
OK, I have to confess this slipped past me.
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On December 27 (I freely admit, not a time I was completely focused on modeling!) EPA published its review of the secondary National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen, oxides of sulfur, and particulate matter (https://www.govinfo.gov/content/pkg/FR-2024-12-27/pdf/2024-29463.pdf). For the modeling world, the only real meaningful change that comes from this is the secondary 3-hour NAAQS for SO2 is now gone, replaced by a secondary Annual NAAQS for SO2.
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You may recall that I blogged about the coming new secondary Annual SO2 NAAQS in April of last year (https://www.blueskymodeling.com/post/the-impending-return-of-annual-so2-naaqs-modeling). That was back when EPA was proposing the new standard, and at the time they were looking at a range of 26 µg/m3 to 39 µg/m3. Naturally, the standard they ended up choosing was the low end of this range—26 µg/m3 (technically it’s 10 ppb, but that comes out to about 26 µg/m3). It’ll go into effect on January 27 of this year.
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What was a surprise to me was the doing away with the secondary 3-hour SO2 NAAQS. Essentially EPA decided that the secondary 3-hour SO2 NAAQS was no longer necessary to protect public welfare and therefore the secondary SO2 NAAQS would become an Annual standard.
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Note that 3-hour SO2 modeling isn’t completely going away with this. There is still a 3-hour PSD Increment, and many states have a 3-hour SO2 standard that will be around for at least a while longer (some states mimic NAAQS in their state standards, but won’t instantly remove it just because of this action).
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Turning back to the new secondary Annual SO2 NAAQS, I pulled down the highest Annual SO2 concentration for each state (plus Washington, D.C. and Puerto Rico) for 2024, see the table below. Even when only looking at these worst-case monitors/concentrations it’s pretty clear that background levels of Annual SO2 aren’t likely to pose much of a hurdle, unlike background levels of Annual PM2.5. Obviously, it will depend on where your project is, but if you do end up having to do Annual SO2 NAAQS modeling there’s a good chance your background is going to be down around 10% or less of the NAAQS.
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Highest Annual SO2Â Concentrations by State
2024Â
State | Annual SO2 | |
ppb | ug/m3 | |
Alabama | 4.34 | 11.34 |
Alaska | 1.58 | 4.13 |
Arizona | 1.26 | 3.29 |
Arkansas | 0.40 | 1.05 |
California | 1.65 | 4.31 |
Colorado | 0.57 | 1.49 |
Connecticut | 0.18 | 0.47 |
Delaware | 0.36 | 0.94 |
District of Columbia | 0.65 | 1.70 |
Florida | 1.81 | 4.73 |
Georgia | 2.30 | 6.01 |
Hawaii | 2.18 | 5.70 |
Idaho | 3.21 | 8.39 |
Illinois | 1.78 | 4.65 |
Indiana | 1.10 | 2.87 |
Iowa | 0.99 | 2.59 |
Kansas | 0.44 | 1.15 |
Kentucky | 2.76 | 7.21 |
Louisiana | 1.25 | 3.27 |
Maine | 0.04 | 0.10 |
Maryland | 0.31 | 0.81 |
Massachusetts | 0.45 | 1.18 |
Michigan | 1.61 | 4.21 |
Minnesota | 2.12 | 5.54 |
Mississippi | 1.28 | 3.35 |
Missouri | 4.34 | 11.34 |
Montana | 1.49 | 3.89 |
Nebraska | 0.68 | 1.78 |
Nevada | 0.68 | 1.78 |
New Hampshire | 0.82 | 2.14 |
New Jersey | 0.86 | 2.25 |
New Mexico | 0.89 | 2.33 |
New York | 4.21 | 11.00 |
North Carolina | 0.20 | 0.52 |
North Dakota | 2.40 | 6.27 |
Ohio | 2.28 | 5.96 |
Oklahoma | 0.51 | 1.33 |
Oregon | 0.26 | 0.68 |
Pennsylvania | 2.25 | 5.88 |
Rhode Island | 0.19 | 0.50 |
South Carolina | 0.28 | 0.73 |
South Dakota | 0.56 | 1.46 |
Tennessee | 2.44 | 6.38 |
Texas | 2.63 | 6.87 |
Utah | 0.73 | 1.91 |
Vermont | 0.13 | 0.34 |
Virginia | 1.37 | 3.58 |
Washington | 0.49 | 1.28 |
West Virginia | 2.68 | 7.00 |
Wisconsin | 0.98 | 2.56 |
Wyoming | 1.46 | 3.82 |
Puerto Rico | 1.58 | 4.13 |
PS--there is no symbolism intended with the broken gavel in the image at the top of this blog. Who said AI image-generation was perfect?
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