How is concentration defined in HYSPLIT and how does it relate to the radiation units of Bacquerel?
Please refer to Technical Memorandum ERL ARL-224 for a complete review of equations used to compute air concentrations. However, briefly ...
The default model simulation as configured on the web (within READY) assumes a horizontal puff distribution and vertical particle dispersion. The horizontal resolution of the concentration grid is defined based on the run duration as chosen by the user (ranging from 0.01 to 0.20). As puffs pass over the concentration grid nodes (grid intersection points) the puff's air concentration (mass divided by puff area) is summed at that node. Summation continues over the duration of the concentration averaging time. Because particle dispersion is assumed in the vertical, any puffs that fall within the predefined concentration layers (value set by the user) are the only ones that contribute to the node summation. Final air concentration is obtained by dividing the sum by the layer depth.
Model input defines the source term as Mass units per hour. Air concentration output becomes mass units per cubic meter. If your mass unit is Bq then output concentrations are Bq/m^3. Mass units are arbitrary and user defined.