Ammonia concentrations and fluxes over a forest in the midwestern USA

We present measurements of ammonia (NH3) over a deciduous forest in souther n Indiana collected during four field campaigns; two in the spring during t he transition to leaf-out and two during the winter. Above canopy NH3 conce ntrations measured continuously using two Wet Effluent Diffusion Denuders i ndicate 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 Fores t 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 in verted concentration gradients of NH3 were observed resulting in an apparen t 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 amm onium nitrate particles may also be partly responsible for the observed inv erted concentration gradients. (C) 2001 Elsevier Science Ltd. All rights re served.