Air quality: EPA Allows States Choice when Defining SO2 Emissions Areas [US]

On August 10th, 2015, the U.S. Environmental Protection Agency (EPA) finalized a rule requiring states to determine air quality in areas with large sources of sulfur dioxide (SO2) emissions by using monitoring or modelling techniques.

Under the rule, states must submit data regarding sources that emit at least 2,000 tons of SO2 per year. The EPA plans to use this data to determine the remaining non-attainment areas of SO2.

Designation decisions

The Clean Air Act authorizes the EPA and the states to make designation decisions regarding ambient air quality standards (NAAQS) using data gathered from air quality monitors across the nation.

As sulfur dioxide is one of the six criteria pollutants regulated under the Clean Air Act, States are therefore required to develop and enforce pollution control plans for “non-attainment” areas that exceed the 2010 one-hour sulfur dioxide limit.


Opponents of the EPA‘s rule contend that modelling overstates the amount of SO2 pollution that a source emits, and is therefore an unreliable source of information on which to base policy decisions. However, the EPA advocates that computer modelling is a less expensive way of assessing an area’s air quality than installing new air quality monitors near SO2 sources.

Final data required

Under this rule, states are required to submit their list of sources that exceed the 2,000-tons-per-year threshold to the EPA by January 15th, 2016. States must notify the EPA about whether they will use modelling or monitoring to assess air quality and submit modelling protocols, if applicable, by July 1st, 2016.  Modelling states must submit their analyses to EPA by January 13th, 2017 and  must ensure their monitors are operational by January 1st, 2017.

This final rulemaking becomes effective 30 days after publication in the Federal Register.

EHS Legalist Red-on-line


Final Rule: Data Requirements Rule for the 2010 1-Hour Sulfur Dioxide (SO2) Primary National Ambient Air Quality Standard (NAAQS), Prepublication Notice, August 10, 2015.

Fact Sheet (PDF)

40 CFR Part 51

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