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 mug m(-3) during the spring and 0.3 mug 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 or nearly 0.2 mg-NH3 m(-2) h(-1). Analyses suggest that this apparent emission flux may be due to canopy emission but evaporation or ammonium nitrate particles may also be partly responsible for the observed inverted concentration gradients. (C) 2001 Elsevier Science Ltd. All rights reserved.