Helena Joyce
As concerns about climate change continue to grow, the need to tackle rising carbon dioxide (CO2) levels in the atmosphere becomes increasingly urgent. Natural processes such as photosynthesis by plants, absorption by the oceans, and geological processes can help remove CO2 from the atmosphere. One of the most effective natural mechanisms for reducing particulate matter in the atmosphere is precipitation, which carries pollutants and particulates such as dust, pollen, and smoke particles away as it falls to the ground. This process, known as wet deposition, is crucial for maintaining air quality. In addition to natural processes, artificial carbon removal techniques, such as carbon capture and storage (CCS), direct air capture (DAC), and soil carbon sequestration, play a crucial role in mitigating climate change by reducing the concentration of CO2 in the atmosphere.
| Characteristics | Values |
|---|---|
| Natural process that removes pollutants from the atmosphere | Precipitation |
| How it works | When rain or snow falls from the atmosphere, it carries with it any particulate matter present in the air. This process is known as wet deposition. |
| Pollutants removed | Dust, pollen, smoke particles, and other pollutants |
| Natural carbon sinks | Forests, plants, and trees |
| How they work | As plants and trees grow, they store carbon in their biomass, including leaves, stems, and roots. Forests can store carbon in their woody tissues for several decades or even centuries. |
| Other natural processes | Photosynthesis, ocean absorption, and geological processes |
| Artificial techniques | Carbon capture and storage (CCS), afforestation, reforestation, direct air capture (DAC), ocean fertilization, and soil carbon sequestration |
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What You'll Learn
Photosynthesis by plants
Photosynthesis is a process by which plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into oxygen and energy in the form of sugar. This process is essential for life on Earth, as it provides most of the chemical energy that flows through the biosphere. Additionally, photosynthesis plays a crucial role in removing carbon dioxide, a greenhouse gas, from the atmosphere, thereby helping to mitigate climate change.
During photosynthesis, plants absorb carbon dioxide (CO2) from the air through tiny pores called stomata in their leaves. Within the plant cell, the water is oxidized, losing electrons, while the carbon dioxide is reduced, gaining electrons. This transformation converts water into oxygen and carbon dioxide into glucose. The plant then releases the oxygen back into the air and stores energy within the glucose molecules.
The oxygen produced by photosynthesis is vital for respiration, the biochemical process that generates energy from food for most organisms on Earth. Additionally, atmospheric oxygen from photosynthesis forms the ozone layer, which protects life on Earth from harmful high-energy ultraviolet (UV) radiation from the Sun.
The amount of organic matter produced by photosynthesizing organisms in an ecosystem is defined as the productivity of that ecosystem. The availability of water impacts the productivity and biomass of the ecosystem, influencing the rate at which water cycles through it.
While photosynthesis is a natural process, human activities such as burning fossil fuels, agricultural practices, and deforestation can affect it. Increasing carbon dioxide levels may enhance photosynthesis rates in some plants, but it can also make them less nutritious. Additionally, rising temperatures and changing precipitation patterns can impact plant growth and make certain species more susceptible to diseases.
To enhance the carbon removal potential of photosynthesis, various strategies can be employed, including reforestation, restocking, silvopasture, and cropland agroforestry. By expanding and restoring tree cover, we can increase carbon uptake and leverage the power of photosynthesis to mitigate climate change.
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Absorption by oceans
Oceans are one of the most valuable natural resources on Earth, covering over 70% of the planet. They play a crucial role in maintaining the health of the planet, from regulating weather patterns to providing a home for most of Earth's life forms. However, human activities have had a detrimental impact on ocean health, with pollution from a variety of sources threatening marine ecosystems.
One of the significant ways in which oceans absorb pollutants is through the process of carbon absorption. Oceans absorb approximately a quarter of all man-made carbon emissions, which may seem beneficial in mitigating climate change, but this absorption has severe ecological consequences. When the oceans absorb carbon dioxide (CO2), it reacts with seawater, reducing the pH level and leading to a process known as ocean acidification. The pace of this acidification is alarming; it is estimated that the pH level of the ocean surface has increased by 30% since the Industrial Revolution, and it could rise by nearly 150% by the end of the century if greenhouse gas emissions continue to increase. This change in the ocean's chemistry has far-reaching impacts on marine life and the coastal economies that depend on these ecosystems.
The oceans also absorb pollutants from the atmosphere through precipitation. When rain or snow falls, it carries with it particulate matter such as dust, pollen, and pollutants, trapping them in water droplets or snowflakes. This process, known as wet deposition, is crucial for maintaining air quality, especially in urban areas. As the droplets fall, they bring the pollutants down to the ocean's surface, contributing to the overall pollution load in the marine environment.
In addition to carbon emissions and particulate matter, the oceans also absorb a variety of other pollutants. Human activities along coastlines and inland contribute significantly to marine pollution. Nonpoint source pollution, for example, occurs due to runoff from sources such as septic tanks, vehicles, farms, and livestock ranches. Point source pollution, on the other hand, results from a single source, such as an oil or chemical spill. Marine debris, including plastic pollution, is another significant issue. Plastic waste, such as single-use plastic bags, bottles, and straws, can persist in the marine environment for centuries, entangling marine life, being ingested, and threatening their habitats.
The impact of human activities on ocean health is evident, and it is crucial to recognize the role oceans play in absorbing pollutants from the atmosphere. While this absorption can help mitigate the effects of pollution on the air we breathe, it comes at a cost to the health of marine ecosystems and the balance of the planet as a whole.
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Geological processes
The Earth has natural processes that remove carbon dioxide (CO2) from the atmosphere, and scientists are also developing innovative artificial techniques to assist in this crucial task. One of the natural processes that remove CO2 from the atmosphere is through geological processes.
