Wikipedia

Direct air capture

(Redirected from Direct carbon capture)

Direct air capture (DAC) is the use of chemical or physical processes to extract carbon dioxide directly from the ambient air.[1] If the extracted CO2 is then sequestered in safe long-term storage (called direct air carbon capture and sequestration (DACCS), the overall process will achieve carbon dioxide removal and be a "negative emissions technology" (NET).

Flow diagram of direct air capture process using sodium hydroxide as the absorbent and including solvent regeneration.
Flow diagram of direct air capture process using sodium hydroxide as the absorbent and including solvent regeneration
An example of what Direct Air Capture could look like and how the process works.

The carbon dioxide (CO2) is captured directly from the ambient air; this is contrast to carbon capture and storage (CCS) which captures CO2 from point sources, such as a cement factory or a bioenergy plant.[2] After the capture, DAC generates a concentrated stream of CO2 for sequestration or utilization. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent[3] or sorbents.[4] These chemical media are subsequently stripped of CO2 through the application of energy (namely heat), resulting in a CO2 stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse.

When combined with long-term storage of CO2, DAC is known as direct air carbon capture and storage (DACCS or DACS[5]). DACCS can function as both a carbon dioxide removal mechanism or a carbon negative technology. As of 2023, DACCS has yet to be integrated into emissions trading because, at over US$1000,[6] the cost per ton of carbon dioxide is many times the carbon price on those markets.[7] For the end-to-end process to remain net carbon negative, DAC machines must be powered by renewable energy sources, since the process can be quite energy expensive. Future innovations may reduce the energy intensity of this process.

DAC was suggested in 1999 and is still in development.[8][9] Several commercial plants are planned or in operation in Europe and the US. Large-scale DAC deployment may be accelerated when connected with economical applications or policy incentives.

In contrast to carbon capture and storage (CCS) which captures emissions from a point source such as a factory, DAC reduces the carbon dioxide concentration in the atmosphere as a whole. Thus, DAC can be used to capture emissions that originated in non-stationary sources such as airplanes.[2]

Methods of capture

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The International Energy Agency reported growth in direct air capture global operating capacity.[10]

There are the three stages of CO2 capture in DAC: the contacting stage, the capture stage, and the separation stage. In the contacting stage, the DAC system transports atmospheric air containing CO2 to the equipment using large-scale fans. Subsequently, in the CO2 capture stage, CO2 rapidly and effectively binds with liquid solvents in chemical reactors or solid sorbents in filters, which must possess binding energies equivalent to that of CO2. Later in the CO2 separation stage, external energy sources facilitate the separation of CO2 from the solvents or sorbents, yielding pure CO2 and regenerated solvents or sorbents. Following the completion of these three stages, the separated pure CO2 is either utilized or stored, while the recovered solvents or sorbents are recycled for reuse in the CO2 capture process.[11]

The low temperature DAC process uses solid sorbents (S-DAC) and the high temperature process utilizes liquid solvents (L-DAC) that feature different properties in terms of kinetics and heat transfers.[12] Currently, liquid DAC (L-DAC) and solid DAC (S-DAC) represent two mature technologies for industrial deployment. Additionally, several emerging DAC technologies, including electro-swing adsorption (ESA), moisture-swing adsorption (MSA), and membrane-based DAC (m-DAC), are in different stages of development, testing, or limited practical application.[11]

More recently, Ireland-based company Carbon Collect Limited[13] has developed the MechanicalTree™ which simply stands in the wind to capture CO2. The company claims this 'passive capture' of CO2 significantly reduces the energy cost of Direct Air Capture, and that its geometry lends itself to scaling for gigaton CO2 capture.

