The U.S. National Ambient Air Quality Standards (NAAQS) for 1-hour SO2, 1-hour NO2, and 24-hour PM2.5use 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 NO2 in compliance with the U.S. National Ambient Air Quality Standards (NAAQS). Keep a lookout for future modeling tips where we discuss modeling with SO2 and PM2.5.
The Format of the 1-Hour NO2 NAAQS Standard for Modeling Purposes
The 1-hour NO2 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 8th highest NO2 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 NO2 1-hour NAAQS design value.
If you are familiar with the format of the general 1-hour NO2 NAAQS standard, you may notice that the steps above do not exactly match the general standard. Specifically, the 8th high value is used rather than the 98th percentile value, and a five-year rather than three-year period is typically modeled. The 8th high requirement is due to the fact that U.S. EPA has provided specific instructions on how to evaluate 1-hour NO2 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 NO2 in AERMOD
How to Model NO2 in AERMOD
If you are modeling for comparison to the 1-hour NO2 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:
1. In the Control options window, under Pollutant select NO2. Click OK.
2. 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.
3. In the Plot Files tab of the Output Options, choose 1-hour 8th 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 standard.
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 8th highest 1-hour NO2 value, and the eight highest-concentration hours in that five-year period all occur on the same day. With AERMOD’s default U.S. NAAQS processing, seven of those eight 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 eight high-concentration hours would count toward the final model result.
Tools for Analyzing U.S. NO2 NAAQS Modeling Results
As outlined above, the U.S. NAAQS for NO2 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.
During the development of this 4TR article, proposed revisions to Appendix W to 40 CFR Part 51 – Guideline on Air Quality Models (Guideline) were published in the Federal Register. As a result of revisions which propose to establish Tier 3 nitrogen dioxide (NO2) atmospheric chemical reaction screening techniques as default methods, applicants will no longer need to gain U.S. EPA approval after the Guideline revisions are finalized. In the meantime, the guidance provided below related to the Tier 3, NO2...
You have seen ALL4 documenting the corresponding evolution of the NAAQS levels since the current U.S. EPA administration started in January 2009, and we will continue to follow them closely and provide you with updates that we think are important to your operations. The following sections provide updates on the most important NAAQS and how you can plan for them today. Discusses SO2, air dispersion modeling and ambient monitoring, ozone, PM2.5.
You don’t need to work in the air consulting arena for very...
EPA Reviews National Ambient Air Quality Standards For Ozone: On December 17, 2014, the U.S. Environmental Protection Agency (EPA) issued a proposed rule revising the air quality criteria for ozone (O3) and related photochemical oxidants and National Ambient Air Quality Standards (NAAQS) for O3. 79 Fed. Reg. 75234. EPA proposed to revise the primary standard to a level within the range of 0.065 to 0.070 parts per million (ppm), and to revise the secondary standard to within the range of 0.065 to 0.070 ppm, which...
Since the 1-hour sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) became effective in August 2010, ALL4 has been tracking it every step of the way. Its implementation has proven to be chaotic at times and quiet at others. As we are ramping up into our air regulatory reporting obligations for 2014, U.S. EPA’s implementation of the 1-hour NAAQS is ramping up as well. This article will discuss where the 1-hour SO2 NAAQS has been, where it is going, and what you can do to plan for 2014 and...
The implementation at the state level of the major New Source Review (NSR) rules that include both prevention of significant deterioration (PSD) and nonattainment new source review (NNSR) have evolved over a long period of time. During the last decade that evolution has been substantially impacted by the “NSR Reform” revisions of 2002; the regulation of particulate matter less than 10 microns (PM10) and particulate matter less than 2.5 microns (PM2.5); and the seemingly routine changes in form and...