Performance and methodological evaluation of a quantum-cascade-laser photoacoustic aerodynamic gradient system for field-scale NH₃ flux measurements

Abstract. Reliable quantification of ammonia (NH₃) surface–atmosphere exchange remains challenging due to the compound’s reactivity, inlet interactions, and the sensitivity of gradient-based flux estimates to instrumental response characteristics. We present the field evaluation and methodological assessment of a quantum-cascade- laser (QCL) based photoacoustic (PA) aerodynamic gradient system for half-hourly NH₃ flux measurements under agricultural conditions. The campaign covered a 54-day post-fertilization period under predominantly dry soil conditions, including the transition from bare soil to a developing winter rapeseed canopy. Instrumental performance was assessed through co-located inlet comparison experiments, yielding a random uncertainty of ±2 ppb (1σ) and demonstrating negligible systematic bias between sampling channels. The system operated continuously under field conditions with active thermal stabilization and humidity management. Fluxes were calculated using Monin-Obukhov similarity theory (MOST) across an ensemble of universal stability functions to evaluate methodological sensitivity. Sensitivity analysis indicated that variability attributable solely to stability-function selection remained small relative to observed diurnal flux amplitudes. Mean NH₃ emission during the investigated period was 1.85 nmol m^–2 s^–1, corresponding to 1.21 kg N ha^–1 or 4.0 % of the applied fertilizer nitrogen. A pronounced diurnal asymmetry was observed, with daytime emissions approximately one order of magnitude higher than nighttime values, reflecting strong coupling between turbulent exchange and radiation-driven surface processes. Complementary machine-learning analysis indicated that incorporating short-term temporal memory substantially improved the representation of NH₃ flux dynamics and revealed distinct daytime and nighttime exchange regimes. The combined QCL–photoacoustic gradient system demonstrated robust field performance and low instrumental bias, supporting its applicability for long-term field-scale studies of reactive trace gas exchange.