Most commercial techniques use a liquid solvent—usually amine-based or caustic—to absorb CO2 from a gas.[14] For example, a common caustic solvent: sodium hydroxide reacts with CO2 and precipitates a stable sodium carbonate. This carbonate is heated to produce a highly pure gaseous CO2 stream.[15][16] Sodium hydroxide can be recycled from sodium carbonate in a process of causticizing.[17] Alternatively, the CO2 binds to solid sorbent in the process of chemisorption.[14] Through heat and vacuum, the CO2 is then desorbed from the solid.[16][18]

Among the specific chemical processes that are being explored, three stand out: causticization with alkali and alkali-earth hydroxides, carbonation,[19] and organic−inorganic hybrid sorbents consisting of amines supported in porous adsorbents.[8]

Other explored methods

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The idea of using many small dispersed DAC scrubbers—analogous to live plants—to create environmentally significant reduction in CO2 levels, has earned the technology a name of artificial trees in popular media.[20][21][22]

Moisture swing sorbent

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In a cyclical process designed in 2012 by professor Klaus Lackner, the director of the Center for Negative Carbon Emissions (CNCE), dilute CO2 can be efficiently separated using an anionic exchange polymer resin called Marathon MSA, which absorbs air CO2 when dry, and releases it when exposed to moisture. A large part of the energy for the process is supplied by the latent heat of phase change of water.[23] The technology requires further research to determine its cost-effectiveness.[24][25][26]

Metal-organic frameworks

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Main article: Carbon dioxide scrubber § Metal-organic frameworks (MOFs)

Other substances which can be used are metal–organic frameworks (MOFs).[27]

Membranes

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Membrane separation of CO2 rely on semi-permeable membranes. This method requires little water and has a smaller footprint.[14] Typically polymeric membranes, either glassy or rubbery, are used for direct air capture. Glassy membranes typically exhibit high selectivity with respect to Carbon Dioxide; however, they also have low permeabilities. Membrane capture of carbon dioxide is still in development and needs further research before it can be implemented on a larger scale.[28]

Environmental impact

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Proponents of DAC argue that it is an essential component of climate change mitigation.[1][18][26] Researchers posit that DAC could help contribute to the goals of the Paris Agreement (namely limiting the increase in global average temperature to well below 2 °C above pre-industrial levels). However, others claim that relying on this technology is risky and might postpone emission reduction under the notion that it will be possible to fix the problem later,[9][29] and suggest that reducing emissions may be a better solution.[15][30]

Opponents of DAC argue that the resources required to operate DAC technologies, are an immense burden that may outweigh the goal of the technology itself.[31] A 2020 analysis revealed that DAC 2 technology may be an unsuitable option to capture the projected 30 Gt-CO2 per year as it requires an enormous amount of materials (16.3–27.8 Gt of NH3 and 3.3–5.6 Gt of EO) [31] The same study found that DAC 1 technology requires at least 8.4–13.1 TW-yr (46–71% TGES), an estimate that was calculated with the exclusion of the associated energy costs for carbon storage.[31]

Energy cost concerns were explored in 2021 and found that In order for DAC technology to maintain a carbon removal of 73-86% per ton of CO2 captured, DAC would demand land occupation and renewable energy equivalent to what is needed for a global switch from gasoline to electric vehicles, with approximately five times higher material consumption.[32]

Some DAC technologies, especially liquid systems, require both high temperature heat and electricity. In these systems the electrical demand is made using natural gas, imported electricity from the grid, and oxyfuel combustion of natural gas.[33] This means that many DAC technologies are powered by fossil fuels, the very thing the technology is meant to eliminate reliance on.[34] The physical scale of the air contactor in any DAC system is a formidable challenge and may also have an impact on the environment. A DAC system meant to combat six million metric tons of CO2 per year, may be sized at about 30 kilometers in length and 10 meters in height[35]

Though using fossil fuel to generate electricity would release more CO2 than captured CO2, the minimum energy required for DAC technologies is estimated to be 250kWh per tonne of CO2m whereas capturing with natural gas and coal power plants requires about 100 and 65 kWh per ton of CO2 [36] This could lead to a new set of environmental impacts in the future.[36]

DAC relying on amine-based absorption demands significant water input. It was estimated, that to capture 3.3 gigatonnes of CO2 a year would require 300 km3 of water, or 4% of the water used for irrigation. On the other hand, using sodium hydroxide needs far less water, but the substance itself is highly caustic and dangerous.[9]

DAC also requires much greater energy input in comparison to traditional capture from point sources, like flue gas, due to the low concentration of CO2.[15][29] The theoretical minimum energy required to extract CO2 from ambient air is about 250 kWh per tonne of CO2, while capture from natural gas and coal power plants requires, respectively, about 100 and 65 kWh per tonne of CO2.[15][1] Because of this implied demand for energy, some have proposed using "small nuclear power plants" connected to DAC installations.[9]

