Tab::Aerosol
Condensing vapours
This file contains all the compounds that are included in the calculation of particle phase condensation and evaporation. Any compounds not found from the chemistry will be ignored, so it is ok to have only one file for any chemistry scheme.
Compound-specific surface tension and wall loss accommodation coefficient
In the Vapour-file one can also set the surface tension and αw. The next column after the parameter B is interpreted as surface tension and the second as αw. If the values are negative, their absolute value will be used as a factor multiplying the common SURFACE_TENSION (0.05 by default; can be overridden in Custom options) and ALPHAWALL.
Vapour elemental composition
If this oiption is chosen and the elemental file is defined, the diffusion diameters are affected by the elemntal composition. This should give a more accurate estimatin of the diffusion coefficient of the molecules than diameters calculated from molecular weight and density.
PSD initialization
Init model for (hrs)
The duration of the initialization. Zero means that the initialization is performed on the first time step only.
Use measured...
The BG particles needs to be defined in tab "General options", and it is used to initialize the model PSD. The initialization will only be made on the times when there is measurement in the file. The time interval is not read from the file, but assumed to be 10 minutes. If your measurements have some other (but still constant) interval, use the Run ARCA→Custom model option "DMPS_TRES_MIN" and set this to whatever interval (in minutes) the file is.
Initialize with multimodal...
If the previous option is off, a parametric initialization can be used. The input is total particle count (#/cm³) and the three mode parameters, separated with spaces: Geometric mean diameter (GMD) in meters, width of the mode (geometric standard deviation) and the relative magnitude of the mode. For example the following input:
10e-9 0.1 1 250e-9 0.2 1
will give the distribution shown in the figure below:
The distribution will be cut off for the couple of last bins too keep them initially empty. This helps to stabilize the simulation.
With the multimodal initialization the simulation PSD is replaced every time step, unlike in the Use measured... option.
Use background particles for the whole run
This will continue using the initialization particles throughout the whole run but only for parts of the size distribution. This can be beneficial for example if the larger particles keep changing due to transportation or other sinks and sources that a box model can not take into account The smaller particles will be very much affected by these changes, and this way the simulation can be focused on them, restricting them with measurements.
Highband lower limit
Unit: meters. Larger particles than this will be replaced with BG particles.
Lowband upper limit
Unit: meters. Smaller particles than this will be replaced with BG particles. If the value is smaller than the smallest diameter (defined in Aerosol structure), the option will be omitted.
Aerosol structure
PSD scheme
There are two methods of keeping track of the particles, Fully stationary (FS) and Fixed grid, moving average (MA) method.
When discussing the differences between FS and MA, we are concentrating on what happens when the particles grow by condensation or coagulation, and how to handle the resulting volume change so that total mass of the particles is conserved.
The schematic difference is shown in the two figures below:
Number of bins
If Fully stationary PSD scheme is used, this number should be high enough to make numerical diffusion insignificant. A value of 100 is a good default (this would also somewhat depend on the diameter range). The Fixed grid, moving average is able to conserve mass well with smaller number of bins, but the output will be more rugged and could be more problematic to analyze. In any case the MA psd scheme will always be faster than FS, when the number of bins is similar.
Particle diameter range
Give the smallest and largest diameter in meters here. Note that if new particles are formed, they will always be assigned the smallest available diameter (and in very small quantities also one to three of the next bins).