FAQ | Umweltbundesamt

A16) How can I estimate the rough order of magnitude of the expected concentration?

In a dispersion calculation, the concentration is influenced by many factors. In addition to the emission rate and the vertically (and possibly horizontally) inhomogeneous meteorological conditions, these include plume rise, deposition, sedimentation (in the case of heavy dusts) and chemical conversions (in the case of gases). In addition, the concentration depends on the distance from the source and the height above the ground. The averaging time (hourly average, annual average) also plays a role.

It is often helpful to make a rough estimate of the order of magnitude in which the calculated concentrations would be expected (e.g. a few µg/m³ or a few mg/m³ ?). Here it is not a factor of 2 or 4 that is important, but rather a factor of 100 or 1000. A rough, but very simple estimate is given below. It neglects deposition and conversion processes and applies to distances from the source that are significantly greater than the effective source height:

• Single plume
  Conservation of mass requires that the mass flow through the plume cross-section is equal to the emission rate: A*c*u = Q (plume cross-section A, mean concentration across the plume cross-section c, mean wind speed u, emission rate Q). This gives the mean concentration c = Q/(A*u). With neutral stratification, the plume expansion is typically in the range of 10 degrees, so the mean radius is r = x*tan(10) with the source distance x. This results in A = pi*r*r = 0.1*x*x and consequently c = 10 * Q/(x*x*u). For particularly unfavorable situations (e.g. low sources and very stable stratification), the pre-factor can be one or two orders of magnitude larger. The estimate is therefore
  
  c = f * Q/(x*x*u)
  Average conditions: f = 10
  Unfavorable conditions: f = 10..1000
  
  
• Annual mean
  If a cylinder with radius r is placed around the source, the mass flow through the cylinder must be equal to the emission rate. For an isotropic wind rose over the year and uniform vertical mixing up to the height h, the mean concentration over this cylinder with the area A = 2*pi*r*h is constant. It results from A*c*u = Q to c = Q/(2*pi*r*h*u). In the limiting case of very large distances, the concentration is vertically homogeneously distributed over the mixing layer height (on average about 800m). If half the mixing layer height is chosen as a reference value for h, then the following results as an estimate
  
  c = Q/(2500m*x*u)

Examples:

• Example calculation x/h50a11: The emission rate of substance xx is 5.56 g/s, for the annual average at a distance of 2 km and a wind speed of 5 m/s, the estimate yields 5.56g/s/(2500m*2000m*5m/s) = 0.2 µg/m³. The calculated value (file xx-j00z.dmna) is between 0.1 and 3 µg/m³ depending on the direction from the source.
• Example calculation x/tower: The emission rate of the substance so2 is 1000 g/s, for the annual average at a distance of 10 km and a wind speed of 5 m/s, the estimate provides 1000g/s/(2500m*10000m*5m/s) = 8 µg/m³. The calculated value (file so2-j00z.dmna) is between 0.2 and 2 µg/m³, depending on the direction from the source.
• Example calculation x/stoer-1: The emission rate of substance xx is 6800 g/s, the wind speed is around 3 m/s with indifferent stratification. For the 3-minute average (single plume) at a distance of 100 m, the estimate yields 10*6800g/s/(100m*100m*3m/s) = 2 g/m³. The calculated value (file xx-002z.dmna) is 0.06 g/m³. This is in the lower range of what would be expected based on the estimate. In this case, the plume is strongly widened due to the release close to the ground in combination with the large roughness length of 1 m.
• Example calculation x/besmax-1: The emission rate of substance xx is 27.8 g/s. For the highest hourly average at a distance of 1000 m (most unfavorable situation), the estimation with f=100 and u=3 m/s provides the value 100*27.8g/s/(1000m*1000m*3m/s) = 0.9 mg/m³. The calculated value (file xx-s00z.dmna) is 0.7 mg/m³.

A15) Why does BESMAX not carry out the calculation?

BESMAX only performs the calculation if the original flag library is available. Check that you have downloaded the flag library in full, unpacked it and copied it to the BESTAL folder. The subfolder "jar" must contain the folder "plumes", it must contain 29 folders and 1,508 files and be 974,663,522 bytes in size. The CRC checksum of the "plumes" folder is 8D48FD38.

A14) Why does AUSTAL show such a high maximum of wet deposition near the source?

