Direct Air Capture

This Protocol provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO₂e) removals from the atmosphere via Direct Air Capture (DAC). This Protocol is developed for application to DAC processes (e.g., solid-sorbent processes,¹ liquid-solvent processes,² membrane processes,³ electro-chemical processes,⁴ etc.), or combinations of processes, in which a cradle-to-grave greenhouse gas (GHG) Statement can be accurately applied and in which the CO₂ captured is stored via physical⁵ or chemical⁶ trapping mechanisms for >1000 years.

The Protocol was developed in line with latest scientific understanding^7,8,9 and industry best-practices^10,11 which inform the quantification of gross CO₂ durably captured and stored via DAC, as well as the accounting of GHG emissions associated with DAC processes. Additionally, the Protocol ensures:

• Consistent procedures are used to measure and monitor all aspects of the process required to enable accurate accounting of net CO₂e removal;
• Consistent system boundaries and calculations are utilized to quantify net CO₂e removal;
• Requirements are met to ensure the CO₂ removals are additional; and
• Evidence is provided and verified by independent third parties to support all net CO₂e removal claims.

Specific standards and protocols which are utilized as the foundation of this Protocol, and which this Protocol is intended to be fully compliant with, are as follows:

• Isometric Standard; and
• ISO 14064-2: 2019 – Greenhouse Gases – Part 2: Specification with guidance at the project level for quantification, monitoring, and reporting of greenhouse gas emission reductions or removal enhancements.

Additional reference standards that inform the requirements and overall practices incorporated in this Protocol include:

• ISO 14064-3: 2019 – Greenhouse Gases – Part 3: Specification with Guidance for the verification and validation of greenhouse gas statements;
• ISO 14040: 2006 - Environmental Management - Life Cycle Assessment - Principles & Framework; and
• ISO 14044: 2006 - Environmental Management - Life Cycle Assessment - Requirements & Guidelines.

This Protocol was developed based on the current state of the art and publicly available science regarding DAC and CO₂ storage. Because DAC is still a developing approach to carbon dioxide removal (CDR), with ever-expanding published literature, the Protocol incorporates requirements that may be more stringent than some current regulations or other protocols related to DAC and CO₂ storage. The approach taken here may be altered in future versions of the Protocol as DAC and CO₂ storage technology and research advance.

This Protocol applies to projects that chemically or physically capture atmospheric CO₂ from ambient air, and store it durably according to the storage Modules associated with this Protocol (see Section 9). A cradle-to-grave GHG Statement must also be able to be accurately applied to all processes within the scope of the Project.

Only DAC projects which meet a zero emissions baseline scenario (see Section 7.2) are eligible under this Protocol. A Project may qualify for this distinction by meeting one of the following conditions:

• No capture facility existed at the capture facility location prior to the start of the project activity (greenfield capture facilities);
• New capture facilities or expanded capture facilities are installed at an existing capture facility location (expansion of existing capture facilities); or
• An existing capture facility would be decommissioned prior to the start of the project activity (refurbishment of an existing capture facility).

• G-XR1V-0 CO₂ storage and pipelines must have monitoring systems in place to detect and identify potential CO₂ leaks, including audible alarms and regular monitoring or remote access to alarm notifications by Project Proponent staff following national/international regulations¹² (see Section 8 and the relevant storage modules for more information).
• G-HCXG-0 The Project Proponent must have a CO₂ leak response plan in place that complies with local or national requirements¹² and covers the Area of Review (AOR). This plan must be shared with local communities and first responders in temporary holding, pipeline, and storage locations.
• G-51S5-0 The Project Proponent must document emissions of solvents and/or sorbents from the DAC Project and demonstrate that emissions of the following types are below regulatory limits where applicable based on solvent and/or sorbent chemistry:
  • Amines (volatilized or in aerosol form);
  • Ammonia;
  • Volatile organic compounds (VOCs);
  • Nitrosamines and nitramines;
  • Particulate matter, and
  • Other hazardous substances (as defined by the authority of the geography where the Project is located or the most stringent of relevant standards worldwide, for example by the EPA) that may be released as a result of sorbent or solvent degradation, volatilization, or aerosolization.
• G-RPMT-0 Emission levels of solvents and/or sorbents must be demonstrated by using, where possible and reasonable, national or international standard test and sampling methods (i.e., NIST, ASTM, EPA), and should be completed by a qualified testing organization. Project Proponents must keep such records on file and must repeat testing when substantial changes to solvent or sorbent formulations occur or substantial changes in operating conditions occur that may impact emissions.
• G-CBGF-0 Project Proponents must comply with all health and safety procedures as required by the authority of the geography where the Project is located or the most stringent of relevant standards worldwide. Proponents must demonstrate that they have addressed any specific hazards associated with the use of the proposed sorbent or solvent, and with high concentrations of CO₂.
• G-00C4-0 Demonstrate that a social risk assessment has been conducted. This must consider environmental and social justice issues for the selected storage site, including a robust Stakeholder Input Process and considerations of any direct or indirect impacts on local communities or indigenous people, including livelihoods, ancestral knowledge, and cultural heritage in line with the applicable human rights laws and United Nations declarations.
• G-KN7S-0 Demonstrate a risk assessment and mitigation plan for safeguarding working conditions, especially regarding safe handling and transportation of solvents/sorbents as well as workers operating the capture and storage facility.
• G-7A3E-0 The Project Proponent must monitor water consumption and water stress, in addition to implementing necessary mitigation activities to ensure water neutrality.¹³

