Introducing the persistency-weighted median flux rate
A new metric to help focus on the sources that matter most.
On the Eye on Methane data platform, IMEO's Methane Alerts and Response System (MARS) team aims to highlight meaningful emission by focusing on sources that are continuously emitting and therefore require greater attention and faster intervention.
To address these challenges, the persistency-weighted flux (PWF) visualization metric was developed. The PWF is designed to be an actionable and representative indicator of emissions based on MARS satellite data.
The PWF connects directly to IMEO’s mission by helping to:
- Identify very high, sporadic emissions that may indicate serious but irregular problems.
- Track lower-level, persistent emitters which may be overlooked by other metrics but cumulatively contribute significantly to emissions.
- Monitor mitigation progress by showing reductions or cessation of emissions over time.
By incorporating both magnitude and frequency of emissions, the PWF offers a stable, transparent, and policy-relevant view of the data – empowering users to focus on the sources that matter most for climate action.
Methodology
The PWF is a measure of methane of methane emissions that combines (1) how often a source is emitting (persistency) and (2) how strong the emissions are when active (flux rate) and above the instrument’s detection limit. It is calculated as:
PWF=P×Q
Where P is the persistence of emissions during the last six months and has a value between 0 and 1 (0 – no emissions and 1 – persistent). Q is the emission flux rate or strength of emissions above the detection threshold. PWF is calculated at the source level.
Persistence is calculated as the number of detections (D) divided by the number of observations (M):
P=D/M
Emissions could go undetected in poor quality observations. Including these observations in the calculation of P would artificially reduce the value of P. Hence, M only considers the good-quality observations (i.e. clear sky, low-noise images), determined based on the sensor’s probability of detection and the source typical emission strength (flux rate median).
If a source has no detections within the last six months, its persistency (P) is 0 and therefore PWF is 0, even if emissions were detected earlier. When there are too few good-quality observations to assess persistency, P is marked as undetermined and PWF cannot be computed. To ensure that these sources are not overlooked, we use minimum thresholds values for P and Q to render them in a faint colour.
Details on parameter setting for PWF in IMEO’s Eye on Methane data platform:
- Q = 50% = Median? Why did we choose the median? The median represents the typical emission rate detected and it is a stable metric over time.
- D – number of detections: How is the sensor detection defined? Number of plumes detected by MARS and attributed to the same source over a six-month period with the available constellation of satellites.
- M - observation sampling: We do not consider observations that have a probability of detection lower than 50 per cent for the typical detected flux rate. We consider only observations with clear skies that have a probability of detection higher than 50 per cent for the typical detected flux rate.
- P – persistency: timeframe – based on six months
Important notes / disclaimers
PWF is intended mainly for visual purposes, not for direct physical interpretation.
The time series of satellite detections involves observations that are irregularly sampled and the exclusion of low-level emissions below the sensor’s detection limit, not the full emission history of the source.