A significant diurnal pattern of ammonia dry deposition to a cropland is detected by an open-path quantum cascade laser-based eddy covariance instrument

Abstract

Large uncertainties and challenges remain in the quantification of land-atmosphere exchanges of ammonia (NH3), mainly due to limitations in measurement techniques. In this study, the eddy covariance (EC) technique based on an open-path quantum cascade laser (QCL) spectrometer was used to measure the NH3 fluxes at a typical cropland in the North China Plain, one of the global NH3 hotspots. It is the first attempt of measuring both NH3 volatilization and deposition fluxes at an agricultural ecosystem using the open-path instrument-based EC technique. During the one-month experimental period, the NH3 fluxes were characterized by enhanced emissions after fertilizer application and persistent dry depositions after ploughing practice, with the flux magnitude varying from −0.293 to 7.29 (median: −0.042) mg N m−2 h−1. Nearly 93% of the half-hourly fluxes (absolute values) exceeded the mean instrumental flux detection limit of 9.6 μg N m−2 h−1. Nitrogen loss rate from the wheat field through NH3 volatilization was 0.57–0.71% relative to the applied nitrogen fertilizers. Ammonia deposition dominated the period after the ploughing practice, showing an average deposition rate of 0.014 kg N ha−1 d−1 with a significant diurnal pattern. Both the local emission sources and meteorological conditions influenced the variation of NH3 dry deposition. The deposition fluxes measured by the EC technique were nearly one third the results modelled by the widely used inferential model method, but the difference needs to be validated by more observations that includes different surface and meteorological conditions. This study suggests that this open-path QCL-based EC instrument is an effective tool for long-term and high-frequency measurements of ecosystem NH3 exchanges. It also enables cross-validation between different methods to improve the understanding in the atmospheric reactive nitrogen cycling.