The following topics are covered briefly in this Protocol due to their inclusion in the Isometric Standard, which governs all Isometric Protocols. See in-text references to the Isometric Standard for further guidance.

Projects must be validated and Project net CO₂e removals verified by an independent third party, consistent with the requirements described in this Protocol, as well as in the Validation and Verification Section of the Isometric Standard.

The Validation and Verification Body (VVB) must consider following requisite components:

1. Verify that storage sites adhere to the requirements listed in the relevant storage module;
2. Verify that the quantification approach and monitoring plan adheres to requirements of Section 8, including demonstration of required records;
3. Verify that the Environmental & Social Safeguards outlined in Section 5 are met; and
4. Verify that the Project is compliant with requirements outlined in the Isometric Standard.

The threshold for Materiality, considering the totality of all omissions, errors, and mis-statements, is 5%, in accordance with the Materiality Section of the Isometric Standard.

Verifiers should also verify the documentation of uncertainty of the GHG Statement, as required by the Uncertainty Accounting Section of the Isometric Standard. Qualitative Materiality issues may also be identified and documented, such as:¹⁴

• Data and document control issues that erode the verifier’s confidence in the reported data;
• Poorly managed documented information;
• Difficulty in locating requested information; and
• Noncompliance with regulations indirectly related to GHG emissions, removals or storage.

Project validation and verification must incorporate site visits to project facilities in accordance with the requirements of ISO 14064-3, 6.1.4.2, including, at minimum, site visits during validation and initial verification, to the DAC Project and (if applicable) storage site. Verifiers should, whenever possible, observe operation of the capture and storage processes to ensure full documentation of process inputs and outputs through visual observation and validation of instrumentation, measurements, and required data quality measures.

A site visit must thereafter occur at least once every 2 years at each location.

VVBs must comply with the requirements defined in Validation and Verification Requirements Section of the Isometric Standard. In addition, teams should maintain and demonstrate expertise associated with the specific technologies of interest, including solvent/sorbent chemistry, electricity procurement, heat/power generation and the relevant CO₂ storage technology.

Competency must be demonstrated in accordance with Isometric's VVB policy, for example based on the relevant sectoral scope accreditations in IAF MD 14, or another demonstration of relevant expertise for this protocol and the selected storage module(s).

CDR via DAC and subsequent storage is often a result of a multi-step process (such as capture, desorption, CO₂ transport, CO₂ temporary holding, CO₂ injection or reaction, etc.), with activities in each step sometimes managed and operated by different operators, companies, or owners. When there are multiple parties involved in the process (e.g. if capture and storage are undertaken by different entities), and to avoid double counting of net CO₂e removals, a single Project Proponent must be specified contractually as the sole owner of the Credits. Contracts must comply with all requirements defined in the Ownership Section of the Isometric Standard.

The Project Proponent must be able to demonstrate additionality through compliance with the Additionality Section of the Isometric Standard. The baseline scenario and counterfactual utilized to assess additionality must be project-specific, and are described in Section 7.2 of this Protocol.

Additionality determinations should be reviewed and completed every Crediting Period, at a minimum, or whenever project operating conditions change significantly, such as the following:

• Regulatory requirements or other legal obligations for project implementation change or new requirements are implemented;
• Project financials indicate Carbon Finance is no longer required, potentially due to, for example:
  • Sale of co-products that make the business viable without Carbon Finance; or
  • Reduced rates for capital access.

Any review and change in the determination of additionality should not affect the availability of Carbon Finance and Verified Credits for the current or past Crediting Periods, but if the review indicates The Project has become non-additional, this should make The Project ineligible for future Credits.¹⁵

The uncertainty in the overall estimate of the net CO₂e removal as a result of the Project must be calculated and transparently presented. The total net CO₂e removed over a Reporting Period ($RP$; see Section 8.2) for a Project, $CO_2e_{Removal,\ RP}$, must be conservatively determined, based on the requirements outlined in the Uncertainty Accounting Section of the Isometric Standard.

