Measurements of Methane Emissions from Landfills Using a Time Correlation Tracer Method Based on FTIR Absorption Spectroscopy BO GALLE* AND JERKER SAMUELSSON IVL Swedish Environm ental Research Institute, P.O. Box 47086, S-402 58 Go ¨teborg, Sweden BO H. SVENSSON AND GUNNAR BO ¨ RJESSON Department of Water and Environmental Studies, Linko ¨ping University, S-581 83 Linko ¨ping, Sweden Methane is an important climate gas contributing significantly to global warming. A large part of the anthropogenic emissions of methane comes from landfills. Due to the biogenic origin of these emissions and the inhomogeneous characteristics of landfills and their soil cover, these emissions show large spatial variation. Thus, development of reliable and cost-effective methods for measurements of these emissions is an important task and a challenge to the scientific community. Traditionally, field chamber methods have been used but also different area integrating methods based on downwind plume measurements. These measurements have been supported by meteorological data either directly from local measure- ments or by controlled release of tracer gas from the landfill providing the dispersion characteristics of the plume. In this paper we describe a method, the Time Correlation Tracer method, combining controlled tracer gas release from the landfill with time-resolved concentration measurements downwind the landfill using FTIR absorption spectroscopy. The method has been tested and used on measurements at a landfill in southern Sweden over the past 1.5 years. The method has proven to be a usable method for measurements of total methane emission from landfills, and under favorable meteorological conditions we estimate an achievable accuracy of 15-30%. The real time analysis capability of the FTIR makes it possible to judge the success of the measurement already on site and to decide whether more measurements are necessary. The measure- ment strategy is relatively simple and straightforward, and one person can make a measurement from a medium sized landfill (1-4 ha) within a few days to a week depending on the meteorological situation. Introduction Methane is a climate gas contributing to the greenhouse effect.The concentration in the atmosphere increasesglobally by 0.6-0.8% per year (1). An important source of methane is bacterial processes. Old landfills with urban waste are known to produce and emit considerable amounts of methane. The IPCC (1) has estimated that more than 10%, 20-70 Tg‚year -1 , of the total anthropogenic methane emis- sions, 385 Tg‚year -1 , originate from landfills (2). Besides developingand implementingnewwaste treatment practices such as incineration and separation at source,it is important to find methods to reduce the methane emissions from future and existinglandfills.The organic carbon available in a landfill is biogenically degraded to CH4 or CO2, about 50% of each. Since CH4 has a GWP (Global Warming Potential) that is 20 times stronger then CO2, a considerable improvement is possible ifa larger fraction ofthe carbon was emitted as CO2. There is in principle two ways to achieve this: by pumping the gas out and burning it or by methane oxidation via bacteria in the top soil. Both these methods are currently under strongdevelopment,and in order to validate the effect of different methods there is a need to be able to quantify the totalemissionsfrom landfills.Aproject hasbeen initiated amingat,via measurementsat representative sites,improving the national budget of methane emissions from landfills in Sweden as well as studying the effects of improved covering of leaking landfills. As a first step in this project, different measurement techniques for methane flux measurements were tested. A traditional and straightforward way to determine methane emissions from landfills is using field chambers (3, 4).Due to the spatialvariabilityin the emissions, a large number ofmeasurements are needed to characterize a site;however,stillthe errorsare considerable.Thus,different methods, integrating the total emission from the landfill by concentration measurements in the plume downwind ofthe site, have been tried in combination with dispersion model- ing. To constrain the model, meteorological measurements as wellas controlled tracer gas releases have been used. One method that has been used, to quantify fugitive emissions from the petrochemicalindustry,is based on a wellcontrolled release ofa tracer gas from locations simulatingthe expected source distribution at the site. The plume is then sampled downwind the site by means of bags or canisters along a transect perpendicular to the wind direction. The concen- tration oftracer CT aswellasthecompound understudy, CM, is then analyzed. With concentrations expressed in a mixing ratio, MT and MM being molecular weights of the respective gas, and knowing the emission rate of the tracer QT, the emission rate of the measured gas QM is obtained from eq 1. Provided that the tracer release simulates the source well, a good estimate ofthe emission can be obtained.The quality of the simulation is confirmed by the variation in ratio between source gas and tracer in the different samples (5, 6). Drawbacks ofthe method are the rather elaborate and time-consuming sampling procedure as wellas the fact that data are not available in real time, although analysis can be made on site. A variation of this method is to use mobile, sensitive, direct monitoring instruments for the plume traverses. Then real time information about the plume characteristics and thus the quality ofthe source simulation is available (7, 8).Atechnique that has proven to have a good potential for flux measurements of climate gases is Long- Path FTIR spectroscopy (9-16). Long-Path FTIR has been used, combined with tracer techniques, for measurements *Corresponding author phone: +46 317256200; fax: +46 317256290; e-mail: Bo.Galle@ivl.se. Q M ) Q T ‚ C M ‚M M C T ‚M T (1) Environ. Sci. Technol. 2001, 35, 21-25 10.1021/es0011008 CCC: $20.00  2001 American Chemical Society VOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 21 Published on Web 12/02/2000