What documentation does CARB require for mandatory GHG reporting under MRR?

California's Mandatory GHG Reporting Regulation requires covered facilities to submit verified annual greenhouse gas emissions reports to the California Air Resources Board. The April 1 deadline is fixed — and the supporting documentation behind that report is what gets examined when CARB audits a submission or a facility disputes a verification finding.

For environmental consultants preparing or supporting MRR compliance, the document trail extends well beyond the submitted report itself.

What does CARB's Mandatory Reporting Regulation actually require?

Cal. Code Regs. tit. 17, § 95100 et seq. establishes the framework. Covered entities — large industrial facilities, electricity generators, fuel suppliers, and others that meet GHG emission thresholds — must submit an annual emissions data report by April 1 covering the prior calendar year.

The report itself must include:

• Source-level emissions data for each covered emissions unit
• Calculation methodologies and underlying data (fuel consumption, process rates, activity data)
• A monitoring plan describing how data is collected and managed
• A verification statement from a CARB-accredited verification body

The monitoring plan is a living document. If a facility changes its monitoring approach mid-year, that change must be documented and retained. Consultants who update monitoring plans on behalf of covered entities should version-control those documents with the same rigor as the report itself.

What records must be retained — and for how long?

CARB's audit authority under MRR extends seven years from the end of the reporting period. That means records supporting a 2025 annual report (submitted April 1, 2026) must be retained through at least 2033.

Retention requirements include:

• Raw data underlying emissions calculations (meter readings, fuel receipts, process rate logs)
• Monitoring plan versions, including any mid-year amendments
• Verifier communications and working papers (to the extent the consultant holds them)
• Emissions factor documentation if site-specific emission factors are used

If a consultant prepares MRR reports on behalf of multiple facilities, the version-specific record for each facility's submission matters. A dispute about whether the submitted report reflects updated monitoring data — or an earlier draft — cannot be resolved from email alone. The submitted file and the supporting data package need to be tied together with a clear chain of custody.

What happens when CARB audits an MRR submission?

CARB conducts both desk audits and site audits. A desk audit will request the supporting calculation workbooks, the monitoring plan, and the verifier's work product. If the consultant prepared the report, the agency may request documentation directly from the facility — or from the consultant if they are the primary contact on the submission.

A common audit trigger is a material difference between two consecutive years' emissions that is not explained by the report narrative. Consultants should document operational changes, facility shutdowns, or equipment modifications that affect emissions in a way that can be produced quickly during an audit response window.

Does the verification requirement affect consultant documentation obligations?

Yes. The third-party verifier conducts their own assessment and issues a verification statement, but they rely on documentation the facility and its consultants provide. If the verifier finds inconsistencies between what was submitted to CARB and the underlying records, the result can be a qualified verification statement — or, in serious cases, a referral for enforcement.

Consultants who maintain a complete, timestamped record of what was delivered to the verifier — and what version of the report was ultimately submitted — are in a much stronger position when a verification dispute arises late in the process.

What about California Cap-and-Trade compliance reports?

MRR is the data foundation for California's Cap-and-Trade program under Cal. Code Regs. tit. 17, § 95800 et seq. Facilities covered by Cap-and-Trade must ensure their MRR submission aligns with their allowance obligations. Errors in the MRR submission can propagate into Cap-and-Trade compliance obligations — making the underlying documentation even more consequential.

The version control problem is specific: the data report submitted to CARB via CITSS must match the version the third-party verifier reviewed and attested to. If the report was revised after the verifier signed off — even to correct a calculation error — the verification statement is no longer valid for that version. Consultants who cannot produce file-level proof that the submitted version is identical to the verified version face a non-compliance finding under § 95133 even if the underlying emissions data is accurate.

Confirm current thresholds, report templates, and verifier accreditation requirements directly with CARB, as program details are periodically updated.

What are the common failure modes in CARB MRR documentation?

Three scenarios create audit exposure specifically for consultants:

Monitoring plan updated post-submission. The consultant updates the monitoring plan after the annual report is filed — correcting a methodology, adding a new emissions unit, or reflecting equipment changes. The CITSS submission references an earlier version. When CARB audits and requests the monitoring plan on file, the consultant cannot quickly establish which version was in effect at the time of submission. The submitted file and the supporting documentation should be version-locked at the moment of CITSS confirmation.

Verifier working papers reference a different version. The third-party verifier reviewed a version of the data report that was subsequently revised before CITSS submission. The verifier's statement attests to a document the consultant can no longer produce as a standalone file. Under § 95131(d), the verification statement must correspond to the submitted version. If there is a discrepancy, the consultant is explaining a version gap under an enforcement inquiry rather than producing a clean record.

Calculation workbooks not tied to the specific annual report. CARB's audit request asks for the underlying calculation workbooks for the 2024 reporting year. The consultant has workbooks, but they span multiple clients and multiple report versions. Without a clear association between the workbook version and the specific CITSS submission, the audit response is reconstruction rather than production. A delivery record that locks the supporting file package to the CITSS confirmation timestamp prevents this scenario.

Mainstay generates a SHA-256 delivery receipt at the moment of each submittal — tying the exact file to a timestamp and recipient record. For MRR submittals, that receipt establishes which version was delivered to the verifier and which version was submitted to CARB, in a record that neither party can alter after the fact.

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Mainstay coordinates the work; it is not a compliance advisor and makes no regulatory determination. Always confirm requirements with the relevant agency.