2.6 Measurements of deposition

2.6 Measurements of deposition

How is the amount of nutrients deposited from the atmosphere quantified? Why is it necessary to quantify it and how can it be done? Basically there are two ways; measurements and modelling. Although these ways may also be combined.

2.6.1 Measurements of deposition

Many different measurement techniques exist, some very complex, resource demanding and expensive in terms of equipment, some much more simple and likewise inexpensive. In general it is less difficult to measure wet deposition than dry deposition due to the nature of the deposition process; the chemical components are confined to the liquid of the rain drops, also after the deposition has taken place, whereas the dry depositing gases are not confined and can undergo physical transformations (e.g. re-emission). In both processes, however, the components can undergo chemical transformations after deposition.

2.6.1.1 Measurements of wet deposition

The basics of measuring wet deposition is to first collect the precipitation, then analyse it for content of different pollutants. There are two methods for collecting precipitation and numerous techniques for the subsequent analysis. The two collection methods are:

• Bulk sampling: The precipitation is collected in a bucket, which is open all the time. In most cases a net prevents leaves etc from falling into the bucket. This sampling method has a slight tendency to cause an overestimation of the wet deposition, because some material is also deposited when it is not raining (dry deposition). Due to the complexity of the dry deposition process, the amount of material dry deposited in the bulk sampler is however much smaller than the dry deposited material in the vicinity of the sampler. An example of a bulk sampler is presented in Figure 7.
• Wet-only sampling: The precipitation is collected in a bucket, which is only open when it rains. A sensor registers whether it is raining and the lid is automatically opened at the beginning of a rain event and closed at the end. Again some kind of net is built into the system to prevent contamination of the samples with organic material. This sampling method may cause underestimation of the wet deposition depending on the sensitivity of the sensor which determines the status of the lid (open or closed). An example of a wet-only sampler is presented in Figure 8.

Once the samples are collected there are many different ways of determining what chemical components they contain. A commonly applied technique is ion chromatography, other examples are atomic absorption methods, spectrophotometric methods or atomic emission methods.

2.6.1.2 Measurements of dry deposition

Measurement techniques for dry deposition can be divided into two categories: direct and indirect. When applying one of the direct methods the flux of the chemical component is determined explicitly. This is done either through a collection of the material at the surface or through a measurement of the vertical flux in the air just above the surface. When applying an indirect method the flux is not measured directly, instead the concentration or the vertical gradient of the concentraion is measured. Some examples of direct and indirect measuring techniques are listed below [411]:

• Surrogate surfaces (direct method): a surrogate surface, such as a filter substrate, is used to collect depositing material. This method is best for particles where the nature of the surface is less important.
• Natural surfaces (direct method): analysis of material deposited on natural surfaces by e.g. leaf washing or snow analysis. This approach is ineffective for gases due to the chemical binding of gases to the surface.
• Chamber method (direct method): deposition to surfaces in a chamber (soil or vegetation) can be calculated by measuring the fluxes in and out of the chamber over a given time interval. This method is suitable for gaseous species, however establishing the chamber is resource consuming both in terms of time and money.
• Eddy Correlation (direct method): statistical correlations of the fluctuations in wind and concentration fields are measured to directly obtain values of the associated vertical fluxes. If the fluctuations correlate, high-speed measurements of vertical velocity and concentration are used to derive the vertical turbulent flux. Assuming theat the turbulent transport is dominating and that chemical reactions are absent, this is a direct measure of the local vertical flux at the measurement point, and the deposition velocity can be obtained by dividing the flux with the mean concentration. This method is suitable for gaseous species, however very fast-response equipment is required.
• Eddy Accumulation (direct method): In this method air is collected in two separate containers with the vertical velocity determining which container receives the sampled air. One container is used for positive vertical velocities and the second is used for negative vertical velocities the instantaneous sampling rate for each container is proportional to the magnitude of the velocity. The collected air is then analyzed for the species of interest and the results are used to calculated the net flux.
• Gradient Method (indirect method): In this method the deposition velocity is determined by measuring the vertical concentration gradient of the depositing chemical component and then the theory of gradient-transport is used to determine the associated deposition flux. An example of a gradient method measuring system is presented in Figure 9.
• Inferential Method (indirect method): Measured ambient concentrations at a particular reference height are multiplied by a deposition velocity assumed to be representative of the local surface to compute the dry deposition rate. This method is dependent on the availability of (routine) measurements of air concentrations and the derived values for the deposition flux are clearly dependent on the validity of the estimated deposition velocity.

Generally the measurement methods related to micrometeorology (e.g. the fluctuations in the wind or the direction of the vertical wind speed) are dependent on the turbulent characteristics of the area where the measurements are performed. In most cases the deposition fluxes derived using these methods are representative for a larger area than just the local area where the measurement was made. In contrast, the methods related to the properties of the surface provide dry deposition fluxes that are representative of the spatial scale of the piece of surface that is analysed. Therefore if the surface characteristics are inhomogeneous the measured deposition flux is not representative of a very large area. In most cases the micrometeorological measuring techniques produce results that are representative for a larger area compared to the surface measuring techniques.

2.6 Measurements of deposition