A micrometeorological mass balance approach for greenhouse gas flux measurements from stored animal manure

Storage of animal manure is estimated to be a significant agricultural source of the greenhouse gases methane (CH4) and nitrous oxide (N2O). These estimates are, however, quite uncertain due to the lack of in situ measurements of fluxes. The micrometeorological mass balance method (MMB) is well suited for non-intrusive flux measurements from these sources. In this paper, we propose a four-tower MMB approach for simultaneously measuring CH4 and N2O fluxes from outdoor animal manure storage tanks. This approach was evaluated in studies at two commercial swine farms in Ontario, Canada, over 2 years. Each of the four air sampling towers used was designated as an upwind or downwind tower depending on wind direction. Mean CH4 and N2O concentration differences between upwind and downwind tower positions were measured simultaneously using two tunable diode laser trace gas analyzers and a 16-air intake sampling system. Tower azimuth and 1 min wind direction means were used to obtain mean half-hourly fetch (path of airflow travel over source). Mean concentration profiles showed the liquid manure storage tanks were significant sources of CH4 but not N2O. The presence of CH4 sources in the area surrounding the manure storage tank affected background (or upwind tower) gas concentrations, and this effect was considered in flux calculations. Four sampling heights were sufficient to capture the profiles. Flux calculations were improved when the following filtering criteria were used to select data: wind veering within 30–90°, wind speed over 2 m s−1 at height 3.5 m, and restricting angles by 30° for tower designation. The simplest method for horizontal flux integration with height (rectangular method) yielded CH4 flux means similar to the other methods evaluated (regression, interpolation, trapezoidal). The four-tower MMB approach allowed for flux measurements regardless of wind direction, and fluxes obtained were representative of contrasting areas within the storage tank, allowing for evaluation of the spatial heterogeneity of the source.