ATMOSPHERIC AMMONIA EXCHANGE ON A HEATHLAND IN DENMARK

Passive flux samplers were used to determine the ammonia exchange on a n inland heath in Denmark over the last 2 years. The samplers measured the horizontal ammonia flux directly. Data were sampled continuously for periods of 1-4 weeks. The micro-meteorological gradient method was used with passive flux samplers and cup anemometers at different heig hts above the vegetation in order to calculate the vertical fluxes of ammonia. First a fixed sampler system was used with tubes mounted in f our orthogonal horizontal directions. This system has been successfull y applied to measure the emission from fertilized crops. Adapting this type of sampler to measure the deposition to heathlands did not prove to be straightforward. The precision of the calculated ammonia concen trations vias too poor to give an acceptable accuracy for the concentr ation gradient. The problems were (a) driving rain which entered the t ubes, (b) too few measuring points in each concentration profile, and (c) too long sampling periods to allow for the low-concentration level s above the heath arsa and the detection limits. Therefore, a passive flux sampler mounted on a wind vane and fitted with a rain shelter was developed. Results from the first 5 weekly periods are very promising , yielding accurate concentration gradients. The advantages of the pas sive flux samplers on the wind vane are (a) the minimum measuring peri od can be approximately halved compared to the fixed samplers,(b) some of the directional correction terms used with the fixed passive flux samplers are dispensed with, and (c) the field and laboratory work is minimized. (C) 1998 Elsevier Science Ltd. All rights reserved.