National Ambient Air Quality Standards: Difference between revisions

#* PM₁₀, coarse particles: 2.5 [[micrometre|micrometers]] (μm) to 10 μm in size (although current implementation includes all particles 10 μm or less in the standard)

#* PM_2.5, fine particles: 2.5 μm in size or less. Particulate Matter (PM) was listed in the 1996 Criteria document issued by the EPA. In April 2001, the EPA created a Second External Review Draft of the Air Quality Criteria for PM, which addressed updated studies done on particulate matter and the modified pollutant standards done since the First External Review Draft. In May 2002, a Third External Review Draft was made, and the EPA revised PM requirements again. After issuing a fourth version of the document, the EPA issued the final version in October 2004.

# [[Carbon monoxide]] (CO): The EPA set the first NAAQS for carbon monoxide in 1971. The primary standard was set at 9&nbsp;ppm averaged over an 8-hour period and 35&nbsp;ppm over a 1-hour period.<ref>{{Cite web|url=http://www.epa.gov/airquality/carbonmonoxide/.|title = Air Quality Planning and Standards}}</ref> The majority of CO emitted into the ambient air is from mobile sources. The EPA has reviewed and assessed the current scientific literature with respect to CO in 1979, 1984, 1991, and 1994.<ref>http://www.epa.gov/ncea/pdfs/coaqcd.pdf. {{Webarchive|url=https://web.archive.org/web/20110212220246/http://www.epa.gov/NCEA/pdfs/coaqcd.pdf |date=2011-02-12 }}</ref> After the review in 1984 the EPA decided to remove the secondary standard for CO due to lack of significant evidence of the adverse environmental impacts. On January 28, 2011 the EPA decided that the current NAAQS for CO were sufficient and proposed to keep the existing standards as they stood. The EPA is strengthening monitoring requirements for CO by calling for CO monitors to be placed in strategic locations near large urban areas. Specifically, the EPA has called for monitors to be placed and operational in CBSA's (core based statistical areas) with populations over 2.5 million by January 1, 2015; and in CBSA's with populations of 1 million or more by January 1, 2017. In addition they are requiring the collocation of CO monitors with NO₂ monitors in urban areas having a population of 1 million for more. As of May 2011 there were approximately 328 operational CO monitors in place nationwide. The EPA has provided some authority to the EPA Regional Administrators to oversee case-by-case requested exceptions and in determining the need for additional monitoring systems above the minimum required.<ref name="CO">{{Cite web|url=http://www.epa.gov/airquality/carbonmonoxide/pdfs/COFactSheet.pdf.|title=Air Quality Planning and Standards}}</ref> The EPA reports the national average concentration of CO has decreased by 82% since 1980.<ref>{{cite web | url=http://www.epa.gov/airtrends/carbon.html | title=National Air Quality: Status and Trends of Key Air Pollutants | date=8 July 2014 }}</ref> The last nonattainment designation was deemed in attainment on September 27, 2010. Currently all areas in the US are in attainment.<ref name="CO"/>

# [[Sulfur oxide]]s (SO_x): SO_x refers to the oxides of sulfur, a highly reactive group of gases. SO₂ is of greatest interest and is used as the indicator for the entire SO_x family. The EPA first set primary and secondary standards in 1971. Dual primary standards were set at 140&nbsp;ppb averaged over a 24-hour period, and at 30&nbsp;ppb averaged annually. The secondary standard was set at 500&nbsp;ppb averaged over a 3-hour period, not to be exceeded more than once a year. The most recent review took place in 1996 during which the EPA considered implementing a new NAAQS for 5-minute peaks of SO₂ affecting sensitive populations such as asthmatics. The Agency did not establish this new NAAQS and kept the existing standards.<ref>{{Cite web|url=http://www3.epa.gov/airquality/sulfurdioxide/.|title = Sulfur Dioxide &#124; Air & Radiation &#124; US EPA}}</ref> In 2010 the EPA decided to replace the dual primary standards with a new 1-hour standard set at 75&nbsp;ppb. On March 20, 2012, the EPA "took final action" to maintain the existing NAAQS as they stood.<ref>[http://www3.epa.gov/airquality/sulfurdioxide/pdfs/20120320factsheet.pdf Factsheet] epa.gov {{Webarchive|url=https://web.archive.org/web/20160415150152/https://www3.epa.gov/airquality/sulfurdioxide/pdfs/20120320factsheet.pdf |date=2016-04-15 }}</ref> Only three monitoring sites have exceeded the current NAAQS for SO₂, all of which are located in the Hawaii Volcanoes National Park. The violations occurred between 2007–2008 and the state of Hawaii suggested these should be exempt from regulatory actions due to an 'exceptional event' (volcanic activity). Since 1980 the national concentration of SO₂ in the ambient air has decreased by 83%.<ref>{{cite web | url=http://www.epa.gov/airtrends/sulfur.html | title=National Air Quality: Status and Trends of Key Air Pollutants | date=8 July 2014 }}</ref> Annual average concentrations hover between 1–6&nbsp;ppb. Currently all ACQR's are in attainment for SO₂.<ref>{{Cite web|url=http://www.epa.gov/airquality/sulfurdioxide/basic.html.|title = Air Quality Planning and Standards}}</ref>

{| class="wikitable"

|-

|-

| rowspan="2" | [[Sulfur dioxide]] (SO₂)

| rowspan="3" | Fine particulate matter (PM_2.5)

| Primary

| annual

| Annual mean, averaged over 3 years

| Primary and Secondary

| 0.12 ppm (235 μg/m³)

| expected number of days per calendar year, with maximum hourly average concentration greater than 0.12&nbsp;ppm, is equal to or less than 1

| {{USCFR|40|50|9|a}}

| {{USCFR|40|50|12|a}}

|}

* Source: [https://www.epa.gov/criteria-air-pollutants/naaqs-table USEPA]