Projects must report a list of all input variables used in the net CO₂e removal calculation and their uncertainties, including:

• Emission factors utilized, as published in public and other databases used;
• Values of measured parameters from process instrumentation, such as metered heat and electricity usage, sorbent/solvent replacement periods and other equipment considerations;
• Laboratory analyses, including that required by selected storage module(s), which could include analysis of carbon content and purity of CO₂, CO₂-containing injectants or carbonated minerals; and
• Summary of data handling, processing, and error propagation approach.

The uncertainty information should at least include the minimum and maximum values of a variable. More detailed uncertainty information should be provided if available, as outlined in the Uncertainty Accounting Section of the Isometric Standard.

In addition, a sensitivity analysis that demonstrates the impact of each input parameter’s uncertainty on the final net CO₂e uncertainty must be provided. Details of the sensitivity analysis method must be provided so that the results can be re-created. Parameters may be omitted from a full uncertainty analysis if a sensitivity analysis can demonstrate that the parameter contributes to 1% change in removal. For all other parameters, information about Uncertainty must be specified.

In accordance with the Data Sharing Section of the Isometric Standard, all evidence and data related to the underlying quantification of the net CO₂e removal will be available to the public through Isometric's platform. This includes:

• Project Design Document;
• GHG Statement;
• Measurements taken;
• Emission factors used; and
• Scientific literature used.

The Project Proponent can request certain information to be restricted (only available to authorized Buyers, the Registry, and VVB) where it is subject to confidentiality. This includes emission factors from licensed databases. However, all other numerical data produced or used as part of the quantification of net CO₂e removal will be made available.

The scope of this Protocol includes the GHG sources, sinks, and reservoirs (SSRs) associated with a DAC Project.

A cradle-to-grave GHG Statement must be prepared encompassing the GHG emissions relating to the activities outlined within the system boundary.

GHG emissions and removals associated with The Project may be direct emissions from a process or storage system, or indirect emissions from combustion of fuels, electricity generation, or other sources. Emissions must include all GHG SSRs within the system boundary, from the construction or manufacturing of each physical site and associated equipment, closure and disposal of each site and associated equipment, and operation of each process (DAC process, CO₂ transportation, storage, and monitoring), including embodied emissions of equipment and consumables used in the project. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities.

Any emissions from sub-processes or process changes that would not have taken place without the CDR Project must be fully considered in the system boundary. Any activity that ultimately leads to the issuance of Credits should be included in the system boundary. This allows for accurate consideration of additional, incremental emissions induced by the carbon removal process.

The system boundary must include all relevant GHG SSRs controlled and related to The Project, including but not limited to the SSRs set out in Table 1. If any GHG SSRs within Table 1 are deemed not appropriate to include in the system boundary, they may be excluded provided that robust justification and appropriate evidence is provided in the PDD.

Figure 1. Process flow diagram showing system boundary for DAC projects

Table 1. Scope of activities to be included in the system boundary for DAC projects

Miscellaneous GHG emissions are those that cannot be categorized by the GHG SSR categories provided in Table 1. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities and must report any outside of the SSR categories identified as miscellaneous emissions.

Emissions associated with The Project's impact on activities that fall outside of the system boundary of The Project must also be considered. This is covered under Leakage in Section 8.5.4.

In line with the GHG Accounting Module v1.1, the Project must:

• Consider all GHGs associated with SSRs, in alignment with the United States Environmental Protection Agency’s definition of GHGs which includes: carbon dioxide (CO₂), methane (CH4), nitrous oxide (N20) and fluorinated gasses such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). For CO2 stored, only CO2 shall be included as part of the quantification. For all other activities all GHGs must be considered. For example, the release of CO2, CH4, and N2O is expected during diesel consumption;
• Quantify emissions in tonnes CO₂ equivalent (t CO₂e) using the 100-year Global Warming Potential (GWP) for the GHG of interest, based on the most recent volume of the IPCC Assessment Report (currently the Sixth Assessment Report); and
• Consider materiality of SSRs in line with Isometric requirements.

The baseline scenario for a DAC Project assumes the activities associated with The Project do not take place and any associated infrastructure is not built.

DAC systems are typically operated continuously, with captured CO₂ being transported and durably stored using a variety of potential processes. Due to the continuous nature of DAC systems, the equations below used to calculate net CO₂e removals will pertain to all CO₂ removals and GHG emissions that occur over an interval of time. This unit of time is defined as the Reporting Period, $RP$, which represents an interval of time over which net CO₂e removals are calculated and reported for verification.

