Modeling Tip – August 2006

In last month's modeling tip we discussed the emission rate and modeled output concentration, when modeling both single and multiple odor producing pollutants. In this tip we will discuss further how the correct units can be achieved in AERMOD and ISCST3, as well as how shorter term averaging periods can be calculated for odor concentrations.

Setting Correct Units in AERMOD and ISCST3

As discussed in the last modeling tip, for a single pollutant, the emission rate can be given in grams per second [g/s] and the output concentration in micrograms per cubic meter [µg/m3]. These are the default values for AERMOD and ISCST3, and hence no changes are needed.

For multiple pollutants, the emission rate is given in terms of odor units per second [OU/s] and the output concentration is given as odor units per cubic meter [OU/m3]. This requires several parameters in AERMOD and ISCST3 to be changed. These parameters can be found in the source pathway, and should be changed as follows:

This may be written in the source pathway as:

CONCUNIT 1 OU/SEC OU/M³

ISC-AERMOD View Users: These changes can be made directly in the Source Pathway under Emission Output Unit.

Calculating Short-Term Odor Concentrations

Because the nose can detect odors very quickly, an averaging period less than 1-hour is more appropriate (e.g., 3-min or 10-min). Therefore, you will need to convert the 1-hour AERMOD and ISCST3 concentration results into the appropriate shorter average period using the formula below:

C_new = C_1-hour (T_1-hourT_new)^q

C_new = Concentration for the shorter time period

C_1-hour = 1-hour concentration

T_new = Shorter average period in seconds

T_1-hour = 3600 (for 1-hour average period)

q = factor (values for q vary, please contact your regulator for guidance)

ISC-AERMOD View Users: This conversion is available in the Concentration Converter under the Tools menu.