Measuring the nocturnal emissions of nitrous oxide (N2O) by N2O accumulation within the stable boundary layer

Micrometeorological methods of measuring trace gases such N2O usually are based on the assumption of turbulent transport. However during the night-time, the temperature gradient suppresses turbulence resulting in frequent periods of no or intermittent turbulence. Nocturnal emissions of N2O were evaluated using four N2O concentration measurement heights (1, 3, 5, and 8 m), three 3D sonic anemometer heights (3, 5, and 8 m) to measure turbulence at a 2.8 m high maize stand at West Lafayette, Indiana in 2013-2014. N2O was measured using a difference frequency generation (DFG) laser-based N2O analyzer with a resolution of 0.3 ppb. Ancillary measurements of soil temperature and moisture at 5 cm and 10 cm depth as well as air temperature, pressure, and humidity at 1.5 m were also made. N2O flux was determined under stable conditions using the time change over 1 ½ hours of three ½ hour mean N2O profiles under conditions with no detectable turbulence at the top of the profile. N2O accumulated most frequently over the period between 2000 and 2400 local time as the stability exceeded a z/L of +1. In general, the maximum measured N2O flux was proportional to the 5 cm soil moisture, declining throughout the study period from 90 g ha-1 d-1 to essentially zero. The N2O flux were also periodically measured using vented static chambers. The chamber-measured fluxes and the maximum accumulation measured flux were comparable.