Modeling Tip – July 2006

The mechanisms of odor dispersion in the atmosphere are the same as the dispersion of other pollutants. When modeling odor concentrations using the U.S. EPA AERMOD or ISCST3 models, there are a few considerations to be made:

Determining Odor Emissions Rates

1. Single Odor-Producing Pollutant:

• Odor Emission Rate [g/s]: specify the pollutant emission rate in g/s as you would specify if doing a normal air dispersion modeling study.


• Model Output Concentration.[µg/m³]: the model predicted concentration will be given in mass per volume (µg/m³). Compare this concentration with the pollutant specific odor threshold which is generally given in parts per million (ppm) and/or µg/m³.

2. Multiple Odor-Producing Pollutants:

When multiple pollutants are emitted, masking and enhancing effects may occur, in this case the relationship between concentration and odor is not well defined and odor must be characterized in terms of an odor detection threshold value (OTV).

• Odor Detection Threshold Value [OTV]: Using a panel of testers, the OTV is the value at which 50% of the panelists can just detect the odor. Odor complaint levels are usually 2 to 3 times higher than the odor threshold levels.


• Odor Concentration [OU/m³]: Odor concentration is expressed in terms of odor dilution ratio or odor units (OU) per cubic meter of air (OU/m³), where the odor is no longer perceptible by 50% of the panelists.


• Example: If an odorous air sample of the exhaust stream was diluted with 1000 volumes of odor-free air to reach the OTV, then the odor concentration of the sample would be given as 1000 OU/m³.



• Odor Emission Rate [OU/s]: The odor emission rate of the source can then be calculated as follows:
  E = V x DTT
  
  E = Odor emission rate (OU/s)
  V = Volumetric flow rate of the emission source (m³/s)
  DTT = Odor concentration in OU/m³ (which is the number of dilutions to od our threshold)



• Model Output Concentration [OU/m^3]: The model predicted odor concentration will be given in terms of OU/m³. These values should than be compared with the odor threshold of 1 OU/m³ (odor detection threshold).

ISC-AERMOD View Users: You can specify the emission unit label of OU/s and the concentration unit label of OU/m³ in the Source Pathway under Emission Output Unit.