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Surprise! Changes to the SO2 NAAQS

bjones148

Updated: Jan 20


A gavel on a table with open books, set against a large window showing lush greenery and sunlight. Warm tones create a peaceful mood.

OK, I have to confess this slipped past me.

 

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.

 

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.

 

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.

 

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).

 

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.

 

Highest Annual SO2 Concentrations by State

2024 

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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