When DAC is combined with a carbon capture and storage (CCS) system, it can produce a negative emissions plant, but it would require a carbon-free electricity source. The use of any fossil-fuel-generated electricity would end up releasing more CO2 to the atmosphere than it would capture.[29] Moreover, using DAC for enhanced oil recovery would cancel any supposed climate mitigation benefits.[9][16]

Applications

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Practical applications of DAC include:

These applications require different concentrations of CO2 product formed from the captured gas. Forms of carbon sequestration such as geological storage require pure CO2 products (concentration > 99%), while other applications such as agriculture can function with more dilute products (~ 5%). Since the air that is processed through DAC originally contains 0.04% CO2 (or 400 ppm), creating a pure product requires more energy than a dilute product and is thus typically more expensive.[23][38] Capture carbon that is used for food typically requires CO2 with higher purity, ranging from 50+% followed by additional chemical processing. [39]

DAC is not an alternative to traditional, point-source carbon capture and storage (CCS), rather it is a complementary technology that could be utilized to manage carbon emissions from distributed sources, fugitive emissions from the CCS network, and leakage from geological formations.[1][30][15] Because DAC can be deployed far from the source of pollution, synthetic fuel produced with this method can use already existing fuel transport infrastructure.[37]

Typical discourse surrounding DAC is relegated to its effectiveness at mitigating climate change/global warming issues.[40] However, the majority of existing DAC facilities are small scale,[41] And operate primarily to sell the captured CO2 for use in other products rather than permanently sequestering it.[42] DAC facilities that sell CO2 for beverage production operate with low recovery rates of around 4.7% and produces 58-tCO2 per day.[43] The use of DAC facilities for commercial purposes, reemphasizes the opinion of naysayers, that DAC is a ploy used by corporations to protect and promote financial interest.[40]

There are also a variety of alternative carbon capture methods similar to but distinct from Direct Air Capture Technologies.

electro-swing adsorption (ESA)

moisture-swing adsorption (MSA)

membrane-based separation (m-DAC)[41]

Given the myriad of DAC applications, proponents of DAC argue that the political utility of the technology lies in its ability to create new employment opportunities.[44]

DAC Projects and their respective processes for Carbon removal and/or storage[45]

Operational/Developing DAC Facilities

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Company, project Process technology
Antecy, Carbon from Air (CAIR™) Solid carbonate sorbent, temperature swing
Carbon Capture™ Zeolite molecular sieves, temperature-vacuum swing
Carbon Collect, MechanicalTrees™ for Passive Direct Air Capture (PDAC™) Solid ion-exchange resin tiles, moisture swing
Carbon Engineering & Greyrock Energy, AIR TO FUELS™, Direct Fuel Production™, GreyCat™ Carbon Engineering DAC with Fischer–Tropsch catalysis
Carbon Engineering & Storegga Geotechnologies Carbon Engineering DAC with geological storage
Carbon Engineering & 1PointFive (Oxy Low Carbon Ventures & Rusheen Capital Management) Carbon Engineering DAC licensed to 1PointFive with EOR and geological storage
Carbyon Thin-film sorbent on porous membrane, temperature swing
Climeworks in partnership with Northern Lights Climeworks DAC with geological storage
Climeworks in partnership with Carbfix, Orca Climeworks DAC with geological storage
CO2Circulair Membrane gas absorption with liquid absorbent and concentration by membrane electrolysis
DACCITY Surface-activated porous carbon composite ceramic monoliths
Global Algae DAC and flue gas capture with algae production
Highly Innovative Fuels DAC with water electrolysis and fuel synthesis
Hydrocell Solid amine sorbent for indoor air quality control
Mission Zero Technologies DRIVE: Direct Removal (of CO2) via Innovative Valorisation using Emissions
Mosaic Materials Metal–organic framework sorbents for indoor air quality control
Nordic Electrofuel Climeworks and Sunfire technologies for synfuel production
Noya Retrofit of building cooling towers with “non-toxic CO2-absorbing chemical blend”
Origen Power Lime-based sorbents with solid oxide fuel cell and oxy-fired calcination
Rolls-Royce Small modular nuclear reactors to power DAC and fuel synthesis (aviation fuel)
Skytree Derived from International Space Station air scrubber technology, deployed in electric vehicles
Sunfire Climeworks DAC with co-electrolysis for syngas production and fuel synthesis
Sustaera Solid alkali metal sorbent on ceramic monoliths
Verdox Solid quinone sorbent, electro-swing adsorption
Zenid Fuel Climeworks DAC with co-electrolysis for syngas production and fuel synthesis (aviation fuel)

