Evaluating development of empirical estimates using two top-down methods at midstream natural gas facilities

12 October 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


To align with climate initiatives, multiple reporting programs are transitioning from generic activity-based emission factors to site-specific measured emissions data to estimate greenhouse gas emissions at oil and gas facilities. This study contemporaneously deployed two top-down (TD) aerial methods across 14 midstream facilities, extending work done in Brown et al.1. Methods produced multiple whole-facility estimates at each facility, resulting in 773 individual paired estimates (same facility, same day), and robust mean estimates for each facility. Mean estimates for each facility, aggregated across all facilities, differed by nearly 2:1 (49% [32% to 68%]). At 6 of 14 facilities, the methods produced mean estimates that differed by more than a factor of two. These data suggest that one or both methods did not produce accurate facility-level estimates, at a majority of facilities and in aggregate across all facilities. Overall results are augmented with two case studies where TD estimates at two pre-selected facilities were coupled with comprehensive onsite measurements to understand factors driving the divergence between TD and bottom-up (BU) emissions estimates. In 3 of 4 paired comparisons between the intensive onsite estimates and one of the TD methods, the intensive onsite surveys did not conclusively diagnose the difference in estimates. In these cases, our work suggests that the TD methods mis-estimate emissions an unknown fraction of the time, for unknown reasons. While two methods were selected for this study, it is unlikely that the issues identified here are confined to these two methods; similar issues may exist for other similar whole-facility methods, on midstream and/or other facility types. These findings have important implications for the construction of voluntary and regulatory reporting programs that rely on emission estimates for reporting, fees or penalties, or for studies using whole-facility estimates to aggregate TD emissions to basin or regional estimates.


Methane emissions
oil and gas
aerial measurements
measurement-informed inventories
bottom-up inventories
emission inventories
top-down measurements

Supplementary materials

Evaluating development of empirical estimates using two top-down methods at midstream natural gas facilities - Supplementary Information
Contains additional details on methods, uncertainty analysis, statistical testing, facilities in case study, and facility data comparing TD methods over both deployments.


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