Dry Deposition Monitoring in Europe

Between 1993 and 1999 two EU funded projects wereexecuted aimed at (i) the development of drydeposition monitoring methods for core sites andlarge scale application, (ii) the installation andrunning of three core sites in Europe and (iii) the improvement and validation of models used forregional application. This article provides anoverview of the development of depositionmonitoring stations and the main results of thethree core sites, which were operated between1995 and 1998. Furthermore, the results of thedevelopment of a low cost monitoring system arepresented. Continuous measurements were made ofboth wet and dry deposition of sulphur andnitrogen components and base cations. The 4 yearsof data show a decrease in sulphur loads and notrend for the other components. It is shown thatthe surface affinities for sulphur depositionalso changed during the years, underpinning theneed for dry deposition monitoring. A conditionaltime average gradient system was successfullydeveloped and tested and provides a good meansfor low cost monitoring of dry deposition fluxes.The costs can be reduced by a factor of 3–4 without losing the accuracy of the annual average gas fluxes.     

Measurement of the dry deposition flux of NH3 on to coniferous forest

The dry deposition flux of NH3 to coniferous forest was determined by the micrometeorological gradient method using a 36 m high tower. Aerodynamic characteristics of the site were studied, using a second tower erected in the forest 100 m from the first. Fluxes and gradients of heat and momentum measured on both towers indicated a fairly homogeneous turbulent flow field over the studied area of the forest. Site specific flux profile functions for heat were derived from continuous measurements of turbulent fluxes and gradients. These functions were used to derive fluxes from the observed gradients of NH3. In total, eighty 90-min NH3 flux runs were performed. The results indicate a strong nonstomatal uptake of NH3 by the forest. A representative dry deposition velocity for NH3 of 3.6 cm(-1) s was derived. The annual average flux was roughly estimated to be equivalent to 50 kg N ha(-1) yr, significantly higher than the critical load for coniferous forest.     

Monitoring and Modelling of Ammonia Concentrations and Deposition in Agricultural Areas of The Netherlands

Passive samplers were used from 1996 to 1999 in a dense network to monitorthe concentrations of ammonia in air, in four agricultural areas in The Netherlands. To show representative patterns, sampling was not made within 50 m of livestock buildings and stores. The concentration of ammonia varies typically between 10 and 40 g m^-3 within a few kilometres in these areas. The interpretation of the measurements was supported by calculations with OPS, a Lagrangian dispersion model. Model calculations were based on a high-resolution database that included estimates of the ammonia emission of each farm in the area and emissions from surface application of manure at a 250 × 250 m scale. The model underestimated the observed ammonia concentrations by nearly a factor of two over most of the area. This result was attributed to underestimation of the ammonia emission in the models. And the ammonia emissions from field application of manure seem to be seriously underestimated. A detailed analysis of model results and measurements showed that the observed decrease of the ammonia concentration in the study period was partly due to changes in meteorological conditions during the study period and partly due to the reduced amount of manure applied in 1998.