Weathering
Weathering is a natural geological process in which atmospheric CO2 dissolved in rainwater attacks rocks and soils, partly dissolving them. The CO2 is converted into hydrogen carbonate ions (alkalinity) that are eventually captured in the ocean, where they are securely stored for more than 100,000 years. On a global scale, weathering removes about 1 Gt of CO2 every year and plays a key role in the long-term regulation of Earth's climate.
Carbon Mineralization
Carbon mineralization is another geological process that removes CO2 from the atmosphere. Some minerals naturally react with CO2, turning carbon dioxide from a gas into a solid and keeping it out of the atmosphere permanently. This process happens very slowly, over hundreds or thousands of years. Scientists are, however, finding ways to speed up the carbon mineralization process, such as by enhancing the exposure of these minerals to CO2 in the air or ocean.
Enhanced Rock Weathering
Enhanced rock weathering is a CO2-removal strategy that amends agricultural soils with crushed calcium- and magnesium-rich silicate rocks and harnesses the photosynthetic energy of the crops to increase weathering rates. The calcium, magnesium, and hydrogen carbonate ions produced by weathering these silicate minerals can subsequently precipitate as carbonate minerals.
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Carbon capture and storage
CCS facilities capture carbon dioxide before it enters the atmosphere. Generally, a chemical solvent or a porous solid material is used to separate the CO2 from other components of a plant’s exhaust stream. Most commonly, the gas stream passes through an amine solvent, which binds the CO2 molecule. This CO2-rich solvent is heated in a regeneration unit to release the CO2 from the solvent. The released solvents are recycled to capture more CO2 from the facility. The purified CO2 stream is then compressed and transported for storage or end-use. After the CO2 has been captured, it is usually compressed into a supercritical fluid and then injected underground.
In mineral storage, captured CO2 is reacted with naturally occurring iron (Fe), magnesium (Mg), and calcium (Ca) minerals. This is called ‘mineral carbonation’ and occurs naturally in the weathering of rock over time. Such minerals are very abundant and very stable. As a result, the re-release of CO2 into the atmosphere does not happen. However, these carbonation reactions are very slow under normal conditions, and to speed them up, energy is required to increase the temperature and pressure to ideal levels.
Bioenergy with carbon capture and storage (BECCS) is the process of extracting bioenergy from biomass and capturing and storing the CO2 that is produced. Under some conditions, BECCS can remove carbon dioxide from the atmosphere. One option is bioenergy with CCS (BECCS), where biomass (like wood or grasses) removes CO2 from the air through photosynthesis as it grows. The biomass is then harvested and burned in a power plant to produce energy, with the CO2 being captured and stored.
CCS could have a critical but limited role in reducing greenhouse gas emissions. However, other emission-reduction options such as solar and wind energy, electrification, and public transit are less expensive than CCS and are much more effective at reducing air pollution. Given its cost and limitations, CCS is envisioned to be most useful in specific niches, including heavy industry and plant retrofits.
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Wet deposition (precipitation)
Wet deposition, or precipitation, is a natural process that effectively removes pollutants from the atmosphere. It occurs when pollutants and particulate matter, such as dust, pollen, and smoke particles, are trapped in water droplets or snowflakes as rain or snow falls from the atmosphere. This process is crucial for maintaining air quality, especially in urban areas where particulate matter is more concentrated.
The process of wet deposition can be divided into two distinct mechanisms: rainout (in-cloud scavenging) and washout (below-cloud scavenging). Rainout refers to the removal of aerosol particles from the atmosphere by acting as cloud condensation nuclei (CCN) or ice nuclei (IN) at altitudes above the cloud base, leading to precipitation. Washout, on the other hand, occurs when aerosol particles collide and combine with falling hydrometers, such as rain, snow, or hail, at altitudes below the cloud base.
Wet deposition plays a significant role in reducing particulate matter and improving air quality. It is relatively easy to collect and evaluate compared to dry deposition, which is a slower and more continuous process influenced by wind turbulence and seasonal factors. However, the efficacy of wet deposition is influenced by particle size and the local availability of precipitation. For example, in regions with low rainfall, dry deposition may be more relevant for air purification over a year.
Wet deposition is also associated with acid rain, which occurs when sulfuric and nitric acids formed in the atmosphere mix with precipitation. These acidic components, resulting from the transformation of pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx), contribute to the acidity of rain, snow, fog, or hail. Acid deposition, including both wet and dry forms, can have harmful effects on soil, water bodies, forests, and other ecological systems.
Overall, wet deposition through precipitation is a crucial natural process for removing pollutants and particulate matter from the atmosphere, playing a vital role in maintaining and improving air quality, especially in urban areas with high concentrations of pollutants.
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Frequently asked questions
Natural processes that remove pollutants from the atmosphere include carbon sinks, photosynthesis by plants, absorption by the oceans, and geological processes. Trees are particularly good at filtering pollutants from the air.
Plants and trees absorb carbon dioxide and emit oxygen through photosynthesis. They also filter atmospheric pollutants like sulphur dioxide and nitrogen dioxide through their leaves.
Yes, precipitation is a natural mechanism that filters and reduces particulate matter in the air, improving air quality. For example, rain can wash away smoke particles from the air after a wildfire.
Silver birch, yew, elder, and conifer trees like pines and cypresses are effective at capturing particles due to the hairs on their leaves. Urban trees can also help reduce air pollution.
While oceans absorb carbon dioxide and capture excess heat, rising greenhouse gas emissions have led to the acidification of seawater. This is an example of a natural process attempting to mitigate climate change, but ultimately being harmed by it.
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