Accurate measurement of landfill emissions has proven to be a challenge given these greenhouse gas emissions occur over a large, decentralized area that can span hundreds of acres.  This decentralization is what has made methane measurement from landfills more challenging than say an oil and gas refinery, where emissions tend to be from piping and equipment used to process these resources. Thus, from a measurement perspective, landfills tend to behave more like a non-point source rather than a point source, which is why accurate emissions measurement has proven elusive.

Modeling Emissions 

Mathematical models have been used for the past few decades as a way to estimate emissions and are still the primary method used by the EPA and other quasi-governmental entities (e.g. Intergovernmental Panel on Climate Control) to quantify emissions.  Significant investment in research by a variety of entities, including the EPA, the Environmental Research & Education Foundation (EREF) and others has advanced the utility and improvement of these models over time.  Today, models tend to be informed by a combination of actual data (e.g. collected landfill gas volume and % methane) and estimated model parameters (e.g. % methane oxidized, rate the waste decays in the landfill).

Modeling offers a number of advantages: the ability to estimate whole landfill emissions over time and the ability to account for landfill behaviors that impact emissions, like changes due to diurnal effects (day vs night) and weather.  Modeling is also relatively inexpensive.  However, estimating emissions using models comes with limitations: the parameters used may not accurately represent site specific conditions and may not characterize variability in emission rates from site to site and over time.  Most models do not account for operational issues, such as a leakage from pipes or seepage from the landfill cover.  Further, the ability to validate the accuracy of models has been challenging, which has led facility owners, in collaboration with researchers, to seek ways to directly measure methane emissions at sites.

Direct Measurement

Motivation to find a more accurate measurement method resulted in the development of a variety of ground-based measurement methods, such as flux boxes, meteorological methods and using lasers with mirror arrays.  Early efforts proved to be cumbersome and expensive.  Measuring emissions from the entire landfill at once also was not possible without additional modeling to account for data gaps, wind dispersion, and other factors.  Within the last decade, advances in methane detection technology, in combination with airborne measurement capabilities (via planes, balloons and drones) propelled direct measurement capabilities forward in a way that had never before been possible.  For the first time, the ability to consider directly measuring methane emission from an entire landfill at once was possible.  Today, an array of direct measurement strategies has come on the scene, with improved ground-based techniques such as tracer correlation, to fixed measurement systems being used, as well as airborne and satellite-based systems.

Direct measurement strategies have been a significant advancement and tend to be more straightforward than modeling since the variables used in modeling are accounted for in the direct measurement approach (e.g. methane oxidation).  From a practical standpoint, there are a number of obvious advantages to direct measurement strategies in that: (1) multiple techniques can be used to provide a comparative or side-by-side evaluation of methane emissions, (2) they allow for methane emission models to be validated and benchmarked against whole site data, and (3) they should be able to achieve a higher degree of accuracy than models.

Challenges and the Road Ahead

Why would the word “should” be used when it seems obvious that directly measuring something should be far superior to a model that only estimates it?  Although cliché, the devil truly is in the details.  While direct measurement offers distinct advantages, the strategy currently has a number of known limitations that still require further evaluation and research for full potential to be realized.  For airborne direct measurement technologies, the further away the measurement device is, the lower the resolution and accuracy tend to be.  For example, a drone flying less than 100 ft in altitude over a landfill would tend to have higher resolution than a satellite detecting a methane plume from space.  In this respect, lower resolution strategies may not be able to separate methane emissions drifting across a landfill from off-site sources (such as nearby agricultural sites, wetlands or other methane-emitters).  Another limitation is that most direct measurement strategies must be utilized during the day in relatively clear and calm weather conditions (especially for aircraft and satellite-based techniques).  Such conditions potentially skew aggregated annual measured values since it is well established that methane emissions are different at night and under different atmospheric conditions. 

Currently, most direct measurements made tend to be at a single point in time.  This creates a “snapshot problem” meaning methane emissions detected from a single for small number of measurements may not be representative of typical methane emission profiles over time.  For example, if direct measurements were conducted when a gas well was being installed or repaired, above average emissions may be detected.  If the measurements are then assumed to be representative and extrapolated to estimate annual emissions, the estimate would likely not be representative of the entire year.  While the snapshot problem is generally well-recognized, this presents a current challenge in that very little data exists and, despite significant advancements, continuous or high frequency measurements are expensive to perform or have not been fully vetted for accuracy.

While it may be perceived that direct measurements allow us to do away with mathematical models or algorithms, this is generally not the case right now.  Most direct measurement techniques still require modeling of atmospheric dispersion, wind and other meteorological attributes to come up with a whole landfill estimate of emissions. A lower resolution is the trade-off of not having to use modeling since such techniques tend to result in measurements that are taken further away from the landfill.

Collectively, the science and technology related to direct measurement strategies is evolving and full-scale deployment is still at an emergent stage.  Accuracy and reliability have not been fully tested/evaluated.  Modeled estimates and direct measurements have not consistently agreed with one another, which is an important factor since regulatory agencies like the EPA still use models in their estimates.  Standardized benchmarking (e.g. tracer correlation) and comparative technology assessments are needed to establish effectiveness, precision and absolute accuracy of direct measurement strategies. 

More data is needed to establish variability in landfill emissions, particularly over larger, annual time frames needed for emissions inventories used by federal agencies and intergovernmental efforts.  Continuous/high frequency measurements across multiple sites representing different geography and climate conditions would help overcome the snapshot problem.  Further, capturing changes in emissions at night is still a potential challenge, even with continuous direct measurement strategies, as most of these methods must be deployed during the day and under fair weather conditions.  In the face of these challenges, methane emissions measurement advancements have still been significant and, with continued efforts underway to address the above challenges, increased accuracy will continue to be realized, as well as broader adoption of direct measurement technology.