Airborne Flux Measurement of Agricultural Methane Emissions

In Canada, methane (CH₄) emissions from manure management and enteric fermentation are estimated to account for about 40% of the agricultural sector's greenhouse gas emissions, or about 3% of national emissions. Bottom-up estimates of national scale agricultural CH₄ emissions are based on a limited number of field measurements and emissions are highly uncertain. Top-down airborne flux measurements of CH₄ emissions are an option to verify and constrain regional scale bottom-up inventory estimates of emissions.

During the summer and fall of 2003 and 2004, as well as spring of 2011, aircraft-based flux measurement of CH₄, water vapour (H₂O), carbon dioxide (CO₂) and heat were conducted above an agricultural, primarily dairying region of Eastern Ontario, Canada, along multiple flight tracks, 10 to 20 km in length. During 2003 and 2004, flux measurements of H₂O, CO₂ and heat were conducted using the eddy covariance (EC) method, with a Rosemount 5-hole probe (model 858AJ28) to measure 3-dimentional wind speed, LI-COR (model LI-6262 and LI-7000) instrumentation for H₂O and CO₂ and Rosemount (model 102EAL) instrumentation for heat. No fast response CH₄ analyzer was available at this time for use on-board an aircraft, and therefore the relaxed eddy accumulation (REA) approach was used exclusively to measure the flux of CH₄. In flight air samples collected in pre-evacuated Teflon sample bags, were returned to the laboratory for analysis post-flight using a Campbell Scientific TGA-100. During 2011, similar instrumentation was used for the measurement of H₂O, CO₂ and heat, whereas EC measurement of CH₄ flux was possible due to the addition of a Picarro G2301 CH₄/CO₂/H₂O fast response analyzer. In 2011, REA air samples were collected in pre-evacuated metallic bi-layered sample bags, which were returned to the laboratory for post-flight analysis using a Picarro G1301 CH₄/CO₂/H₂O analyzer.

Instrumentation employed in 2011 offered many improvements over that used in 2003-2004. First, having a fast-response analyzer on-board the aircraft not only allowed for the EC measurement of CH₄ flux, but also allowed for the identification of confounding sources of emissions. In 2003-2004, a waste treatment facility was hypothesized to be a source of CH₄ emissions, though the extent of its influence was unknown. However, with the fast-response analyzer installed in 2011, an increase in CH₄ concentration above background on the order of > 1 ppm was detected at a distance of 0.5 km downwind of the waste treatment facility, which decreased to approximately 40 ppb above background at a distance of 25 km downwind. Second, the bi-layered metallic REA sample bags employed in 2011 were more conservative than the Teflon bags employed in 2003-2004. During testing in 2011 using the Picarro G1301, sample contamination was detected during the sample analysis time period (approximately 36 hours). However, this contamination could be eliminated through the use of the bi-layered metallic sample bags, which were used throughout 2011. Third, ‘null' testing, where samples of equal CH₄ concentration were added to two REA sample bags, then analyzed expecting a null concentration difference between samples, it was found that the Picarro G1301 offered comparable accuracy to that of the Campbell TGA 100, but about two times the precision. For a series of 10 null tests using the Picarro G1301, mean concentration difference was 28 ppt CH₄, with a confidence interval of 85 ppt.

Considering the advances made between 2003-2004 and 2011, we will restrict the following discussion to the 2011 measurements. An inter-comparison between the EC and REA methods was made using water vapour. Flight track averaged water vapour fluxes agreed well between the EC and REA approaches (R² = 0.78). Measurement of CH₄ flux was complicated by a waste treatment facility, a source of CH₄ emissions. Methane flux by the REA approach for flight tracks that were unaffected by the waste treatment facility (n=9) ranged from 0.2 to 1.6 kg CH₄ km^-2 hr^-1 and averaged 0.8 kg CH₄ km^-2 hr^-1. Average CH₄ emissions estimated using the EC approach were comparable, 0.9 kg CH₄ km^-2 hr^-1, with a range from 0.4 to 1.5 kg CH₄ km^-2 hr^-1, though agreement between EC and REA estimates was poor (R² = 0.2). The aircraft-based, top-down estimates of CH₄ emissions are approximately double the estimated rate of CH₄ emissions based on a bottom-up agricultural inventory approach.

It is hypothesized that the timing (spring, midday) of the aircraft-based measurements positively biased the EC and REA CH₄ emissions, but not sufficiently to account for the discrepancy between bottom-up and top-down measurements. Other reasons for the difference between top-down and bottom-up estimates, including systematic underestimation of emission factors, and unaccounted sources in the bottom-up approach are possible. Recent field scale research in the area of the aircraft-based measurements has found emissions from dairy manure management systems almost double the value currently used in the bottom-up approach. If the larger manure management factors are used, the top-down:bottom-up emissions ratio of decreases from 1.9 to 1.3. Therefore, although the bottom-up and top-down approaches better agree, the potential for unaccounted sources in the bottom-up approach cannot be ignored. This work demonstrates the value of aircraft-based measurements for constraining trace gas budgets and emphasizes the importance of a multi-scale, multi-method measurement approach to estimate agricultural trace gas emissions.