GHG emission calculations must include all emissions related to the project activities that occur within the Reporting Period. This includes:

• any emissions associated with project establishment allocated to the Reporting Period (See Section 8.5.1);
• any emissions that occur within the Reporting Period (See Section 8.5.2);
• any anticipated emissions that would occur after the Reporting Period that have been allocated to the Reporting Period (See Section 8.5.3); and
• leakage emissions that occur outside of the system boundary that are associated with the Reporting Period (See Section 8.5.4).

Total net CO2e removal is calculated for each Reporting Period, and is written hereafter as $CO_2e_{Removal,\ RP}$. The final net CO2e removal quantification must be conservatively determined, giving high confidence that at a minimum, the estimated amount of CO2e was removed.

In line with the Isometric Standard, this Protocol requires that Removal Credits are issued ex-post. Credits may be issued once CO₂ has been durably stored in the identified storage reservoir.

Net CO₂e removal for a process utilizing DAC must be calculated as follows for a Reporting Period, $RP$:

$$CO_2e_{Removal,\ RP} = CO_2e_{Stored,\ RP}\ –\ CO_2e_{Counterfactual,\ RP}\ -\ CO_2e_{Emissions,\ RP}$$

(Equation 1)

Where;

• $CO_2e_{Removal,\ RP}$ = the total net CO₂e removed for a given $RP$, in tonnes of CO₂e.
• $CO_2e_{Stored,\ RP}$ = the total CO₂ removed from the atmosphere and durably stored over the $RP$, in tonnes of CO₂e. See Section 8.3.
• $CO_2e_{Counterfactual,\ RP}$ = the total counterfactual CO₂ removed from the atmosphere and durably stored in the absence of The Project over the $RP$, in tonnes of CO₂e. Note, unless otherwise specified in the applicable storage module, the counterfactual for DAC projects is typically zero. See Section 8.4.
• $CO_2e_{Emissions,\ RP}$ = the total GHG emissions associated with the $RP$, in tonnes of CO₂e. See Section 8.5.

It should be noted that any potential reversals of CO₂ storage in the final storage location occur after Credits have been issued so are not included in this equation. See the Reversal and Buffer Pool Section of the Isometric Standard for further information. Risk of reversal information is given in Appendix 1: Risk of Reversal Questionnaire, with further information provided within the relevant storage module.

$CO_2e_{Stored,\ RP}$ represents the cumulative total CO₂ sequestered in all durable storage reservoirs over a Reporting Period. It is calculated as:

$$CO_2e_{Stored,\ RP} = \sum_{i}^{N} CO_{2}e_{Storage,i}$$

(Equation 2)

• $CO_{2}e_{Storage,i}$ is the total amount of $CO_{2}$ sequestered in a durable storage reservoir, i, following an applicable storage Module, over the Reporting Period, RP, in tonnes $CO_{2}e$. This term must be measured following the requirements in the respective storage Module.
• $N$ is the total number of durable storage reservoirs used for storage of captured $CO_{2}$.

Quantification of $CO_2e_{Storage,\ RP}$ measurements, and monitoring requirements for the different storage pathways are detailed within the respective Modules.

Unless otherwise specified in the applicable storage module, the counterfactual ($CO_2e_{Counterfactual,\ RP}$) for eligible projects is considered to be zero, as outlined in Section 4 and Section 7.2.

$CO_2e_{Emissions,\ RP}$ is is the total quantity of GHG emissions associated with a given Reporting Period, $RP$. This can be calculated as:

$$CO_{2}e_{ Emissions,\ RP} = CO_{2}e_{Establishment,\ RP} + CO_{2}e_{Operations,\ RP} \\ + CO_{2}e_{End-of-life,\ RP} + CO_{2}e_{Leakage, RP}$$

(Equation 3)

Where:

• $CO_{2}e_{ Emissions,\ RP}$ = the total GHG emissions for the Reporting Period, $RP$, in tonnes of CO₂e.
• $CO_{2}e_{Establishment,\ RP}$ = the total GHG emissions associated with project establishment for the $RP$, in tonnes of CO₂e, , see Section 8.5.1
• $CO_{2}e_{Operations,\ RP}$ = the total GHG emissions associated with operational processes for the $RP$, in tonnes of CO₂e, see Section 8.5.2
• $CO_{2}e_{End-of-life,\ RP}$ = the total GHG emissions that occur after the $RP$ and are allocated to the $RP$, in tonnes of CO₂e, see Section 8.5.3
• $CO_{2}e_{Leakage,\ RP}$ = the GHG emissions associated with the Project’s impact on activities that fall outside of the system boundary of a Project, over a given $RP$, in tonnes of CO₂e, see Section 8.5.4

The following sections provide an overview for each variable.

