Dayle K. McDermitt

Impact of changes in barometric pressure on landfill methane emission

Landfill methane emissions were measured continuously using the eddy covariance method from June to December 2010. The study site was located at the Bluff Road Landfill in Lincoln, Nebraska, USA. Our results show that landfill methane emissions strongly depended on changes in barometric pressure; rising barometric pressure suppressed the emission, while falling barometric pressure enhanced the emission, a phenomenon called barometric pumping. There was up to a 35-fold variation in day-to-day methane emissions due to changes in barometric pressure. Wavelet coherence analysis revealed a strong spectral coherency between variations of barometric pressure and methane emission at periodicities ranging from 1 day to 8 days. Power spectrum and ogive analysis showed that at least 10 days of continuous measurements was needed in order to capture 90% of the total variance in the methane emission time series at our landfill site. From our results, it is clear that point-in-time measurements taken at monthly or longer time intervals using techniques such as the trace plume method, the mass balance method, or the closed-chamber method will be subject to large variations in measured emission rates because of the barometric pumping phenomenon. Estimates of long-term integrated methane emissions from landfills based on such measurements could yield uncertainties, ranging from 28.8% underestimation to 32.3% overestimation. Our results demonstrate a need for continuous measurements to quantify annual total landfill emissions. This conclusion may apply to the study of methane emissions from wetlands, peatlands, lakes, and other environmental contexts where emissions are from porous media or ebullition. Other implications from the present study for hazard gas monitoring programs are also discussed.

Novel design of an enclosed CO2/H2O gas analyser for eddy covariance flux measurements

This study describes design and field performance of a new enclosed CO2/H2O gas analyser, LI-7200. Unlike present closed-path analysers, this new instrument is designed for operation with short intake tubes, with the intention to maximize strengths and to minimize weaknesses of both traditional open-path and closed-path approaches. The study provides description of the instrument, shows the principles of its operation, and explains advantages of a new design. Field results are provided from three field experiments with the prototypes, and cover such parameters as high frequency air temperature and pressure fluctuations inside the sampling cell versus ambient conditions, instantaneous concentrations and cospectra for CO2 and H2O in comparison with open-path instrument, and eddy covariance hourly CO2 and H2O fluxes in comparison with both open-path and closed-path instruments. Field data loss inventory is also provided in comparison with open-path and closed-path gas analysers. The new enclosed design results in little data loss during precipitation and icing, similar to the closed-path design, but with a low power consumption and high field stability comparable to open-path instruments.