Micrometeorological Gradient Research Articles

The dry deposition of ammonia onto a Douglas fir forest in the Netherlands

The dry deposition flux of ammonia (NH 3 ) to forest was measured using the micrometeorological gradient method. The measurements were carried out on a 36 m high tower in the forest, an approximately 18 m high Douglas fir stand. The deposition flux of NH 3 was calculated from the turbulent exchange coefficient and the concentration gradient of NH 3 measured between 20 and 36 m. The results indicate a strong uptake of NH 3 by the forest, especially under moist or wet conditions. The uptake rate of NH 3 by the canopy was significantly higher than could be explained by stomata] uptake alone. The canopy resistance for NH 3 correlates with water vapour pressure deficit and is a factor of five to ten lower than the stomata] resistance. Only at vapour pressure deficits higher than 10gm −3 is absorption of NH 3 less efficient. The observations can be explained by assuming a strong uptake by the leaf surface under wet or moist conditions and a less strong, possibly stomatal, uptake under dry conditions. To estimate the annual flux to the canopy, the experimental results were parameterized using a resistance layer model. This model was run using the results of detailed meteorological measurements carried out continuously on the site in the year 1989. The annually averaged deposition velocity to the forest was between 2 and 3 cm s −1 . Based on these calculations, and taking all uncertainties into account, the dry deposition flux of NH 3 was estimated to lie between 21 and 44 kg N ha −1 yr −1 These estimates are a little higher than previous estimates made on the basis of large-scale dispersion model calculations and significantly higher than the critical N load to coniferous forest (10–20 kg N ha −1 yr −1 ).

Continuous measurements of surface exchange of SO 2 and NH 3; Implications for their possible interaction in the deposition process

Simultaneous measurements of SO 2 and NH 3 dry deposition using the micrometeorological gradient method were performed continuously at two heathlands. It is concluded that dry deposition of both SO 2 and NH 3 at the two locations in general is limited by aerodynamic resistance during conditions with wet surfaces. The results of the measurements are used to investigate the influence of pollution climates on the dry deposition of SO 2 and NH 3. It is demonstrated that low surface resistances of SO 2 and NH 3 will occur in humid regions, as surface wetness is found to be the most important parameter. A high flux of one gas relative to the other can increase its surface resistance through saturation of the surface and lower the resistance for the other gas. During dry conditions, SO 2 and NH 3 uptake is controlled by stomatal behaviour of the heather plants. Upward fluxes of NH 3 have been observed during these periods. It is furthermore suggested that fog composition has a large influence on R c values, resulting in both high and low values. Frost was demonstrated to increase R c values of both gases.

Fluxes of physical quantities and ozone concentration within a fruit tree

Implicit to an understanding of the eco-physiological behaviour and yield of cultivated plants is a knowledge of air-vegetation interaction. One of the basic factors controlling the quantity and quality of plant growth is the turbulent exchange of such physical quantities as temperature and humidity, a fact making the impact assessment of gaseous pollutant deposition also important. Oxidants such as ozone are potentially harmful to plants via metabolic responses and cell injury and, once the threshold of adaptation is exceeded, the death of the plant may even ensue. Experimental application of the micrometeorological gradient method to a peach tree so as to determine the in-canopy transfer coefficients and to derive the fluxes of physical quantities and ozone is reported.

Monitoring the dry deposition of SO2 in the Netherlands: Results for grassland and heather vegetation

Abstract An automated system based on the micrometeorological gradient technique has been developed to measure the dry deposition of SO 2 on a routine basis. Measurements were made at two locations in the Netherlands. From these results dry deposition fluxes, dry deposition velocities and surface resistances for a heathland and for an agricultural grassland site were estimated using a selected set of data and a calculation procedure based on micrometeorological considerations. An extensive analysis was made to determine uncertainties in the resulting deposition parameters. From this analysis it has been concluded that the uncertainty in these parameters is almost completely determined by the random errors in measured concentrations. The meteorological surface exchange parameters can be estimated sufficiently accurately ( −1 were calculated for wet and dry conditions, respectively. At the heathland site, a similar distinct difference between R c values for wet and dry conditions was found. These values are 20(±21) and 70(±90) s m −1 , respectively. The yearly average dry deposition flux for SO 2 at the grassland site amounts to 585(±330) mol ha −1 yr −1 , while at the heathland site the yearly average flux was 300(±270) mol ha −1 yr −1 . The yearly average dry deposition velocity at 4 m height was 1.2(±0.3) cm s −1 at the grassland site and 0.8(±0.4) cm s −1 at the heathland site.

Measurement of the dry deposition flux of NH 3 on to coniferous forest

The dry deposition flux of NH 3 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 NH 3. In total, eighty 90-min NH 3 flux runs were performed. The results indicate a strong nonstomatal uptake of NH 3 by the forest. A representative dry deposition velocity for NH 3 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.

A micrometeorological investigation of surface exchange parameters over heathland

An automated system for measuring the dry deposition fluxes of SO2 on a routine basis based on the micrometeorological gradient technique has been developed for application over grassland and other short vegetation. The ability of the system to determine turbulent exchange parameters such as the friction velocity u* and the heat flux H was investigated by comparison with an automated eddy correlation system.