Measurement of short-range dispersion and deposition of ammonia over a maize canopy

This study reports extensive measurements of atmospheric ammonia (NH3) concentration within and above a maize canopy, downwind of a controlled line source placed at the top of the canopy emitting between 830 and 1510 μg NH3 m−1 s−1. The NH3 horizontal fluxes and the cumulated deposition were assessed with mass-balance masts located at different distances from the source. Thirty acid-coated denuders were placed between x=12 and 162 m, where x is the downwind distance from the source. Standard micro-meteorological variables as well as canopy-specific variables were measured. Eight series of measurements are reported in total. The measured concentration decreased sharply from between 30 and 550 μg NH3 m−3 at x=12 m to between 2 and 23 μg NH3 m−3 at x=162 m. Wind speed profiles are comparable to previously reported measurements within and above canopies, and temperature profiles exhibit unstable stratification at the top and the bottom of the canopy, but seem to show a temperature inversion within the canopy. The mass balance estimate of the total horizontal flux at x=12 m compares well with the source strength, although this indicates an overestimation of the mass balance method, since the flux above the mast was not taken into account. The cumulated deposition between x=12 m and 162 m was estimated to range between 5 and 30% of the emitted NH3, although the mass balance technique was proved to be very uncertain due to difficulties in estimating both vertical and horizontal fluxes. The dataset presented here constitutes a reference against which within-canopy short-range NH3 deposition models can be validated, and sensitivity studies can be done to better understand the underlying processes.