MEASUREMENTS OF THE HORIZONTAL GRADIENT OF AMMONIA OVER A CONIFER FOREST IN DENMARK

Horizontal gradients of ammonia have been measured in order to determi ne the spatial variation of the concentration above a forest as a func tion of distance from ammonia emitting areas. Measurements were carrie d out in June 1994 during conditions with winds from north and west. C oncentration levels of ammonia were generally low, ranging from 0.04 t o 0.7 mu g NH3-N m(-3). At northerly winds the concentration levels we re about four times higher than at westerly winds, which relates well to more intensive and larger areas of ammonia emission towards north o f the forest. The ammonia concentration decreases in the forest; in ge neral the percentage decreases were lower during northerly winds (16%) than those obtained during westerly winds (67%). This is probably due to the larger extension of emitting areas towards north than towards the west. Model calculations are comparable with the measured ammonia concentration gradients, although calculated concentration levels may be higher than measured due to the use of yearly average emissions as input to the model calculations. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.