Greenhouse Gases: Air Pollution's Invisible Menace

Greenhouse gases are gases that trap heat in the atmosphere, causing global warming and climate change. They include carbon dioxide, methane, nitrous oxide, ozone, and water vapor. The release of these gases into the atmosphere is referred to as greenhouse gas emissions or climate pollution. Human activities, such as burning fossil fuels and deforestation, have significantly increased the volume of greenhouse gases in the atmosphere, leading to an enhanced greenhouse effect and global warming. This, in turn, contributes to air pollution, which has detrimental effects on human health and the planet.

Carbon dioxide from burning fossil fuels

Carbon dioxide (CO2) is a key greenhouse gas that drives global climate change. It is released into the Earth's atmosphere through the burning of fossil fuels, such as coal, oil, and natural gas. These fossil fuels are formed over millions of years from the burial of photosynthetic organisms. For example, plants on land form coal, while plankton in the oceans form oil and natural gas. As these organisms grow, they remove carbon dioxide from the atmosphere and the ocean, and their burial inhibits the movement of carbon through the carbon cycle. However, when these fossil materials are burned, the carbon is released back into the atmosphere as carbon dioxide at a much faster rate than it was buried, and faster than it can be removed by the carbon cycle.

The burning of fossil fuels is a major contributor to the increase in atmospheric carbon dioxide concentrations. In 1750, at the beginning of the industrial era, the concentration of carbon dioxide in the atmosphere was approximately 278 parts per million. By 2020, this number had risen to 417 parts per million and continues to increase at a rate of almost 3 parts per million per year. According to estimates from an international team of scientists, carbon dioxide emissions from fossil fuels rose again in 2023, reaching record levels. The early analysis for that year shows that emissions from fossil fuels rose by 1.1% compared to 2022 levels, resulting in a total of 36.8 billion metric tons of carbon dioxide emitted from fossil fuels alone.

Carbon dioxide, as a greenhouse gas, has far-ranging environmental and health effects. It contributes to global warming by trapping heat from the sun and preventing it from escaping into space. This phenomenon is known as the greenhouse effect. While greenhouse gases have kept the Earth's climate habitable for humans and millions of other species, the current levels of these gases are out of balance and threaten the survival of living things on the planet. The increase in greenhouse gases, including carbon dioxide, leads to higher concentrations in the atmosphere, which intensifies the greenhouse effect and further warms the planet.

In addition to carbon dioxide, the burning of fossil fuels also releases other greenhouse gases such as nitrous oxide (N2O) and methane (CH4). These gases have different global warming potentials, which measure how much energy the emissions of one ton of a gas absorb over a given period. Gases with higher global warming potentials absorb more energy and contribute more to warming the Earth. The net effect of burning fossil fuels is warming due to the long-lasting presence of greenhouse gases in the atmosphere compared to the shorter suspension time of cooling airborne particles.

Methane from landfills and agriculture

Greenhouse gases are gases that trap heat in the Earth's atmosphere, causing global warming and climate change. They include carbon dioxide, water vapour, methane, nitrous oxide, and ozone, among others. While carbon dioxide is the most prevalent and dangerous greenhouse gas, methane is far more potent and a major contributor to global warming.

Methane (CH4) is released from landfills, natural gas and petroleum industries, and agriculture. Landfills are one of the three main sources of human methane pollution, along with livestock and the oil and gas industry. The decomposition of organic waste such as food scraps, wood, and paper in landfills produces methane. The Environmental Integrity Project (EIP) reported that in 2021, over 1,100 municipal landfills in the United States emitted at least 3.7 million metric tons of methane, which had the same climate-warming impact as 66 million gasoline-powered vehicles driving for a year. The Sampson County Landfill in Roseboro, North Carolina, was the biggest polluter, releasing over ten times the average landfill's methane emissions.

The Environmental Protection Agency (EPA) has struggled to accurately quantify methane emissions from landfills. While the EPA has developed methods to estimate methane emissions, critics argue that these methods are outdated and result in significant under-reporting. For instance, the EPA's data on methane emissions from landfills are based on mathematical formulas rather than pollution monitoring or sampling. Aerial surveys have also detected large leaks at landfills, indicating potential gaps in the EPA's methodology.

To address these challenges, the EPA proposed revising its methodology and has reached an agreement with the EIP to review and revise its methods for reporting landfill methane emissions. The EIP recommends stronger regulations, such as requiring more landfills to have methane collection systems and improved operational standards to ensure the effectiveness of these systems in reducing pollution. Additionally, the EIP urges the EPA to adopt new technology for the direct measurement of methane emissions from landfills, including ground-based and aerial monitoring systems.

Agriculture is another significant source of methane emissions. In particular, methane is produced in the digestive systems of grazing animals. While methane molecules do not remain in the atmosphere as long as carbon dioxide molecules, reducing methane emissions can have an immediate impact on curbing climate change.

