Nitrous oxide emissions from a dung heap measured by chambers and plume methods

Dung heaps provide a large, spatial and temporal variable, source of the greenhouse gas N2O. In this paper emission rates measured by static and flow through chamber methods, which enclose only a small area of the heap, were compared with Gaussian plume and tracer ratio methods, which measure the emissions from the entire dung heap. The dung heap was a 300 m3 heap, composed of material from nearby cattle sheds. From the flow through and static chambers it was estimated that the dung heap emitted 315 and 51 g N2ON m−3 day−1, respectively. The spatial variability between the chambers and chamber methods was large. Standard deviations of the mean fluxes were >75% of the average flux. The smaller emissions were measured on the slopes of the heap and the larger emissions on the ridge. The plume of N2O was measured downwind of the dung heap by (1) tunable diode laser spectroscopy and calculation of the N2O source strength of the heap using Gaussian plume theory and (2) tracer ratio method releasing SF6 from the heap summit and capture in Tedlar bags downwind with subsequent analysis by gas chromatography. The Gaussian plume theory calculated an average N2O source strength of 5.3 g N2ON m−3 day−1 (1.4-6.7 g N2ON m−3 day−1). The tracer ratio method calculated a slightly larger average emission rate of 14.4 g N2ON m−3 day−1 (7.4-38.6 g N2ON m−3 day−1). Both methods were successfully validated by point release of SF6 and N2O, which suggests that the micrometeorological methods provided a good estimate of the source strength of the heap, whereas the few chamber measurements overestimated its source strength.