A Methane Emissions Reconciliation Exercise: Comparing Sub-Site Measurement-Based Emission Factor Estimates with Site-Level Measurements at Two LNG Facilities

This study presents the results from a comparison of measurement quantification methods of methane emissions from two onshore liquefied natural gas (LNG) export terminals, comparing site-level measurements, made using an in situ airborne technique, and estimates based on emission factors (EFs) derived from measurements using a remote sensing, ground-based, differential absorption LIDAR (DIAL) technique. The methane emissions from each site were quantified at an approximately one-year interval for each of the two techniques. DIAL was used to measure emissions at the sub-site, functional element (FE) level and calculate EFs for each FE using the specific FE activity data (AD). The total site methane emissions during the airborne measurements were estimated for each site using these EFs and the AD at the time. The results show the estimated methane emissions and the airborne measurements are close to agreement when considering the average of all the flight curtains (down to a 7% difference between uncertainty limits), whilst individual curtains were potentially significantly different. These results highlight the importance of fully characterising the methodology and uncertainty of both approaches. Using up-to-date, site-specific EFs or comparing over a statistically large sample size should improve agreement by reducing unknown emission uncertainties associated with site changes affecting the emission profile. Understanding each FE emission profile across a range of AD is critical to address potential differences due to non-linearity. It is important that accurate, specific and up-to-date AD is obtained to give a reliable estimate of emissions. The potential of the concept to estimate methane emissions from the FE EFs is demonstrated.