What are they?

High Priority Emission Plumes (HPEPs) are methane (CH4) plumes identified using satellites and located near public areas that may pose a short-term safety hazard due to their flammability risks.

Are HPEPs common?

No, HPEPs are rare and the risk to the public is low. However, the potential consequences are serious enough to warrant systematic monitoring and rapid coordination with emergency response agencies when needed.

How are HPEPs detected?

• Each methane plume detected by satellite is reviewed by CARB scientists.
• CARB estimates the maximum distance (radius) that could reach flammable levels, which only occur within certain atmospheric conditions.
• If this area overlaps with homes and public spaces, the plume is flagged as an HPEP.

How does the radius work?

• The radius represents the maximum distance methane could reach flammable concentrations.
• The size of the radius depends on how much methane is being released. Smaller plumes have a flammable radius of only about 10 meters, while the largest plumes could be up to 200 meters.
• A built-in safety buffer accounts for uncertainty in satellite data, ensuring any potential hazard is captured.

An HPEP is found. What next?

• Once an HPEP is identified, CARB works with local air districts and emergency response agencies to ensure the situation is addressed quickly and safely.
• Local air districts and CalOES work together to assess the site, determine appropriate safety measures, and mitigate any hazards.
• These types of plumes have often gone undetected in the past, but by pairing satellite methane detection with CARB’s HPEP protocol, the state can now protect communities while reducing harmful methane emissions at their source.

A more detailed technical description of how HPEPs are identified is available at California Satellite Methane Project (//ww2.arb.ca.gov/our-work/programs/california-satellite-methane-project).

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