11.5 Wet Deposition for Particles




As before, start by retrieving the previously saved captex_control.txt and captex_setup.txt settings into the GUI menu. Set the run duration to 25 h, change the number of vertical levels from 1 to 2, and add the level height of 0 preceding the 100.

  1. Wet deposition of particles is divided into two processes, those in which the polluted air is continuously ingested into a cloud from a polluted boundary layer and those in which rain falls through a polluted layer. Starting with HYSPLIT version 657, the particulate wet removal equations have been simplified to use the same computational approach for below- and within-cloud removal. Wet removal is defined as a scavenging coefficient expressed directly as a rate constant, modified by the precipitation rate (P in mm/h). The rate constant (β) is given by,
    • βwet = 8x10-5 P0.79.

    The default scavenging coefficient (8x10-5), applied both below- and within-cloud, should be applicable to a wide range of particles and is defined in the CONTROL file. The coefficients and precipitaion exponents can vary for different pollutants as well as scavenging processes (rain or snow). For instance, see Sportisse, 2007 (doi:10.1016/j.atmosenv.2006.11.057) for examples.

  2. Open the Setup Run / Deposition menu and reconfigure the emissions for particles by first setting the particle radiobutton and then toggling the wet deposition radiobutton to set the default scavenging coefficients for particles. However, to avoid computing dry deposition, force the dry deposition (and settling) to a very small negative value (-1.0E-10) so that we can see only the effects of wet deposition. A negative value in this field is a special case where mass is not lost to dry deposition.

  3. Save to close, run the model, and display the results. The final particle deposition pattern looks very similar to the gaseous pattern but with much more deposition. The MESSAGE file shows a total mass of 196174 grams at the end of the run, a few kilograms less than the gaseous wet deposition simulation. However from the deposition graphics, we can see that the particle wet deposition amounts were a factor of 10,000 greater than the gaseous wet deposition.

  4. Deposition can be difficult to interpret and the previous results seem inconsistent. However, a rough examination of the deposition region shows the extent to be about one-degree square, or about 1010 m2. The high particle wet deposition region showed a peak of about 105 pg/m2 compared with the gaseous deposition maximum of about 10 pg/m2. The additional particle deposition translates into about 1015 pg of deposition or about 1 kg of the 200 kg released.

Dry deposition rates may be small but they occur everywhere at all times resulting in large losses over longer time periods, while large losses by wet deposition are limited to areas with rainfall and over much shorter time periods. However, model derived rainfall predictions may be very uncertain and access to observed precipitation such as from NASA TRMM may be useful in the interpretation of modeling results.