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National Ambient Air Quality Standards (NAAQS)

Sulfur Dioxide (SO2) Primary Standards -
Table of Historical SO2 NAAQS

History of the National Ambient Air Quality Standards for Oxides of Sulfur
During the Period 1971-2010

Final Rule/Decision Primary/ Secondary Indicator (1) Averaging Time Level (2) Form

1971

36 FR 8186
Apr 30, 1971

Primary

SO2

24-Hour

0.14 ppm

Not to be exceeded more than once per year

Annual

0.03 ppm

Annual arithmetic average

Secondary

3-Hour

0.5 ppm

Not to be exceeded more than once per year

Annual (3)

0.02 ppm

Annual arithmetic average

1973

38 FR 25678
Sept 14, 1973

Secondary

Secondary 3-hour SO2 standard retained, without revision; secondary annual SO2 standard revoked.

1996

61 FR 25566
May 22, 1996

Primary

Existing primary SO2 standards retained, without revision.

2010

75 FR 35520
Jun 22, 2010 (4)

Primary

SO2

1-hour

75 ppb

99th percentile, averaged over 3 years (5)

Primary annual and 24-hour SO2 standards revoked.

(1) SO2 = sulfur dioxide

(2) Units of measure are in parts per million (ppm) and parts per billion (ppb).

(3) The 1971 final rule also included a secondary 24-hour SO2 standard of 0.1 ppm, maximum 24-hour concentration not to be exceeded more than once per year, as a guide to be used in assessing implementation plans to achieve the annual standard.

(4) The 1-hour SO2 standard added in 2010 is a primary standard.  The revision of the SO2 NAAQS in 2010 did not address the secondary standard, which remains a 3-hour standard with a level of 0.5 ppm.

(5) The form of the 1-hour standard is the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO2 concentrations.

 

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