As an implementation of the dispersion model of TA Luft 2021, AUSTAL 3 takes wet deposition into account according to the procedures of the standards VDI 3945 Part 3 and VDI 3782 Part  5. Wet deposition is quantified by the parameter washout rate. The rate depends on the pollutant properties and the rate of precipitation. It is often determined empirically, i.e. derived from measurements, and describes the proportion of a gaseous or particulate substance that is washed out of the atmosphere and deposited in the soil per unit of time during a precipitation event. In the convention of the standard VDI 3782 Part 5 (April 2006 edition), which AUSTAL applied up to version 3.1.2, the input into the soil is calculated exactly under the position at which the substance is washed out of the atmosphere. In numerical models with horizontal mesh sizes of several hundred meters and more, this approximation has no significant influence on the spatial distribution of deposition as calculated by the model.

However, this convention can lead to artifacts in the vicinity of a tall chimney if the mesh size is very small, as is usually used to improve the resolution of buildings: The model then records a sharply localized high maximum of wet deposition immediately next to the chimney as well as in the adjacent grid cells. In reality, however, the raindrops are drifted by the wind and the wet deposition therefore occurs further away from the source and is distributed over a larger area. In order to be able to compare the parameters of the additional deposition load or the total additional deposition load at assessment points for high sources with the immission values for pollutant depositions without further spatial averaging, the drift of the raindrops should be taken into account in the dispersion model when designating wet deposition. Such a procedure is to be described in the new version of the standard VDI 3782 Part 5 and has been implemented in a preliminary from as an option in AUSTAL Version 3.2 (NOSTANDARD option WETDRIFT).

A13) How to classify the critics by R. Schenk in journal "Immissionsschutz" 01/2015?

The regulation TA Luft states how the public is to be protected from hazardous air pollution in the context of plant licensing according to the Federal Immission Control Act. Part of the regulation is a dispersion model based on guideline VDI 3945 Part 3, for which the reference implementation AUSTAL2000 has been developed on behalf of the Federal Environmental Agency. Entitled "AUSTAL 2000 ist nicht validiert" (AUSTAL 2000 not validated), R. Schenk provides in journal "Immissionsschutz" 01/2015 several examples to falsify AUSTAL2000 and guideline VDI 3945 Part 3.

A more detailed analysis reveals that the results by AUSTAL/AUSTAL2000 are correct and that the contradictions highlighted by R. Schenk are based on fundamental errors in his argumentation. A detailed explanation is provided in "Immissionsschutz" 03/2015 (Alfred Trukenmüller, Wolfgang Bächlin, Wolfram Bahmann, André Förster, Uwe Hartmann, Heike Hebbinghaus, Ulf Janicke, Wolfgang J. Müller, Jost Nielinger, Ralf Petrich, Nicole Schmonsees, Uwe Strotkötter, Thomas Wohlfahrt, Sabine Wurzler: Reply to the critics of Schenk to AUSTAL2000 in Immissionsschutz 01/2015 - AUSTAL2000 is verified and validated; Erwiderung der Kritik von Schenk an AUSTAL2000 in Immissionsschutz 01/2015 - AUSTAL2000 ist verifiziert und validiert).

A12) What must be considered for the spatial average of odor hour frequencies?

The existance of an odor hour is detected by internal comparison of the hourly mean concentration in a given grid cell  with the threshold value. For this purpose the concentration must be spatially resolved  with a sufficient accuracy. For hourly means, in general a finer spatial resolution is required than for long time means. This aspect can be of particular importance at small distances to near-ground point sources. As a rule of thumb for such a case, at least 5 grid meshes should be between the source and the point of interest. If the grid meshes are too large, the hourly concentration plume is not well resolved and the resulting odor hour frequency may be under- or overestimated, depending on the emission rate.

Based on the results of a sufficiently resolved dispersion calculation, larger spatial averages can be derived. For example the auxiliary program A2KArea.jar can be used for this purpose. This distinctive feature of spatially averaging odor hour frequencies is due to the non-linear relationship between odor hour and concentration (yes/no decision: concentration above or below a threshold). For the spatial average of concentration in contrast, the values provided by the dispersion program on a coarse calculation grid can be directly used.

A11) What must be considered for the specification of coordinates?

1. All coordinate specifications are in meters.
2. All coordinate specifications except for the reference point must be of absolute value smaller 200000. The reference point must be chosen accordingly.
3. In order to avoid internal rounding errors, specification of calculation grid (left and lower border, mesh width), of the reference point, and of anemometer position should be made in integer meters.

A10) How are dust components specified and evaluated?