Cost

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One of the largest hurdles to implementing DAC is the cost of separating CO2 and air.[38][46] Although DAC implementation was initially and optimistically estimated to cost around $100-$300 per tonne, As of 2023 it is estimated that the total system cost is over $1,000 per tonne of CO2. Large-scale DAC deployment can be accelerated by policy incentives.[47] There is discourse surrounding the actual cost of globalized usage of DAC technology as cost values reported by private companies tend to be lower than academic estimates.[48] The Department of Energy estimated costs per tonne to be under $100, while other sources have estimated the cost to be much larger.As of 2023 it is estimated that the total system cost is over $1,000 per tonne of CO2.[6] Large-scale DAC deployment can be accelerated by policy incentives.[49]

Under the Bipartisan Infrastructure Law, the U.S. Department of Energy will invest $3.5 billion in four direct air capture hubs. According to the agency, the hubs have the potential to capture at least 1 million metric tonnes of carbon dioxide (CO2) annually from the atmosphere. Once captured, the CO2 will be permanently stored in a geologic formation.[50]

The Department of Energy invested $1.2 billion to further developments of direct air capture facilities in Texas and Louisiana. These projects are the result of initial selections from President Biden's Bipartisan Infrastructure Law [51]

Development

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Carbon Engineering

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Main article: Carbon Engineering

Carbon Engineering is a commercial DAC company founded in 2009 and backed, among others, by Bill Gates and Murray Edwards.[37][30] As of 2018, it runs a pilot plant in British Columbia, Canada, that has been in use since 2015[18] and is able to extract about a tonne of CO2 a day.[9][30] An economic study of its pilot plant conducted from 2015 to 2018 estimated the cost at $94–232 per tonne of atmospheric CO2 removed.[18][3]

Partnering with California energy company Greyrock, Carbon Engineering converts a portion of its concentrated CO2 into synthetic fuel, including gasoline, diesel, and jet fuel.[18][30]

The company uses a potassium hydroxide solution. It reacts with CO2 to form potassium carbonate, which removes a certain amount of CO2 from the air.[37]

Climeworks

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Main article: Climeworks

Climeworks's first industrial-scale DAC plant, which started operation in May 2017 in Hinwil, in the canton of Zurich, Switzerland, can capture 900 tonnes of CO2 per year. To lower its energy requirements, the plant uses heat from a local waste incineration plant. The CO2 is used to increase vegetable yields in a nearby greenhouse.[52]

The company stated that it costs around $600 to capture one tonne of CO2 from the air.[53][14][need quotation to verify]

Climeworks partnered with Reykjavik Energy in Carbfix, a project launched in 2007. In 2017, the CarbFix2 project was started[54] and received funding from European Union's Horizon 2020 research program. The CarbFix2 pilot plant project runs alongside a geothermal power plant in Hellisheidi, Iceland. In this approach, CO2 is injected 700 meters under the ground and mineralizes into basaltic bedrock forming carbonate minerals. The DAC plant uses low-grade waste heat from the plant, effectively eliminating more CO2 than they both produce.[9][55]

On May 8, 2024, Climeworks activated the world's largest DAC planet named Mammoth in Iceland. It will be able to pull 36,000 tons of carbon from the atmosphere a year at full capacity, according to Climeworks, equivalent to taking around 7,800 gas-powered cars off the road for a year.[56]

Global Thermostat

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Global Thermostat is private company founded in 2010, located in Manhattan, New York, with a plant in Huntsville, Alabama.[37] Global Thermostat uses amine-based sorbents bound to carbon sponges to remove CO2 from the atmosphere. The company has projects ranging from 40 to 50,000 tonnes per year.[57][verification needed][third-party source needed]

The company claims to remove CO2 for $120 per tonne at its facility in Huntsville.[37][dubious – discuss]

Global Thermostat has closed deals with Coca-Cola (which aims to use DAC to source CO2 for its carbonated beverages) and ExxonMobil which intends to start a DAC‑to‑fuel business using Global Thermostat's technology.[37]

