China during winter. The primary source of BrCl and other bromine gases was found to be widespread coal burning in rural households and a photo-assisted process. Reactive halogen atoms (chlorine and bromine) were found to significantly impact air quality in continental regions where uncontrolled coal combustion is prevalent.

Bromine and chlorine are both part of the halogen element group. They have similar chemical and geochemical properties, but distinct differences in their association, behaviour, and occurrence in coal have been observed. Chlorine is an impurity in minerals and inorganic phases such as clay minerals, mica, and feldspars, while bromine has a characteristic association with illite and, to a lesser extent, with mica and kaolinite.

The presence of chlorine and bromine in coal may occur in various forms, including organic compounds, impurity components in crystalline and amorphous inorganic constituents, fluid constituents, and discrete minerals. The behaviour of these elements during coal burning can have significant environmental impacts, influencing the atmospheric chemical composition and affecting the budget of ozone and the fate of pollutants such as hydrocarbons and mercury.

China's coal consumption and the resulting emissions have raised concerns about their impact on climate change. David Sandalow, a senior energy official in the Obama and Clinton administrations, emphasized that reducing China's coal combustion is essential for addressing global climate challenges. The country's future decisions regarding the operation of its coal-fired plants will be crucial in mitigating its environmental footprint.

Magnesium refinery in Utah

Air pollution containing chlorine and bromine has been linked to a magnesium refinery in Utah. The US Magnesium refinery, located on the Great Salt Lake, has been identified as a significant contributor to the winter "brown clouds" or inversions observed over northern Utah, particularly in Salt Lake City. The refinery's emissions of chlorine and bromine, known as halogenated compounds, have been shown to account for a substantial portion of the area's particulate matter, specifically PM 2.5, during pollution episodes. These microscopic solids or liquid droplets in the air can pose serious health risks if inhaled.

US Magnesium, the only such refinery in the country, is the nation's largest chlorine emitter, according to regulatory records. While the company has emphasized its commitment to environmental responsibility and reducing its environmental footprint, the study's lead researcher, Carrie (or Caroline) Womack, noted that emissions of chlorine have not significantly decreased over the last five years. During research flights near the plant, scientists encountered air that was noticeably different due to the high levels of chlorine emissions, describing it as having a bleach-like odour.

The magnesium production process at the refinery involves extracting metal from the brine of the Great Salt Lake. This process results in the release of chlorine and bromine compounds, which contribute to air pollution in the region. The study by the Cooperative Institute for Research in Environmental Sciences, based on data from a period of winter air pollution in 2017, revealed that the refinery's emissions were responsible for 10-25% of the particulate matter in the area during these episodes.

The findings have sparked discussions about the role of industry in Utah's air quality, particularly in the Salt Lake Valley, where winter pollution levels of PM 2.5 exceed national air quality standards on average for 18 days per year. The state's response to the issue indicates a balance between economic considerations and environmental goals. While initially supporting the refinery project for economic and national security reasons, Utah's public lands office later retracted its support following public backlash, demonstrating a growing emphasis on environmental concerns.

Ozone depletion

The ozone layer in the Earth's stratosphere plays a critical role in absorbing ultraviolet radiation from the sun. In recent decades, it has been discovered that the ozone layer is depleting due to anthropogenic pollutants. One of the most significant causes of ozone depletion is the release of halogen radicals, specifically chlorine and bromine. These halogen atoms can catalytically destroy ozone molecules, leading to a reduction in the ozone layer's ability to protect us from harmful UV-B radiation, which can cause skin cancer and genetic damage.

Bromine, with the chemical symbol Br, is a rare element that accumulates in the oceans due to the high solubility of the bromide ion. While free bromine does not occur naturally, it can be found as soluble crystalline mineral halide salts. Bromine has various industrial applications, including its use as a fire retardant due to its ability to terminate free-radical chemical chain reactions. However, one of its unwanted side effects is its contribution to ozone depletion.

