Ammonia concentrations and fluxes over a forest in the midwestern USA

We present measurements of ammonia (NH3) over a deciduous forest in southern Indiana collected during four field campaigns; two in the spring during the transition to leaf-out and two during the winter. Above canopy NH3 concentrations measured continuously using two Wet Effluent Diffusion Denuders indicate mean concentrations of 0.6–1.2 μg m−3 during the spring and 0.3 μg m−3 during the winter. Measurements suggest that on average the forest act as a sink of NH3, with a representative daily deposition flux of 1.8 mg-NH3 m−2 during the spring. However, on some days during the spring inverted concentration gradients of NH3 were observed resulting in an apparent upward flux of nearly 0.2 mg-NH3 m−2 h−1. Analyses suggest that this apparent emission flux may be due to canopy emission but evaporation of ammonium nitrate particles may also be partly responsible for the observed inverted concentration gradients.