Community Air Monitoring Near Aliso Canyon Natural Gas Leak

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This page was last reviewed on July 28, 2016

July 26th, 2016. After several months of typical ambient methane levels in the Porter Ranch community the Air Resources Board removed their 5 methane monitors (Sites 1, 2, 5, 7, 8) in late July 2016. The South Coast Air Quality Management District will continue to operate 3 real time monitors, and will operate one of the two benzene monitors previously run by ARB. The Air Quality Criteria will now be updated weekly based on real time data from the three SCAQMD sites and the canister samples that will be collected once every six days. ARB continues to make methane measurements in the Los Angeles basin along with its collaborators, and also intends to make additional downwind flights to measure total methane from the Aliso Canyon Natural Gas Storage Facility. The website will be updated whenever new data is available.

Background

The Aliso Canyon natural gas storage facility leak that started on October 23rd is emitting large quantities of natural gas into the atmosphere. Where this natural gas goes once released is highly dependent on the wind conditions and both general and local meteorology. When northerly winds prevail the natural gas can be carried to the nearby Porter Ranch community.

Natural gas is comprised mainly of methane, often 90% or more. Methane is an odorless and colorless gas that would not be noticed at the levels seen from this leak. In addition odorants, in the form of mercaptans, are added so leaks can be detected. These odorants are not toxic at the levels found in the Porter Ranch community, but the odors have resulted in nausea, headaches and nosebleeds.  Other constituents in the gas also have health impacts, such as benzene, although they have not been measured at levels that are considered a health issue. For more information on the health effects of these toxics and the natural gas in general please visit the California Governor’s Office of Emergency Services main page on the Aliso Canyon leak: http://www.caloes.ca.gov/ICESite/Pages/Aliso-Canyon.aspx.

In order for the public to be informed about methane concentrations that may be related to the natural gas plume, and specifically the potential distribution into the Porter Ranch community, ARB and the South Coast Air Quality Management District (SCAQMD) have combined their continuous methane monitoring efforts to display the combined near real time methane monitoring data in the community. Hourly results and previous day maximum results are available. The results from these monitors alone cannot be used to infer health effects, but they do provide a general sense of how much natural gas is in the community at a given time.

Methane is measured in parts per million, or ppm. Normal background is approximately 2 ppm. Results greater than 3 ppm suggest some additional source of methane, and results greater than 10 ppm suggests a considerable additional amount of methane is present. The highest level of methane measured in the community since the leak is 230 ppm, although such high levels have not been seen with any regularity. For comparison oxygen comprises approximately 20% of the atmosphere, or 200,000 ppm.

Map of Community Air Monitoring Sites

Real-time Methane Concentrations in the Porter Ranch Community:

The methane analyzer at Site #8 has been temporarily removed for maintenance and calibration.
 

  • Final Reviewed Methane Datasets (1-minute averages)**: 


** Final reviewed datasets have been reviewed by the CARB staff and corrected for instrument error and non-sampling activities (including changing of filters, challenging instruments with known standards, etc.).  External influences (such as cigarette smoking) are not known to us and - if they exist - are not excluded.  

Real-time Benzene Concentrations in the Porter Ranch Community:

*The benzene analyzer has a specified lower level of detection of 0.4 ppb.  Any reported measurements below 0.4 ppb have less accuracy.
 

  • Final Reviewed Benzene Datasets (15-minute averages)**: 
  • Final Reviewed Benzene Datasets (Hourly averages)**: 


** Final reviewed datasets have been reviewed by the CARB staff and corrected for instrument error and non-sampling activities (including changing of filters, challenging instruments with known standards, etc.).  External influences (such as cigarette smoking) are not known to us and - if they exist - are not excluded.

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