At high temperatures, organobromine compounds can convert to free bromine atoms, which are highly effective in depleting the ozone layer. This discovery has led to the discontinuation of many common volatile brominated organics, such as the pesticide methyl bromide. The formation of free bromine atoms in the atmosphere can occur through the action of sunlight on volatile organobromine compounds.

Chlorine, on the other hand, is also a potent catalyst for ozone depletion. Chlorofluorocarbons (CFCs), when exposed to high-energy photons in the stratosphere, can release chlorine radicals. These chlorine radicals then catalytically react with ozone molecules, destroying them and contributing to the depletion of the ozone layer. The use of CFCs and the excessive burning of fossil fuels are significant anthropogenic factors in ozone depletion.

The impact of halogen atoms on ozone depletion is not limited to bromine and chlorine. Other halogens, such as bromine chloride (BrCl), have also been found to influence ozone levels. Uncontrolled coal combustion, particularly in regions like North China, has led to the release of unprecedented levels of BrCl, further contributing to the deterioration of air quality and the depletion of the ozone layer. Understanding the sources and significance of these pollutants in continental regions is crucial for mitigating their impact on the environment and human health.

Hydrocarbons and mercury

Bromine and chlorine are both halogens, and they play a significant role in air pollution. They are particularly prevalent in the wintertime air quality of continental regions, where uncontrolled coal combustion is common.

In a study conducted in the North China Plain (NCP), high levels of bromine chloride (BrCl) were observed. This is a reactive halogen compound produced from coal burning and other photochemical reactions. The presence of BrCl and other halogens was found to alter ozone production, hydrocarbon oxidation, and the conversion of elemental mercury to a soluble form in the atmosphere. This has a significant impact on air quality, particularly in densely populated regions with high levels of industrial and agricultural activity.

Halogen atoms, including bromine and chlorine, affect the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Their sources and significance in polluted continental regions are not yet fully understood. However, it is known that bromine atoms play a central role in atmospheric reactive halogen chemistry, depleting ozone and elemental mercury. This enhances the deposition of toxic mercury into the ecosystem, particularly in the Arctic near-surface troposphere.

The oxidation of mercury is influenced by the presence of bromine atoms in the mid-latitude atmosphere. This process is known as bromine-induced oxidation and has been observed in regions with high levels of anthropogenic emissions, such as China.

Additionally, organobromine compounds, which are easily converted to free bromine atoms at high temperatures, contribute to ozone depletion. While these compounds have industrial uses, their role in ozone depletion has led to a decrease in their use in certain applications, such as pesticides.

Wintertime air quality

The presence of these halogen atoms has a detrimental effect on wintertime air quality. They increase the abundance of conventional tropospheric oxidants (OH, HO2, and RO2) by 26-73% and enhance the oxidation of hydrocarbons by nearly a factor of two. This, in turn, leads to a 55% increase in net ozone production. The impact of this process is particularly evident in continental regions with high levels of uncontrolled coal combustion, such as North China.

The oxidation of mercury is another critical aspect of wintertime air quality. Bromine-induced oxidation of mercury in the mid-latitude atmosphere has been observed, leading to the depletion of ozone. High levels of molecular chlorine in the Arctic atmosphere have also been detected, further contributing to ozone depletion. The reactions of gaseous mercury with atomic and molecular halogens have been studied, and the kinetics and atmospheric implications are being explored.

Additionally, the presence of reactive bromine and chlorine can modify the oxidative capacity by influencing the levels of the hydroxyl radical (OH) and hydroperoxyl radical (HO2). They can also remove methane, a climate-forcing agent. The impact of these halogen atoms on atmospheric chemistry, especially in continental regions, is a growing area of interest. The existence of anthropogenic chloride sources that can form Cl atoms has shifted the focus from previous studies primarily concerned with the stratosphere and the marine boundary layer.

In conclusion, wintertime air quality is significantly impacted by the presence of reactive bromine and chlorine, which affect ozone levels, mercury oxidation, and the oxidation capacity of the atmosphere. The sources and significance of these halogen atoms in polluted continental regions are still not fully understood, and further research is needed to address this complex issue.

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