GHG emissions associated with $CO_{2}e_{Establishment,\ RP}$ should include all historic emissions incurred as a result of project establishment, including but not limited to the SSRs set out in Table 1.

Project establishment emissions occur from the point of project inception through to before the first removal activity takes place. GHG emissions associated with project establishment may be amortized over the anticipated project lifetime, or per output of product. Rules on amortization are outlined in Section 7 of the GHG Accounting module .

GHG emissions associated with $CO_{2}e_{Operations,\ RP}$ should include all emissions associated with operational activities including but not limited to the SSRs set out in Table 1.

$CO_{2}e_{Operations,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis, in agreement with Isometric.

$CO_2e_{End-of-Life,\ RP}$ includes all emissions associated with activities that are anticipated to occur after the Crediting Period until the end of the Project Commitment Period. This includes activities related to ongoing monitoring for Reversals.

$CO_2e_{End-of-Life,\ RP}$ must be estimated upfront and allocated in the same way as set out for calculation of $CO_2e_{Establishment,\ RP}$.

Given the uncertain nature of $CO_2e_{End-of-Life,\ RP}$ emissions, assumptions must be revisited at each Reporting Period and any necessary adjustments made.

$CO_2e_{Leakage,\ RP}$ includes emissions associated with a Project's impact on activities outside the system boundary of The Project. This includes instances where The Project causes an increase in GHG emissions by diverting material from other uses or incentivizing increased production activity.

It is the Project Proponent's responsibility to identify potential sources of leakage emissions. For a DAC Project, market leakage emissions associated with the replacement of consumables used must be considered as a minimum. Project Proponents may also consider the impact of project operations on potential increased use of rare-earth materials for the production of sorbents, land use change, and increased strain on existing CO₂ transportation and storage infrastructure.

The Energy Use Accounting Module v1.3 provides requirements on how energy-related emissions must be calculated for The Project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed for intensive facilities and non-intensive facilities and acceptable emission factors.

Energy emissions are those related to electricity or fuel usage. They may include, but are not limited to:

• DAC Process:
  • Electricity used in process operations, including renewable energy, such as:
    • Sorbent/solvent or other regeneration process (electrically heated, electrochemical, or other);
    • electricity for pumps, motors, drives, etc;
    • electricity for instrumentation and controls; and
    • electricity for building operation and management for DAC process buildings and direct support buildings (noting that research and development and administrative facilities are not included).
  • Fuel combustion for thermal energy generation (heat/steam) such as:
    • Sorbent/solvent or other regeneration process (thermal); and
    • Heat for DAC process buildings and operations.
  • Heat utilization for thermal processes;¹⁶ and
  • Cryogenic processes for CO₂ purification or liquefaction.
• CO₂ Transportation:
  • Electricity or fuel used for operation of a pipeline or similar non-mobile CO₂ transportation process.
• CO₂ Storage:
  • Electricity used for operation of any CO₂ conversion processes, such as ex-situ carbonate production and handling;
  • Electricity used for injection operations, including any pumps, compressors (including for compression into supercritical CO₂), or related equipment inside the injection facility gate; and
  • Fuel used for heat generation or other purposes at the conversion or injection sites.
• CO₂ Monitoring:
  • Electricity used for monitoring equipment operation, including analyzers, instrumentation, on-site laboratories specifically for monitoring activities;
  • Electricity used for sampling pumps, sampling systems, or other similar monitoring activities;
  • Electricity used for off site analytical laboratory operation and sample analysis;
  • Electricity used for monitoring system installation (if not accounted for in Project embodied emissions) and operation, such as electricity used for temperature control of monitoring systems (heat trace);
  • Electricity for building operation & management for monitoring facility buildings;
  • Fuel used for sampling system operation, such as any pumps or heating systems;
  • Fuel used for any handling equipment, such as fork trucks or loaders, which are used during sample collection and processing; and
  • Fuel used during monitoring system installation, operation or closure, such as that used by drill rigs.

The GHG Accounting Module v1.1 provides requirements on how transportation and embodied emissions must be calculated for The Project so that they can be subtracted in the net CO₂e removal calculation.

