### Time Step and Concentration Calculation

Posted:

**June 4th, 2019, 9:30 am**Dear all,

I would like to clarify something on dispersal modelling.

I am aware that time steps vary a lot during simulation. Some time steps are 6 minutes while others can be 12 minutes. The minimum time step is 1 minute.

Question1: Does HYSPLIT calculate concentration for a grid cell based on each time step? If so, how are time steps defined? Do time steps depend on meteorological data files?

According to Section 12.1 in the online tutorial, one unit of mass can be allocated to each particle released, by configuring Mass (same as emission; CMASS=1) in Advanced/Configuration Setup/Concentration menu #8 the Concentration Grid Packing Method.

The mass of particles will be summed, as they pass through the grid cells. If we define a 3-hour averaging sampling period for the grid cells, a single particle may reside in the cell for multiple time steps.

For instance, a ‘concentration’ value of 10 suggests a range of possibilities. One particle may have resided in the cell for 10 time steps or 10 particles may have been in the cell only for one time step. In such a case, concentration calculation only results in summation of particles’ mass in each cell, and the final result is no longer divided by the cell volume.

Question2: Is mass calculated based on summation of particles’ mass in a cell over a certain sampling period (e.g. 24 hours) that contains multiple time steps, if CMASS=1 [Mass (same as emission)] is set? If so, does it override sampling type [averaging (type=0)] specified in the control file?

Question3: Is concentration calculated based on averaging particles’ mass in a cell over a certain sampling period (e.g. 24 hours) that contains multiple time steps, and then being divided by the cell volume, if sampling type [averaging (type=0)] and CMASS=0 [Concentration (mass/m3)] are set?

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

I would like to clarify something on dispersal modelling.

I am aware that time steps vary a lot during simulation. Some time steps are 6 minutes while others can be 12 minutes. The minimum time step is 1 minute.

Question1: Does HYSPLIT calculate concentration for a grid cell based on each time step? If so, how are time steps defined? Do time steps depend on meteorological data files?

According to Section 12.1 in the online tutorial, one unit of mass can be allocated to each particle released, by configuring Mass (same as emission; CMASS=1) in Advanced/Configuration Setup/Concentration menu #8 the Concentration Grid Packing Method.

The mass of particles will be summed, as they pass through the grid cells. If we define a 3-hour averaging sampling period for the grid cells, a single particle may reside in the cell for multiple time steps.

For instance, a ‘concentration’ value of 10 suggests a range of possibilities. One particle may have resided in the cell for 10 time steps or 10 particles may have been in the cell only for one time step. In such a case, concentration calculation only results in summation of particles’ mass in each cell, and the final result is no longer divided by the cell volume.

Question2: Is mass calculated based on summation of particles’ mass in a cell over a certain sampling period (e.g. 24 hours) that contains multiple time steps, if CMASS=1 [Mass (same as emission)] is set? If so, does it override sampling type [averaging (type=0)] specified in the control file?

Question3: Is concentration calculated based on averaging particles’ mass in a cell over a certain sampling period (e.g. 24 hours) that contains multiple time steps, and then being divided by the cell volume, if sampling type [averaging (type=0)] and CMASS=0 [Concentration (mass/m3)] are set?

Your help would be much appreciated. Thanks.

Kind Regards,

Ming