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GHG Assessment and Research
Learn about emission research studies in the Bay Area that contribute to an improved understanding of emissions inventories.
On occasion, the Air District conducts short-term emissions studies in the San Francisco Bay Area to:
- Improve the accuracy of emissions estimates of certain source categories,
- Assess the benefits of emissions reduction measures, and
- Produce emissions for local-scale air pollution assessments
View information about recent emissions studies below.
Methane Airborne Emissions Quantification Study
In a follow-up to a few top-down emissions assessment studies that indicated significant underestimation of methane emissions in bottom-up inventories in the San Francisco Bay Area, the Air District conducted a 3-year long campaign to determine facility-level methane emission rates.
An airborne mass balance technique was applied from a low-flying aircraft to derive methane emission rates from 92 airborne observations collected over 23 facilities, including 5 refineries, 10 landfills, 4 wastewater treatment plants (POTWs), 2 composting operations, and 2 dairies. Annual measurement-based sector-wide methane emissions were determined from these emission rates and compared to bottom-up and facility-reported estimates. The measured emissions for San Francisco Bay Area refineries ranged from 4 to 23 times larger than the corresponding inventory estimates (see figure below), while measurement-derived landfill and POTW estimates were approximately twice the current inventory estimates. Complementary evidence from airborne remote sensing imagery over the San Francisco Bay Area indicated that atmospheric venting from refinery hydrogen plants, landfill working surfaces, composting stockpiles, etc., was among the specific source types responsible for the observed discrepancies.
This study highlights the value of multi-tier measurement approaches to accurately estimating facility-scale methane emissions and performing source attribution at sub-facility scales to guide and verify effective climate mitigation policy and action.
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Refinery 1
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Refinery 2
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Refinery 3
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Refinery 4
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Refinery 5
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USAEPA reported (2015-17)
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BAAQMD reported (2016-17)
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CARB reported (2015-16)
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Sci. Av. flights (2015-18)
Comparison of airborne measurement-based estimates of methane from Bay Area’s five refineries with bottom-up estimates reported to various agencies.
Last Updated: 4/1/2026