Embodied emissions are those related to the life cycle impact of equipment and consumables. They may include, but are not limited to:

• Equipment, including:
  • DAC Process:
    • DAC Process equipment, including fans, scrubbers, adsorbers, or other contact and/or sorbent regeneration equipment;
    • Sorbent, solvent, or other material handling systems, such as pumps, conveyors, augers, feed bins, and related equipment;
    • Heat transfer equipment;
    • Captured CO₂ purification equipment;
    • CO₂ compression and storage equipment (on-site); and
    • Preparation or mixing equipment for sorbents, solvents, or other materials.
  • CO₂ transportation:
    • Equipment used for transportation of CO₂, including pipelines, and any pumps or compressors.
  • CO₂ storage:
    • Ex-situ CO₂ conversion or reaction equipment (i.e. for carbonate production), including all vessels, pumps, storage, and other process equipment;
    • Closed-system temporary holding of CO₂ at the injection site; and
    • CO₂ injection equipment, including compressors, pumps, and all wellbore equipment and materials.
  • Monitoring:
    • Monitoring wells and all associated materials (steel casing, concrete, etc.);
    • On-line analyzers, measurement equipment, or other such devices; and
    • Buildings and associated equipment utilized for monitoring purposes (e.g., on-site laboratories).
  • General equipment used along the value chain:
    • Pumps, piping, and related equipment;
    • Storage tanks;
    • Support structures, facilities, and infrastructure, including steel platforms, framing, supports, concrete footings, building structures, offshore rigs where applicable etc.; and
    • Instrumentation, controls, and other process management equipment.

Transportation emissions are those related to transportation of products and equipment. They may include, but are not limited to:

• Emissions associated with transportation of compressed gaseous or liquid CO₂, or CO₂ containing injectant (such as a carbonate slurry), or carbonated minerals via freight transportation services, such as rail, truck, or maritime transport; and
• Transportation of samples for lab analysis.

Quantification of these emission sources in a given $RP$ should be undertaken in line with the requirements set out in the GHG Accounting Module v1.1 and the Energy Use Accounting Module v1.3, where appropriate.

$$CO_{2}e_{DirectEmissions} = \sum_{t=1}^{T} m_{em,t} \cdot\ C_{GHG,t} \cdot\ GWP_{GHG}$$

(Equation 4)

Where:

• $m_{em,t}$ = the mass of direct emission (in tonnes) during period $t$.
• $C_{GHG,t}$ = the measured concentration as weight percent (%wt) of the relevant GHGs in direct emissions.
• $GWP_{GHG}$ = the global warming potential of the relevant GHGs for a 100-year time interval.
• $t$ = the time index, ranging from 1 to $T$.
• $T$ = $RP/Δt$, the number of time units in Reporting Period, $RP$.
• $Δt$ = the time interval the average is taken over.

The total quantity of direct emissions can be measured by various acceptable methods, including:

• Use of calibrated flow meters to provide continuous volumetric or mass flow measurement of a release from a process. Any flow meter must be calibrated for the composition and density of tail gas, or use appropriate conversion factors;¹⁷
• Use of flow data and curves from tail gas emissions testing and pressure drop measurement (i.e. pitot tubes) in the tail gas stream. Such testing data should be produced by a qualified emissions testing company, accredited to the Stack Testing Accreditation Council for ASTM D7036, ISO 17025, or approved by the authority of the geography where the Project is located or the most stringent of relevant standards worldwide. Testing should be completed under representative process operating conditions;
• Calculation of tail gas amount by a carbon material balance calculated based on direct measurement of other process streams;
• Measurement of a storage vessel pressure and temperature at beginning and end of a defined period within the Reporting Period, $RP$. Calculation of total mass of gas can be completed based on gas composition data and temperature and pressure data to determine if release has occurred; and
• Weight of a storage vessel as determined by calibrated weigh scale or load sensor at beginning and end of a defined period within the Reporting Period, $RP$.

In instances where direct measurement of concentration is not appropriate (e.g. pressure relief valve actuation), Projects may estimate the quantity of emissions according to best available knowledge, for example based on actuation duration, system pressure change, valve sizing, and typical concentrations of the system being depressurised.

This Protocol provides multiple options for durable storage of CO₂. The Project Proponent can choose from available options when submitting their Project for verification:

Isometric would like to thank following contributors to this Protocol and relevant Modules, or previous versions:

• Tim Hansen (350 Solutions); Direct Air Capture Protocol and Energy Use Accounting, Transportation Emissions Accounting and Embodied Emissions Accounting Modules.
• Chris Holdsworth, PhD (University of Edinburgh); CO₂ Storage in Saline Aquifers and CO₂ Storage via In-Situ Mineralization in Mafic and Ultramafic Formations Modules.
• Wilson Ricks (Princeton University); Energy Use Accounting Module.
• Grant Faber (Carbon Based Consulting); Transportation Emissions Accounting Module.

