Methane exchange in a boreal forest estimated by gradient method

Forests are generally considered to be net sinks of atmospheric methane (CH4) because of oxidation by methanotrophic bacteria in well-aerated forests soils. However, emissions from wet forest soils, and sometimes canopy fluxes, are often neglected when quantifying the CH4 budget of a forest. We used a modified Bowen ratio method and combined eddy covariance and gradient methods to estimate net CH4 exchange at a boreal forest site in central Sweden. Results indicate that the site is a net source of CH4. This is in contrast to soil, branch and leaf chamber measurements of uptake of CH4. Wetter soils within the footprint of the canopy are thought to be responsible for the discrepancy. We found no evidence for canopy emissions per se. However, the diel pattern of the CH4 exchange with minimum emissions at daytime correlated well with gross primary production, which supports an uptake in the canopy. More distant source areas could also contribute to the diel pattern; their contribution might be greater at night during stable boundary layer conditions

Improved temporal resolution in process-based modelling of agricultural soil ammonia emissions

An emerging environmental issue in Canada is how to quantify the contribution of agricultural soil emissions of ammonia (NH3) to environmental pollution. Emission inventories are essential to predict these emissions and their subsequent atmospheric transportation, transformation, and deposition. Due to the high spatial and temporal variability associated with NH3 emissions, emission inventories based on measurements become expensive and emission factors lose accuracy. Process-based models are capable of accounting for the complex soil interactions, but current models lack temporal refinement and few models consider NH3 emissions. This paper presents the development of a one-dimensional (vertical), time-dependent model capable of predicting NH3 emissions from a slurry applied to a bare soil. The model is based on chemical, physical and biological relationships that govern soil heat, moisture, and nitrogen movement. Processes considered include convection, diffusion, decomposition, nitrification, denitrification, and surface to atmosphere transport. The model is tested with experimental data from Agriculture and Agri-Food Canada which conducted NH3 measurements following application of dairy cattle slurry to a bare field. An investigation into the sensitivity of emissions to pH and slurry infiltration rate is conducted and model predictions are best fit to measurements based on this investigation. Testing demonstrated the model's ability to predict the large NH3 emissions immediately following application and subsequent emission trends associated with diurnal patterns that emission factors cannot capture. Results showed that model performance could benefit from a more in depth measurement program and empirical or process models of surface pH. Potential exists for the model to become a useful tool in predicting emissions on local, regional, or national scales.13 page(s

Towards standards for measuring greenhouse gas fluxes from agricultural fields using instrumented towers

This is a discussion of the available technology for measuring turbulent fluxes using instrumented towers. This review focuses on the flux measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) for agricultural systems and the development of standards and protocols for measuring them. Agroecosystems present unique challenges because they undergo large leaf area index (LAI) and canopy architecture changes in a relatively short period of time (i.e., months) coupled with the fact that many of the greenhouse gas sources are diffuse. This review examines all aspects of the theory and application of the micrometeorological techniques, with focus on the flux gradient, eddy accumulation and eddy covariance techniques. Instrument placement, sens or response and noise characteristics are also explored. Innovative applications of micrometeorological methods are discussed for closed- and open-path trace gas sensors and commonly used meteorological instrumentation. The use of fast response single-pass optical tunable diode laser (i.e., CH₄, N₂O) and infrared gas analyzers (i.e., CO₂, H₂O) is described. Consideration is also taken of the trace gas sensors’flow system design, mixing ratio measurement, and data acquisition and reduction requirements for micrometeorological flux measurement. Procedures are outlined for the meteorological instrumentation necessary for eddy covariance-based energy budget measurement including ultrasonic anemometry.28 page(s

Application of a tunable diode laser to the measurement of CH₄ and N₂O fluxes from field to landscape using several micrometeorological techniques