Fluorinated gases from air conditioning and refrigeration

Fluorinated gases (F-gases) are a group of synthetic gases that contain fluorine. They are powerful greenhouse gases with a strong global warming potential. F-gases are used in a variety of applications, including refrigeration, air conditioning, heat pumps, fire suppression, electronics, and the aerospace industry.

The most common F-gases are hydrofluorocarbons (HFCs), which contain hydrogen, fluorine, and carbon. HFCs are used in commercial and industrial refrigeration, air-conditioning systems, heat pump equipment, and blowing agents for foams, fire extinguishants, aerosol propellants, and solvents. HFC-134a (1,1,1,2-Tetrafluoroethane) has become the most abundant HFC in the Earth's atmosphere as of 2015.

F-gases are emitted in smaller quantities than other greenhouse gases, but they are potent and can trap substantially more heat than CO2. They have high Global Warming Potential (GWP) values, which measure their impact relative to one tonne of carbon dioxide (CO2) over a certain period (usually 100 years). The higher the GWP, the worse the consequences for the climate. For example, SF6 has a GWP of 24,300 over 100 years, making it 24,300 times more effective at trapping heat than CO2 during that period.

Emissions of F-gases can occur during production, transportation, storage, filling into products or equipment, leakage during their lifetime, or disposal. In the context of air conditioning and refrigeration, F-gases are used as refrigerants. An example is R134a, which has been used in mobile air conditioning systems. However, due to its high global warming potential, there has been a push to replace it with alternative refrigerants, such as 1234yf or CO2.

To address the potential global warming effects of F-gases, various regulations and international agreements have been put in place. The European Union, for instance, has passed legislation such as the F-gas Regulation and the Mobile Air Conditioning Directive to control the use of F-gases and reduce their emissions. The Kigali Amendment under the Montreal Protocol also mandates a phase-down of HFCs. These efforts reflect a growing recognition of the need to mitigate the impact of F-gases on climate change.

Nitrous oxide from industrial manufacturing

Greenhouse gases are gases that trap heat in the Earth's atmosphere, causing climate change. They have far-ranging environmental and health effects, contributing to respiratory disease from smog and air pollution, extreme weather, food supply disruptions, and increased wildfires.

Nitrous oxide (N2O) is one such greenhouse gas. In 2022, it accounted for 6% of all US greenhouse gas emissions from human activities. Human activities such as agriculture, fuel combustion, wastewater management, and industrial processes are increasing the amount of N2O in the atmosphere.

N2O is generated as a byproduct during the production of chemicals such as nitric acid, which is used to make synthetic commercial fertilizer, and in the production of adipic acid, which is used to make fibres, like nylon, and other synthetic products. It is also emitted from its use in other applications such as anaesthesia and semiconductor manufacturing.

In the industrial manufacturing of N2O, ammonium nitrate is decomposed in melters to give a mixture of nitrous oxide and superheated steam. This process also produces impurities, including ammonium nitrate fumes, nitrogen, and other oxides of nitrogen. The steam and impurities are then removed by "scrubbing" with water, caustic soda, and sulphuric acid in sequence. Nitrogen, present in traces, is removed by bleeding from the top of storage vessels where nitrous oxide is stored, after compression by a compressor. The molten ammonium nitrate flows into a combustion pot where it is decomposed—this reaction is highly sensitive and must be carefully controlled. If the temperature is allowed to rise above a certain point, N2O is not formed.

Water vapour and ground-level ozone

Water vapour is the world's most abundant greenhouse gas. While it is not directly emitted by human activity, human activities do influence its concentration in the atmosphere. Water vapour plays a crucial role in the greenhouse effect, absorbing and re-emitting infrared radiation, which warms the Earth's surface and the lower atmosphere. This contributes to global warming and subsequent climate change.

Ground-level ozone, on the other hand, is a harmful air pollutant that is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. These precursor gases are emitted from cars, power plants, industrial boilers, refineries, and other sources that burn fossil fuels. Ground-level ozone is a major component of smog and can trigger a variety of health problems, especially for vulnerable individuals such as children, the elderly, and people with lung diseases. It aggressively attacks lung tissue, leading to respiratory issues and potentially contributing to premature death.

Ozone can be "'good" or "bad" depending on its location in the atmosphere. Stratospheric ozone, found in the upper atmosphere, is beneficial as it forms a protective layer that shields us from the sun's harmful ultraviolet rays. In contrast, ground-level ozone is considered "bad" due to its negative impact on human health and the environment.

Both water vapour and ground-level ozone contribute to the overall pollution of the air and have far-reaching consequences. While water vapour is the most abundant greenhouse gas, ground-level ozone poses more immediate health risks, especially in urban areas with high levels of air pollution.

Frequently asked questions