Isometric would like to thank following reviewers of this Protocol and relevant Modules, or previous versions:

• James Campbell, PhD (Herriot Watt University); Direct Air Capture Protocol and CO₂ Storage via In-Situ Mineralization in Mafic and Ultramafic Formations Module.
• Grant Faber (Carbon Based Consulting); Energy Use Accounting and Embodied Emissions Accounting Modules.

This risk assessment identifies the pathway specific risk factors relevant to a carbon removal project. The relevant risk factors identified as part of a risk assessment are included in the monitoring plan requirements for the project, with details included in the Project Design Document. Project specific risk factors inform the required duration of monitoring along with the monitoring requirements set out in the Protocol and the requirements set out in the Monitoring Section of the Isometric Standard.

Projects using this Protocol have the option of a number of storage modules. The typical buffer pool contributions and the rationale are indicated in the relevant storage module: typically, geologic storage is considered Very Low Risk Level (leading to a 1% buffer pool).

If Reversals are not directly observable (i.e., all storage is as carbonated materials in the built environment and/or DIC in an open system), the Project's Risk of Reversal is automatically "No observable risk." Such Projects do not need to complete this questionnaire, but must still maintain a monitoring plan in accordance with the requirements of the relevant Protocol. Please note storage as carbonated materials in the built environment also requires a Project-specific calculation of reversal risk and uncertainty discount.

The risk score, as determined by the Risk of Reversal Questionnaire, will determine a project’s buffer pool contribution. Projects must re-assess their reversal risk at the renewal of each crediting period, or if monitoring identifies a reversal-related risk, or if an actual reversal event takes place.

The Risk of Reversal Questionnaire questions that pertain to this protocol, drawn from the programme-level Risk of Reversal Questionnaire defined in the Isometric Standard, include the following:

Note the Risk Score at any step cannot be negative.

Risk Score Categories:

• 0: Very Low Risk Level (1% buffer)
• 1-2: Low Risk Level (5% buffer)
• 3-4: Medium Risk Level (7% buffer)
• 5+: High Risk Level (10-20% buffer)

Project specific risk factors will depend on the form of carbon being stored (i.e., organic vs. inorganic), the method of storage (e.g., mineralization, encapsulation), the location of carbon storage (e.g., subsurface, ocean), and the proximity of that carbon to potential agents of reversal.

For projects with carbon storage as inorganic carbon, the presence of the following risk factors must be reflected in the risk score corresponding to question 10:

• Acidic fluid
• Alkaline fluid (if stored as dissolved inorganic carbon)
• Temperatures in excess of 800 degrees celsius

For projects with any form of subsurface carbon storage, the presence of the following risk factors must be reflected in the risk score corresponding to question 10:

• Seismicity
• Subsurface migration

California Air Resources Board. (2022). Carbon Sequestration: Carbon Capture, Removal, Utilization, and Storage. https://ww2.arb.ca.gov/our-work/programs/carbon-sequestration-carbon-capture-removal-utilization-and-storage

Environment and Climate Change Canada. Clean Fuel Regulations: Quantification Method for CO2 Capture and Permanent Storage Version 1.0. (2022) https://publications.gc.ca/collections/collection_2022/eccc/En4-474-2022-eng.pdf

Intergovernmental Panel on Climate Change. (2005). IPCC Special Report on Carbon Dioxide Capture and Storage https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_wholereport-1.pdf

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International Organization for Standardization. (2006). ISO 14044:2006 Environmental management — Life cycle assessment — Requirements and guidelines. https://www.iso.org/standard/38498.html

International Organization for Standardization. (2011). ISO 14066:2011 Greenhouse gases — Competence requirements for greenhouse gas validation teams and verification teams. https://www.iso.org/standard/43277.html

International Organization for Standardization. (2017). ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratories. https://www.iso.org/standard/66912.html

International Organization for Standardization. (2019). ISO 14064-2:2019. Greenhouse Gases - Part 2: Specification With Guidance At The Project Level For Quantification, Monitoring And Reporting Of Greenhouse Gas Emission s Or Removal Enhancements. ISO. https://www.iso.org/standard/66454.html

International Organization for Standardization. (2019). ISO 14064-3:2019. Greenhouse gases — Part 3: Specification with guidance for the verification and validation of greenhouse gas statements. ISO. https://www.iso.org/standard/66455.html

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1. Shi, X., Xiao, H., Azarabadi, H., Song, J., Wu, X., Chen, X., and Lackner, K. S. (2020). Sorbents for the Direct Capture of CO2 from Ambient Air. Angewandte Chemie International Edition, 59, 6984–7006. https://doi.org/10.1002/anie.201906756 ↩

