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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 4.1 Tracer Gas Correlation

4.1 Tracer Gas Correlation

The tracer gas correlation method is used to measure the total emissions of a pollutant, such as methane, over a large area by introducing a tracer gas with a known emission rate.  First, a tracer gas is selected that is chemically stable, is expected to fully mix with the pollutant, and is assumed to behave similarly to the pollutant under expected meteorological conditions.  Next, the tracer gas is released at a known emission rate at or upwind from the source.  Concentration measurements are taken downwind of the source, far enough to ensure adequate mixing of the gases.  Optical remote sensing technologies, most commonly FTIR or CRDS, are used to measure the ratio of the pollutant concentration in exceedance of background levels to the tracer gas concentration in exceedance of background levels.  This ratio, multiplied by the known tracer gas emission rate yields the estimated emission rate of the pollutant (USEPA 2011).  The following figures illustrate the orientation of the pollutant source and measurement location with respect to the wind direction.  

Wind direction 2

Tracer Gas Correlation Setup: (Monster et al. 2014) 

The advantages of the tracer gas correlation method are that its field units are lightweight, rugged, and easy to operate.  Also, the method takes weather conditions and other meteorological effects into consideration, yielding relatively high precision (USEPA 2011).  The limitations of the method are that the polluting source must be strong enough to be detected at significant distances downwind in order to allow for adequate mixing of the gases (Czepiel et al. 1996).  Also, tracer gas cylinders can be expensive, and the method requires that there is a road nearby the site that runs perpendicular to the wind direction from with to take the downwind measurements (USEPA 2011).  Lastly, some tracer gases that are typically used to quantify methane emissions, such as sulfur hexafluoride, are potent greenhouse gases; therefore, releasing these gases into the atmosphere is undesirable (Galle et al. 2001).

There are many researchers that have used tracer gas correlation methods to estimate methane emissions from a municipal solid waste landfill.  The following studies used tracer gas correlation with extractive FTIR as the optical remote sensing technique to measure the gas concentrations: Galle et al. 2001; Börjesson et al. 2009; and Scheutz et al. 2011 while the following used CRDS to measure the gas concentrations: Green et al. 2010; Mønster et al. 2014; and Taylor et al. 2016.  Additionally, Mønster et al. 2015 used both FTIR and CRDS optical remote sensing techniques for concentration measurements.  Of these studies, Galle et al. 2001; Scheutz et al. 2011; and Mønster et al. 2015 used nitrous oxide as a tracer gas, Green et al. 2010; Mønster et al. 2014; Mønster et al. 2015; and Taylor et al. 2016 used acetylene as a tracer gas, Czepiel et al. 1996 used sulfur hexafluoride as a tracer gas, and Scheutz et al. 2011 used carbon monoxide as a tracer gas.

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