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Methane Emissions from Free-Ranging Cattle: Comparison of Tracer and Integrated Horizontal Flux Techniques

^a Dep. of Chemistry, Univ. of Wollongong, Wollongong, NSW, Australia
^b National Inst. of Water and Atmospheric Research (NIWA), Wellington, New Zealand

View larger version (14K): [in this window] [in a new window]   Fig. 1. Schematic layout of the test plot site. Scale is approximate.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Time series of N2O (dashed lines) and CH4 (solid lines) mixing ratios and wind speed on 9 Sept. 1999. Data point labels are the measurement heights (m) for the corresponding point. The rightmost point in each series is the upwind mixing ratio.

View larger version (11K): [in this window] [in a new window]   Fig. 3. Methane emission rates for 9 Sept. 1999 calculated using the tracer and integrated horizontal flux (IHF) methods. Closed circles, tracer method by ratio; open circles, tracer method by regression; open squares, IHF method. The individual points are offset on the x axis for clarity. The error bars are the relative precision (1) errors on each point propagated through Eq. [1] assuming absolute errors of 0.5 nmol mol–1 for each N2O measurement, 4 nmol mol–1 for CH4, and 5% for the N2O release rate.

View larger version (9K): [in this window] [in a new window]   Fig. 4. Methane vs. nitrous oxide mixing ratios for the profile period beginning at 1320 h on 9 Sept. 1999. The labels are sample inlet heights and the error bars are the analytical errors, 4 nmol mol–1 for CH4, and 0.5 nmol mol–1 for N2O.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Sample profiles of [N2O(z)] and u(z) x [N2O(z)] on 9 and 16 Sept. 1999. The horizontal lines delineate the slabs whose areas are added to calculate the vertically integrated u x C product.

View larger version (9K): [in this window] [in a new window]   Fig. 6. Comparison of integrated horizontal flux (IHF)-derived N2O emission rate (points) and the release rate recorded at the capillaries assuming a constant emission at the daily average rate or a linearly decreasing rate between initial and final measured rates (lines).

View larger version (11K): [in this window] [in a new window]   Fig. 7. Times series of (a) vertical profiles of CO2 (solid lines) and N2O (dashed lines) and (b) calculated CO2 emission rates for 9 Sept. 1999.