Methane and nitrous oxide fluxes were measured at the field to landscape scale by combining tunable diode laser (TDL) technology with micrometeorological techniques. The three following micrometeorological measurement platform were used: (1) tower-based eddy covariance and flux-gradient techniques for measuring methane fluxes over the Canadian boreal forest and nitrous oxide over agricultural fields; (2) blimp-based nocturnal boundary layer (NBL) budget technique over agricultural areas for measuring nitrous oxide fluxes and (3) the NRC Twin Otter aircraft-based relaxed eddy accumulation (REA) to measure methane fluxes over the Canadian boreal forest. The techniques were applied using the TDL in situ (field applications) as well as in the laboratory (environmentally controlled conditions for very high resolution). The latter is accomplished by decoupling in situ air sample collection using PTFE bags from the actual air sample analysis using the TGA100. The application of the TDL to REA and NBL flux measurement is unique. Its versatility is illustrated using results obtained from experiments carried out at scales varying from field to farms to landscape. The results of these experiments clearly demonstrate the importance of knowing the variability of fluxes temporally and spatially to broach issues of scaling up to obtain fluxes representative of large areas.15 page(s

Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia

A compact, fast-response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of ammonia (NH<sub>3</sub>) has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 0.5 L multiple pass absorption cell with an effective path length of 76 m. Detection is achieved using a thermoelectrically-cooled Mercury Cadmium Telluride (HgCdTe) infrared detector. A novel sampling inlet was used, consisting of a short, heated, quartz tube with a hydrophobic coating to minimize the adsorption of NH<sub>3</sub> to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles, and additional ports for delivering NH<sub>3</sub>-free background air and calibration gas standards. The level of noise in this instrument has been found to be 0.23 ppb at 1 Hz. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser Absorption Spectrometer (TDLAS) during a laboratory intercomparison. The effect of humidity and heat on the surface interaction of NH<sub>3</sub> with sample tubing was investigated at mixing ratios ranging from 30–1000 ppb. Humidity was seen to worsen the NH<sub>3</sub> time response and considerable improvement was observed when using a heated sampling line. A field intercomparison of the QC-TILDAS with a modified Thermo 42CTL chemiluminescence-based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May–July 2008. Background tests and calibrations using two different permeation tube sources and an NH<sub>3</sub> gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation at 1 min time resolution (<i>R</i><sup>2</sup> = 0.93) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC-TILDAS. An overall good correlation of <i>R</i><sup>2</sup> = 0.85 was obtained over the entire two month data set, where the majority of the spread can be attributed to differences in inlet design and background subtraction methods

Tools for quantifying N₂O emissions from agroecosystems

The importance of constraining the global budget of nitrous oxide (N₂O) has been well established. The current global estimate of the contribution of N₂O to total anthropogenic greenhouse gas emissions from agriculture is about 69%. Considerable progress has been made over the past few years in developing tools for quantifying the emissions from agricultural sources, at the local and field scale (i.e., chamber and tower-based measurements) as well as at the landscape and regional levels (i.e., aircraft-based measurement and modelling). However, aggregating these emissions over space and time remains a challenge because of the high degree of temporal and spatial variability. Emissions of N₂O in temperate climate are largely event driven, e.g., in Eastern Canada, large emissions are observed right after snowmelt. The average emissions during the snowmelt period vary considerably, reflecting the influence of many controlling factors. Cumulative emissions reported here range from 0.05 kg N₂O-N ha−1 in Western Canada to 1.26 kg N₂O-N ha−1 in Eastern Canada, values that reflect differences in climatic zones and fertilizer management practices. This paper describes the tools for refining the global N₂O budget and provides examples of measurements at various scales. Tower-based and aircraft measurement platforms provide good data for quantifying the variability associated with the measurements. Chamber-based methods lack the temporal and spatial resolution required to follow the event driven nature of N₂O fluxes but provide valuable information for evaluating management practices. The model DeNitrification and DeComposition is an example of a technique to estimate N₂O emissions when no data is available.17 page(s