Soletair Power

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Soletair Power is a startup founded in 2016, located in Lappeenranta, Finland, operating in the fields of Direct Air Capture and Power-to-X. The startup is primarily backed by the Finnish technology group Wärtsilä. According to Soletair Power, its technology is the first to combine Direct Air Capture with buildings' HVAC systems. The technology captures CO2 from the air running through a building's existing ventilation units inside buildings for removing atmospheric CO2 while reducing the building's net emissions. The captured CO2 is mineralized to concrete, stored or utilized to create synthetic products like food, textile or renewable fuel. In 2020, Wärtsilä, together with Soletair Power and Q Power, created their first demonstration unit of Power-to-X[58] for Dubai Expo 2020, that can produce synthetic methane from captured CO2 from buildings.

Prometheus Fuels

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Main article: Prometheus Fuels

Is a start-up company based in Santa Cruz which launched out of Y Combinator in 2019 to remove CO2 from the air and turn it into zero-net-carbon gasoline and jet fuel.[59][60] The company uses a DAC technology, adsorbing CO2 from the air directly into process electrolytes, where it is converted into alcohols by electrocatalysis. The alcohols are then separated from the electrolytes using carbon nanotube membranes, and upgraded to gasoline and jet fuels. Since the process uses only electricity from renewable sources, the fuels are carbon neutral when used, emitting no net CO2 to the atmosphere.

Heirloom Carbon Technologies

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Heirloom's first direct air capture facility opened in Tracy, California, in November 2023. The facility can remove up to 1,000 U.S. tons of CO2 annually, which is then mixed into concrete using technologies from CarbonCure. Heirloom also has a contract with Microsoft in which the latter will purchase 315,000 metric tons of CO2 removal.[61]

Other companies

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Innovations in Research

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Within the research domain, the ETH Zurich team's development of a photoacid solution for direct air capture marks a significant innovation. This technology, still under refinement, stands out for its minimal energy requirements and its novel chemical process that enables efficient CO2 capture and release. This method's potential for scalability and its environmental benefits align it with ongoing efforts by other companies listed in this section, contributing to the global pursuit of effective and sustainable carbon capture solutions.[66]

Political discourse

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Environmentalist Opposition

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In the United States there is conflict between politicians and politically unaffiliated environmental advocates on Direct Air Capture as it relates to economic benefit and efficiency in improving climate change associated risks.

One of the main grievances climate campaigners have is in regards to how DAC is perceived to be at best, a costly irrelevance to the more pressing need to cut emissions and, is a ploy that is utilized to maintain the fossil fuel industry's status quo, and perpetuate pollution [67] The Stratos Project, was purchased by Occidental Petroleum for $1.1 billion. This investment is regarded by some as an attempt to extend the longevity of the fossil fuel industry. The Stratos project is ultimately owned by Occidental Petroleum, an American oil company that bought Carbon Engineering for $1.1bn last month and views carbon removal as a sort of future-proofing for its industry.[68] Jonathan Foley, executive director of Project Drawdown (a research-based plan to reverse global warming and stop climate change)[69] regards DAC technology as a greenwashing exercise, that mitigates climate change issues but does not seek to solve them.[67] The Consumer's Association of Penang perceive DAC to be something that exacerbates the climate crisis, and is fundamentally against the principle of climate justice.[70]

A study conducted in 2024, analyzed the conditional support of DAC technologies in the United States. The study revealed that most of the participants who were familiar with DAC technology and had concerns about climate change had questions regarding the moral hazards of DAC technology.[44] Participants expressed disdain for the possibility that DAC might allow companies to continue pollutive practices while greenwashing their public image was raised across all focus groups.[44] Other participants worried that DAC technology would be used as a front by fossil fuel corporations, to create the illusion that something was being done to combat climate change without contributing real benefit to the environment.[44]

Environmentalist opposition to DAC often concerns the ecological impacts of the associated energy infrastructure.[71] Complications associated with the impact DAC may have on air quality in specific communities are called into question as well.[71] Some critics of DAC are in opposition to the technology because of the locations they tend to be placed in, as some feel that these projects are always developed in poor areas, objectors expressed that they feel "experimented on."[72]