For the substances

• pm (dust, unspecified),
• as (arsenic),
• pb (lead),
• cd (cadmium),
• ni (nickel),
• hg (mercury),
• tl (thallium),
• xx (unknown)

the diameter classes 1 to 4 and class "unknown" according to Annex "Dispersion Calculation" of TA Luft can be applied by appending "-1", "-2", "-3", "-4", or "-u" to the substance name (e.g. xx-1).  The components "1" and "2" define the parts of PM-10. The components "3", "4", and "u" are handled inside the dispersion calculation separately from components "1" and "2" because they have different settling velocities.

The concentration files (e.g. pb-y00a.dmna) contain the concentration sums of components "1" and "2", because the imission values of TA Luft refer to PM-10 (for substances hg and xx, the gaseous component is also included in the sum). The deposition files (e.g. pm-depa.dmna) contain the sum over all components.

If one is interested in the concentration of one of the components "3", "4", and "u", the dispersion calculation can be carried out with the gaseous component "xx" and explicit setting of the sedimentation and deposition velocity using NOSTANDARD options. Example for the output of the concentration for diameter class 3:

    os "NOSTANDARD;Vs=0.04;Vd=0.05"
    xx  100

Note that the sedimentation and deposition velocity specified in the NOSTANDARD options is used for all substances that are included in the dispersion calculation.

S19) What to consider when modifying the settings file?

In general, the settings file (austal.settings, or austal2000.settings, or austal2000n.settings) should not be modified. If so, carefully consider the modification and check the resulting effects in detail. Special aspects:

• The evaluation of highest hourly means is implemented for gaseous substances only (according to the requirements of the current TA Luft). Hence the program aborts if it does not encounter a gaseous component during this evaluation. Even if it does, the evaluation is not useful because no automatic addition of the dust components 1 and 2 takes place as it is the case for annual and daily means. If for a substances highest hourly means is desired, the spectrum of allowed components must be set to "0-0".

S18) How should calms be defined in a AKTerm?

According to the specification of the German Weather Service DWD, calms (hours with no wind) are encoded in a AKTerm file by wind direction 0 (wind from north is defined by 360 deg). In addition, the wind speed should be set to 0: first for obvious plausibility, second to allow a consistent treatment of the wind direction redistribution according to paragraph 8.2 of TA Luft, Annex 3. Thus, a wind direction 0 should only occur together with wind speed 0 and vice versa. This is also the convention followed by the DWD. AUSTAL/AUSTAL2000 detects calms at hand of wind speed 0.

S17) What must be considered when applying externally generated wind fields?

• The wind fields must be defined on the Arakawa-C net.
  
  
• Before the actual dispersion calculation, any divergence in the provided wind field is internally removed by a simple adjustment ofthe z-components. To keep this modification at a minimum, the provided wind fields should be already sufficiently free of divergence.
  
  
• Buildings: Buildings are resolved on the calculation grid. A cell is interpreted as buildingcell if the vertical component Vs of the wind vector at the bottom of the cell is less or equal -99 m/s. Below a building cell there may beno free cell (this would result either in an error message or in incorrect concentration values).
  
  
• Terrain profile and nested grids: AUSTAL/AUSTAL2000 adjusts the z-values specified in files srfa0li.dmna for the two boundary stripes of inner grids at hand of the values ofthe next coarser grid. This "tuning" guarantees a smooth transition between the grids. The resulting values are written to the wind fieldsgenerated by AUSTAL/AUSTAL2000. If external fields are provided, they must containthese tuned z-values. AUSTAL/AUSTAL2000 checks the z-values of the wind field files with the internally created ones andaborts with an according error message if they do not agree.

S16) Why is a quality level of 2 in some situations not sufficient to faithfully calculate the frequency of odor hours?

A sufficiently large number of simulation particles is required to avoid systematic effects in the calculation of odor hour frequencies (see the discussion in the annex of the program manual). Usually (one or several sources near ground and close together) a quality level of 2 is sufficient for time series calculations. However, if the sources are far apart or if a source, which is dominant with respect to the emission, has almost no contribution to the total concentration near ground due to its construction height and/or plume rise (but nevertheless takes most of particles, which are assigned to the sources according to their emissions), a considerably larger rate may be required. Once the systematic effect is avoided, the listed (absolute) statistical uncertainty is a faithful measure also for odor hours. The quality level (parameters qs, default value 0) can be set to a value up to 4 in standard operation. Higher values require the NOSTANDARD setting in the option string (parameter os).