The U.S. National Ambient Air Quality Standards (NAAQS) for 1-hour SO2, 1-hour NO2, and 24-hour PM2.5 use a very particular format. It is very important for all modelers (both within and outside of the U.S.) to understand how AERMOD handles this. In particular, U.S. users need to be familiar with the special tools available to help them analyze these results, and non-U.S. modelers (or anyone using AERMOD for a purpose other than the U.S. NAAQS standards) need to know how to disable AERMOD's U.S. NAAQS-specific processing when necessary. Non-U.S. modelers and those modeling for a purpose other than comparison to the U.S. NAAQS standards should pay special attention to the “How to Turn off Special Processing for Non-U.S. or Non-Standard Modeling”section at the end of this modeling tip.
In this modeling tip we will discuss how to select the appropriate outputs for modeling with SO2 in compliance with the U.S. National Ambient Air Quality Standards (NAAQS). You may also want to take a look at the previous modeling tip where we discuss modeling with NO2.
The Format of the 1-Hour SO2 NAAQS Standard for Modeling Purposes
The 1-hour SO2 NAAQS standard uses a unique data analysis method consisting of 3 steps:
- At each receptor, the model selects the highest-concentration hour from each day, and the other 23 hours from each day are discarded.
- From this pool of 365 1-hour values (one from each day), the highest values are discarded such that the 4th highest SO2 value from each year is selected, leaving one value per year.
- These values (usually 5, one from each year of a 5-year period) are averaged together to produce the final number that can be compared to the SO2 1-hour NAAQS design value.
If you are familiar with the format of the general 1-hour SO2 NAAQS standard, you may notice that the steps above do not exactly match the general standard. Specifically, the 4th high value is used rather than the 99th percentile value, and a five-year rather than three-year period is typically modeled. The 4th high requirement is due to the fact that U.S. EPA has provided specific instructions on how to evaluate 1-hour SO2 for modeling purposes, and the typical five-year requirement is due to the fact that in most cases, a full NAAQS analysis requires five years of representative meteorological data. Exceptions to these rules are possible, such as in the case of one year of on-site meteorological data being used for modeling. If in doubt, consult your regulatory agency before proceeding.
How to Model SO2 in AERMOD
If you are modeling for comparison to the 1-hour SO2 U.S. NAAQS standards, BREEZE AERMOD makes this very simple. After a few basic setup steps, the model will automatically perform all of the special processing described above, and will provide an output that can be directly compared to the NAAQS standards. To enable this special processing in AERMOD, follow the steps below:
- In the Control options window, under Pollutant select SO2. Click OK.