Progress Report

Air Quality

Sulfur Dioxide and Nitrogen Oxides Trends Figures

Source: EPA, 2023
Last updated: 03/2023

Related Figures

National SO₂ Air Quality
Regional Changes in Air Quality


National SO₂ Air Quality

  • Based on EPA’s air trends data, the national average of SO₂ annual mean ambient concentrations decreased from 12.0 parts per billion (ppb) to 0.7 ppb (94 percent) between 1980 and 2021.
  • Since the first year of the ARP, three years have seen reductions of greater than 20 percent: 1994–1995 (22 percent); 2008–2009 (21 percent); and 2014–2015 (23 percent).

Regional Changes in Air Quality

  • Regional average ambient SO₂ concentrations declined in the eastern U.S. by 95 percent from the 1989–1991 observation period to the 2019–2021 observation period.
  • Average ambient particulate sulfate concentrations have decreased by 49 to 84 percent in observed regions from 1989–1991 to 2019–2021.
  • Average annual ambient total nitrate concentrations declined 59 percent from 1989–1991 to 2019–2021 in the eastern U.S., with the most significant decreases occurring after 2002 coinciding with the implementation of the NOₓ Budget Trading Program, followed by CAIR, CSAPR, and CSAPR Update.

Background Information

Sulfur Dioxide

Sulfur oxides are a group of highly reactive gases that can travel long distances in the upper atmosphere and predominantly exist as sulfur dioxide (SO₂). The primary source of SO₂ emissions is fossil fuel combustion at power plants. Smaller sources of SO₂ emissions include industrial processes, such as extracting metal from ore, as well as the burning of high sulfur-containing fuels by locomotives, large ships, and non-road equipment. SO₂ emissions contribute to the formation of fine particle pollution (PM₂.₅) and are linked with adverse effects on the respiratory system.1 In addition, particulate sulfate degrades visibility and, because sulfur compounds are typically acidic, can harm ecosystems when deposited.

Nitrogen Oxides

Nitrogen oxides are a group of highly reactive gases including nitric oxide (NO) and nitrogen dioxide (NO₂). In addition to contributing to the formation of ground-level ozone and PM₂.₅, NOₓ emissions are linked with adverse effects on the respiratory system.2, 3 NOₓ also reacts in the atmosphere to form nitric acid (HNO₃) and particulate ammonium nitrate (NH₄NO₃). HNO₃ and nitrate (NO₃), reported as total nitrate, can also lead to adverse health effects and, when deposited, cause damage to sensitive ecosystems.

Although the ARP and CSAPR programs have significantly reduced NOₓ emissions (primarily from power plants) and improved air quality, emissions from other sources (such as motor vehicles and agriculture) contribute to total nitrate concentrations in many areas. Ambient nitrate levels can also be affected by emissions transported via air currents over wide regions.

More Information


  1. Katsouyanni, K., Schwartz, J., Spix, C., Touloumi, G., Zmirou, D., Zanobetti, A., Wojtyniak, B., Vonk, J.M., Tobias, A., Pönkä, A., Medina, S., Bachárová, L., & Anderson, H.R. (1996). Short term effects of air pollution on health: a European approach using epidemiologic time series data: the APHEA protocol. J. of Epidemiol Community Health, 50: S12–S18.
  2. Peel, J.L., Tolbert, P.E., Klein, M., Metzger, K.B., Flanders, W.D., Todd, K., Mulholland, J.A., Ryan, P.B., & Frumkin, H. (2005). Ambient air pollution and respiratory emergency department visits. Epidemiology, 16: 164–174.
  3. Hong, C., Goldberg, M.S., Burnett, R.T., Jerrett, M., Wheeler, A.J., & Villeneuve, P.J. (2013) Long-term exposure to traffic-related air pollution and cardiovascular mortality. Epidemiology, 24: 35–43.