On June 2, 2010, the EPA issued a new primary National Ambient Air Quality Standard for sulfur dioxide. The new standard is based on a criterion for the highest one-hour average per sampling day. Monitors for which the 99th percentiles of such daily maximums averaged over three consecutive years exceeds 75 parts per billion are likely to be deemed not attaining this standard. This supersedes primary standards based on annual arithmetic mean one-hour average and the highest 24-hour average of the year.

To aid in planning for attainment of the 1-hour SO₂ standard, the following North Carolina maps have been drawn showing the 3-year-average 99th percentile of maximum 1-hour concentrations in each county. Click on links for printer-friendly (PDF) version.

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