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Estimating methane emission durations using continuous monitoring systems
Daniels, William S.
Daniels, William S.
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2024-04
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Abstract
Updates to the EPA's Greenhouse Gas Reporting Program Subpart W will come into effect in January 2025, which include a requirement to report all "maintenance or abnormal emission events." Estimating the duration of emission events is critical for accurate reporting, as the total emitted volume depends heavily on the length of the emission. If an operator is unable to estimate a start and end date for a given emission, a duration of 6 months must be assumed. Infrequent sampling surveys via, e.g., an airplane can provide rough estimates of emission duration, but the minimum duration estimate from this method is bounded by the sampling frequency, which is often quarterly at best. Continuous monitoring systems (CMS), on the other hand, measure methane concentrations in near-real time and hence provide a promising avenue for more robust, measurement-informed emission duration estimates. Here we present a method for creating duration estimates using CMS data. Our proposed method uses a gradient-based spike detection algorithm to cluster enhancements in the concentration time series into events and quantifies uncertainty by assessing the information content of the underlying concentration data as a function of wind direction. We present an evaluation of the method on controlled release data and apply it to a production oil and gas site in the Appalachian basin. We compare duration estimates from our method to estimates provided by infrequent aerial sampling.
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