2. Custelcean, R. (2022). Direct Air Capture of CO2 Using Solvents. Annual Review of Chemical and Biomolecular Engineering, 13, 217–234. https://doi.org/10.1146/annurev-chembioeng-092120-023936 ↩

3. Fujikawa, S., and Selyanchyn, R. (2022). Direct air capture by membranes. MRS Bulletin, 47, 416–423. https://doi.org/10.1557/s43577-022-00313-6 ↩

4. Renfrew, S. E., Starr, D. E., and Strasser, P. (2020). Electrochemical Approaches toward CO2 Capture and Concentration. ACS Catalysis, 10, 13058–13074. https://doi.org/10.1021/acscatal.0c03639 ↩

5. Al Hameli, F., Belhaj, H., and Al Dhuhoori, M. (2022). CO2 Sequestration Overview in Geological Formations: Trapping Mechanisms Matrix Assessment. Energies, 15, Article 20. https://doi.org/10.3390/en15207805 ↩

6. Rochelle, C. A., Czernichowski-Lauriol, I., and Milodowski, A. E. (2004). The impact of chemical reactions on CO2 storage in geological formations: A brief review. Geological Society, London, Special Publications, 233, 87–106. https://doi.org/10.1144/GSL.SP.2004.233.01.07 ↩

7. Ricks, W., Xu, Q., and Jenkins, J. D. (2023). Minimizing emissions from grid-based hydrogen production in the United States. Environmental Research Letters, 18, 014025. https://doi.org/10.1088/1748-9326/acacb5 ↩

8. Goeppert, A., Czaun, M., Prakash, G. K. S., and Olah, G. A. (2012). Air as the renewable carbon source of the future: An overview of CO2 capture from the atmosphere. Energy & Environmental Science, 5, 7833–7853. https://doi.org/10.1039/C2EE21586A ↩

9. Sanz-Pérez, E. S., Murdock, C. R., Didas, S. A., and Jones, C. W. (2016). Direct Capture of CO2 from Ambient Air. Chemical Reviews, 116, 11840–11876. https://doi.org/10.1021/acs.chemrev.6b00173 ↩

10. Terlouw, T., Treyer, K., Bauer, C., and Mazzotti, M. (2021). Life Cycle Assessment of Direct Air Carbon Capture and Storage with Low-Carbon Energy Sources. Environmental Science & Technology, 55, 11397–11411. https://doi.org/10.1021/acs.est.1c03263 ↩

11. Erans, M., Sanz-Pérez, E. S., Hanak, D. P., Clulow, Z., Reiner, D. M., and Mutch, G. A. (2022). Direct air capture: Process technology, techno-economic and socio-political challenges. Energy & Environmental Science, 15, 1360–1405. https://doi.org/10.1039/D1EE03523A ↩

12. For example, 49 CFR §195.402 - Transportation of Hazardous Liquids via Pipeline: Procedural manual for operations, maintenance, and emergencies, and 40 CFR §146.94 - Class VI Wells: Emergency and remedial response. ↩ ↩²

13. Water neutrality is defined as: the total demand for water should be the same after new development is built, as it was before. That is, the new demand for water should be offset in the existing community by making existing infrastructure and homes in the area more water efficient. ↩

14. ISO 14064-3:2019, Section 5.1.7 ↩

15. Carbon Credit Quality Initiative. Methodology for assessing the quality of carbon credits, Version 3.0 (May 2022). https://carboncreditquality.org/methodology.html ↩

16. Lyons, L., Kavvadias, K. and Carlsson, J., (2021). Defining and accounting for waste heat and cold. EUR 30869 EN, Publications Office of the European Union, Luxembourg. doi:10.2760/73253. https://publications.jrc.ec.europa.eu/repository/handle/JRC126383 ↩

17. Flow meters must be calibrated to national traceable standards by an ISO 17025 accredited metrology laboratory. Flow meters may include critical nozzle flow meters (i.e. ISO 9300:2022 compliant meters), coriolis mass flow meters, and other applicable meters for mixed gas flows, as long as properly calibrated and maintained. ↩

18. Dinh, T.-V., Choi, I.-Y., Son, Y.-S., and Kim, J.-C. (2016). A review on non-dispersive infrared gas sensors: Improvement of sensor detection limit and interference correction. Sensors and Actuators B: Chemical, 231, 529–538. https://doi.org/10.1016/j.snb.2016.03.040 ↩

19. Sandoval-Bohorquez, V. S., Rozo, E. A. V., and Baldovino-Medrano, V. G. (2020). A method for the highly accurate quantification of gas streams by on-line chromatography. Journal of Chromatography A, 1626, 461355. https://doi.org/10.1016/j.chroma.2020.461355 ↩