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.
Modeling for NAAQS Part 1: How to select the appropriate output for 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.
- In the Output options, select Receptor, day, and maximum tables for the output. At this time, users can also select additional optional results files including Plot (contour) files, Daily maximum contribution files, Daily maximum files, and/or Daily maximum by year files.
- In the Plot Files tab of the Output Options, choose 1-hour 4th high. Click OK and run the model normally.
With these steps, the receptor table output (and the standard BREEZE Results Summary) in AERMOD can be directly compared to the U.S. NAAQS. As usual, AERMOD will calculate the high concentrations at every receptor and the one receptor with the highest concentration result should be compared to the NAAQS standards.
How to Turn Off Special Processing for Non-U.S. or Non-Standard Modeling
If you are not modeling for comparison to the U.S. NAAQS standards, you need to select the “Disable the special analysis for SO2, NO2, or PM2.5 NAAQS” box in the Control options window.
This will cause the model to revert to the standard processing it uses for all other pollutants/averaging periods. This is a critical distinction, as the same AERMOD output options could produce much less conservative results if the U.S. NAAQS processing is used. Think of a hypothetical example in which 5 years were modeled to evaluate the 4th highest 1-hour SO2 value, and the four highest-concentration hours in that five-year period all occur on the same day. With AERMOD's default U.S. NAAQS processing, three of those four high-concentration hours would be thrown out and lower concentrations from other days would replace them in the calculation. If the “Disable special analysis” option is selected to force AERMOD to use more straightforward processing, all four high-concentration hours would count toward the final model result.
Tools for Analyzing U.S. SO2 NAAQS Modeling Results
As outlined above, the U.S. NAAQS for SO2 pollutants consist of a multi-year average of (typically) 5 values. This makes it more complicated to determine the how and why of high concentrations:
- What times of day and meteorological conditions were responsible?
- Which sources were responsible?
Fortunately, specialized output file types in AERMOD have been added that help to answer these questions. The most useful of these are described below.
Date and Time Information
The Plot (contour) output file type is modified to help users identify the dates and times (and thus the specific situations/meteorological conditions) that contributed to their high concentrations. When viewed as a text file in the Reports tab of BREEZE AERMOD, the Plot files will show not only the NAAQS-format high concentration values (which are typically an average of five values, one from each year) - they will also show the five individual concentrations that were averaged together to get that overall high value, as well as the date and time at which those five key concentrations occurred.
This output file type can be enabled simply by selecting Plot (contour) files in the Output options, and then choosing the 4th high 1-hour for SO2. The image below shows the correct selections for 1-hour SO2.
Source Contribution Analysis
The Maximum Daily Contribution (MAXDCONT) output file type helps users identify the particular sources that contributed to the overall high concentrations in a model run. Normally, the multi-year averaging (and the fact that for each source group, different dates/times will be selected by AERMOD for this averaging) make it hard to do a source contribution analysis for these NAAQS cases. The MAXDCONT output file type does the hard work of pairing everything up in time and space, and shows exactly what each source contributed to the overall concentration you are comparing to the NAAQS.
This output file type can be enabled simply by selecting Max Daily Contribution box in the Output options window, and then choosing the desired range of high values, usually at least 4th high for SO2. The example below shows proper setup for 1-hour SO2 modeling.
These standards and the proper use of AERMOD's related processing and output options can be confusing. Do not hesitate to reach out to BREEZE support with any questions!
PLEASE NOTE: The information provided in this modeling tip describes the generally appropriate model options and configuration for modeling against the U.S. NAAQS, as of September 2017. State and local regulatory agencies may have their own unique requirements, and U.S. EPA could change its general guidance in the future or require a different approach in some circumstances. Always consult your regulatory agency if in doubt as to the standards that apply to your modeling cases, or how to properly evaluate them in a modeling analysis.