Agricultural gas emissions during the spring thaw: Applying a new measurement technique

A new micrometeorological technique is applied to measure gas emissions from soils. The technique relies on a single open-path FTIR sensor (OP-FTIR) with motorized aiming to give gas concentrations along vertically separated paths (not necessarily parallel with each other). Emission rates are inferred from the vertical difference in concentration using two alternative methods: flux-gradient and inverse dispersion calculations. Our objective is to assess the capability of the technique in a field study measuring nitrous oxide (N2O) and ammonia (NH3) emitted from cattle overwintering areas during the spring thaw. Two field configurations were examined: a slant path configuration in which the OP-FTIR is aimed directly at high and low reflectors at the far end of the path (average vertical path separation similar to 1 m), and a periscope configuration where the lower FTIR path was directed closer to ground along the whole path (average path separation similar to 1.5 m). Measured emission rates were generally above the detectability threshold of the system and consistent with the scientific literature showing an emission rise during thawing. At one of our sites the pulse of N2O emitted during thawing was among the largest reported (9.9 kg N-N2O ha(-1) during April). Of the two alternatives tested for calculating emissions, the inverse dispersion approach is more flexible, but with a computation time that can be prohibitive. With large measurement fetches the flux-gradient approach can be equally good and computationally faster. We conclude that the open-path gradient system provides a practical option for studying emissions in difficult environments. (C) 2016 Elsevier B.V. All rights reserved.