Air Quality Criteria

This page was last reviewed on August 16, 2016

July 26th, 2016. After several months of typical ambient methane levels in the Porter Ranch community, 5 methane monitors (Sites 1, 2, 5, 7, 8) ,previously operated by CARB, have been removed from service in late July 2016. SCAQMD will continue to operate real time monitors in accordance to the Aliso Canyon Air Monitoring Plan, and will operate one of the two benzene monitors previously run by CARB. The Air Quality Criteria will now be updated weekly based on real time data from the remaining monitoring sites and the canister samples that will be collected once every six days. CARB 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 CARB website will be updated whenever new data of that type is available.


South Coast Air Quality Management District (SCAQMD) and California Air Resources Board (CARB) staff have jointly developed a set of criteria to determine when air quality in the surrounding communities of the Aliso Canyon facility has returned to typical conditions – a checklist to determine when the air quality is no longer being impacted by the leak.  

The Air Quality Criteria are based mainly on measurements of the chemicals in natural gas of concern: methane, hydrogen sulfide, mercaptans, and benzene.

To view measurement results, click SCAQMD data and CARB data.

Why Do Some Lab Samples Not Meet the Criteria?

The laboratory samples shown in yellow and marked “Did Not Meet Criteria” represent 12-hour samples collected by SoCalGas since February 12 that have methane greater than 3 parts per million. All other laboratory samples met their respective criteria, specifically for mercaptans, benzene and hydrogen sulfide. Continuous monitoring for benzene, methane and hydrogen sulfide also met all criteria.

The map below shows the dates and locations of samples exceeding the 3 ppm criterion. Methane exposure at these detected levels is not considered to be a health concern. The chart below shows the measured methane levels for all 12-hour samples collected by SoCalGas.

It is possible that continued off-gassing of residual methane in the soil near well SS-25 is causing higher measurements near the facility fence line (AA-01 through AA-06). However, the reason for the slightly higher than criteria levels of methane at community sites located further south is unclear. ARB and SCAQMD are investigating to determine the cause, including reviewing potential sources and quality assurance between laboratories.

Map of 12 hour samples with methane concentrations >3ppm
This chart shows the 12 hour methane concentrations from SoCalGas sampling compared to the Criteria. The Night samples are collected from 6:00 pm to 6:00 am the following day. A Night sample >3ppm therefore require the designation of two days as ‘Not Meeting Criteria’. Each dot represents a sample collected at one of the sites shown on the map above.