Another study focusing on perceptions of DAC technology from climate concerned persons from the United States and the United Kingdom found similar results. A theme across all groups was the perception of DAC as a technology that is incongrous with the vision for a sustainable society.[40] Participants reported DAC to be "reactionary" to climate change as opposed to a viable solution to it.[40] A consistent theme across all workshops was the idea that CDR does not necessarily reflect people's ‘vision’ for a sustainable future society: “The survey also showed that "very few people believed that CDR deals with the root cause of emissions." [40] The study revealed that the overall perception was that DAC is merely an intervention that fails to address the root cause of climate change and instead sustains the contributors to the crisis itself.[40]'

Political opposition to DAC technology has also been related to doubts in the feasibility of DAC development and deployment at scale. Technologies analogous with DAC such as CCS and BECCS have been subject to immense public opposition.[45] These technologies have also been characterized by multiple failures and aborted projects, contributing to the already persistent doubt regarding the credibility of DAC projects.[73]

Biden's Bipartisan Infrastructure Law

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Some environmentalists believe that the 3.5 billion investment in DAC is a "dangerous gamble" that puts the lives of frontline communities at risk.[74] The Insititute of Policy studies regards this decision to be risky because "the promise of DAC may never materialize" and should the deployment of this technology fail, the result will be only harm on frontline communities in "new and unacceptable ways".[75] Surveys revealed that among those against DAC Trust in local government was generally low, in addition to mistrust in fossil fuel companies who sponsor DAC development.[44] Environmentalists lack of faith in the bipartisan infrastructure law grew after a 2020 Treasury Department Inspector General investigation revealed that 90% of the tax credits used for carbon capture operations were done so without verifying that any carbon was being captured.[75] Additionally, the IRS decision to not release information about which companies are benefiting from these new investments in DAC increases uncertainty among people who are concerned how their taxes are paying for DAC development.[75]

Partisan Perception in the United States

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A poll taken in 2023 assessing the opinions on Direct Air Capture based on political party affiliation found that, 42% of Democrats were strongly in favor of DAC, 34% of independent voters were in favor while only 28% of Republicans indicated their fervent support of DAC technology.[72] However, despite the negative response from the climate conscious community, politically, DAC technology has received Bipartisan support in government.[76]

The reason for Bipartisan support for DAC seems to be due to two merits, the environmental benefit of DAC and the potential economic advantages.[77] Republicans argue that DAC can provide economic advantages to the countries and local areas hosting these facilities through job creation, increase tax revenue and economic diversification.[78] The economic utiltiy DAC also provides is protection for fossil fuel industries as many including ExxonMobil have donated generously to DAC research and development.[77] Bipartisan support stems from the perception of DAC as a solution that satisfies economic and environmental concerns.[72] However, despite bipartisan support for DAC in congress, a survey conducted in 2024 revealed that "Republicans and Independents were significantly less likely than Democrats to support the development of DAC in and near their communities and in the U.S."

Much of the discourse surrounding DAC comes from environmental activists,[44] and though there are discrepancies in how Republicans and Democrats view DAC, these differences are generally relegated to the perception of the benefits DAC offers.[79] Some view DAC as a feasible solution to combat global warming (primarily Democrats), whereas Republicans support for DAC lies in the way the technology will not interfere with the economic interests of fossil fuel companies.[80]

Previous direct air capture shortcomings

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BECCS Project

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BECCS has come under scrutiny for a variety of reasons but primarily because the technology is energy intensive, requires large land changes/usage and has the potential to leak carbon dioxide back into the atmosphere.[81] Environmentalists argued that BECCS was an infeasible option because of the emissions that the project would produce.[81] BECCS is proposed as a solution based on the assumption that bioenergy would be carbon neutral. This assumption was found to be incorrect because many believe that the deforestation, logging and land required to accommodate this technology would offset the amount of carbon the technology removes.[81] Individuals concerned with protecting animal life also argue that increasing demand for land for BECCS would be an additional threat to biodiversity. Opponents also argue that the risk of a Carbon Dioxide leak outweighs the potential benefits if the technology functions properly. Carbon dioxide that is stored underground has a high risk of leakage, and the consequences of a major leak could be catastrophic.[82] "Atmospheric CO2 levels could spike significantly, especially if a leak were to occur from a major storage site."[81] Anxiety surrounding the possibility of a CO2 leakage is common worry among those who doubt DAC.[44]

See also

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References

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