At Earth's surface, ozone (O₃) is an air pollutant that is not directly emitted into the air, but instead it is formed through chemical reactions in the atmosphere when ultraviolet (UV) radiation from the sun interacts with nitrogen oxides (NO_x = NO₂ + NO) and volatile organic compounds (VOCs). Both compounds are released to the atmosphere through human actions, such as the burning of fossil fuels. (Surface O₃ pollution is not to be confused with the stratospheric "Ozone Layer," which filters out most harmful ultraviolet (UV) rays from the sun.)

When breathed in, O₃ chemically reacts with lung tissue, causing respiratory issues and injury to lung tissue that accumulates over time with continued exposure. (In fact, O₃ is such a highly reactive gas that is used to disinfect water for drinking.) Exposure to O₃ is associated with about 250 thousand deaths annually worldwide (Cohen et al., 2017). O₃ is also harmful to vegetation and even reduces yields for some crops (e.g., soybeans), costing billions annually. It enters a plant’s stomata (i.e., tiny pores on leaves) and chemically reacts with plant cells. O₃ injury to plants is evident often as a reddish stippling pattern on the upper side of the leaf surface that accumulates throughout the growing season. For more information on the impacts of O₃ on health and plants, visit the Impacts tab.

Assessing the Efficacy of Environmental Efforts to Improve Air Quality

Monitoring O₃, NO_x and VOCs levels over time allows air quality managers to assess the efficacy of environmental efforts, such as the U.S. Clean Air Act, to reduce unhealthy O₃ levels. NO₂ and many VOCs are considered air pollutants as they are unhealthy for humans to breathe, though both are primarily regulated to reduce the formation of unhealthy levels of O₃.  For instance, Zhu et al. (2017) used satellite observations of HCHO to estimate that 6,600-13,200 people in the US will develop cancer over their lifetimes by exposure to outdoor formaldehyde (HCHO). For more information, visit the Health Impacts tab.

What can we observe from space?

• O₃: With today's technology, it is not currently feasible to monitor surface levels of O₃ from space. However, it is possible to observe NO_x and VOCs levels from space.
• NO_x: NO₂ serves as an effective proxy for NO_x and is observable from space. For more information on NO_x, visit the Nitrogen Dioxide tab.
• VOCs: HCHO, a VOC, is observable from space. It an effective proxy for many VOCs as HCHO is produced when they are oxidized (i.e., broken down) in the atmosphere via chemical reactions. There are typically thousands of VOCs in polluted areas, especially cities. Many natural VOCs, such as isoprene, are emitted from vegetation and make a much larger contribution to total VOCs than man-made VOCs in many areas of the Eastern U.S., particularly the Southeast U.S. In fact, most HCHO observed from space is associated with the atmospheric oxidation of isoprene. Plants are thought to emit isoprene in response to environmental stresses, such as high temperatures. Therefore, HCHO levels tend to be higher during hotter days and summers than cooler ones.

Satellite-based observations of the NO₂ to HCHO ratio give air quality managers an indication of the most effective way to reduce surface O₃ levels.  Should they reduce emissions of NO_x, VOCs, or both to lower surface O₃ levels? Abundant natural sources of VOCs, such as isoprene, typically make controlling anthropogenic NO_x emissions the only viable way to reduce surface O₃ in the Eastern U.S. during summer. That is, NO_x is the limiting reagent for O₃ formation (i.e., NO_x-limited), except in urban areas with very high NO_x concentrations; in these areas, controlling VOC emissions is often also effective at reducing O₃ formation (i.e., radical-limited). However, satellite data indicate that the extent of radical-limited areas has dwindled in the Eastern U.S. given the substantial reduction (20-60%) of NO_x emissions over the last 15 years (Jin et al., 2017; NASA Earth Observatory story). For more information on how satellite data of HCHO and NO₂ are being used to monitor O₃ formation sensitivity, visit the AQ Managers tab to access a tutorial on using the data.

The Benefits of the U.S. Clean Air Act for O₃ of NO_x Emissions Reductions & Human Health

Benefits for O₃: NO_x emissions have decreased by about 37% in the Eastern U.S. from 2002 to 2011, with reductions as high as 60% in some areas. A model study by Loughner et al. (2014) estimated that the NO_x emissions reductions led to potentially 9-13 fewer O₃ exceedances (using the 2011 maximum eight hour average O₃ standard of 75 ppbv) throughout much of the Ohio River Valley and 3-9 fewer O₃ exceedances throughout much of the Washington, DC – Baltimore, MD metropolitan area in July 2011. To assess the impact on surface O₃ levels, Loughner et al. ran the CMAQ air quality model once with anthropogenic emissions appropriate for 2011 and again with anthropogenic emissions appropriate for 2002. In both simulations, the meteorology for July 2011 was used. They chose to simulate July 2011, one of the hottest months ever recorded in the Mid-Atlantic, as O₃ levels typically correlate well with temperature. That is, July 2011 was an extreme meteorological event which gives an indication of the maximum benefit of the NO_x emission reductions that occurred between 2002 and 2011.

(left) A CMAQ model simulation of the number of O₃ exceedences using July 2011 meteorology with 2002 air pollutant emissions. An exceedance occurs when the maximum eight hour average O₃ standard of 75 ppbv. (middle) The same as the left image, but with 2011 air pollutant emissions. Observed O₃ values are shown in circles. (right) The difference in the simulated number of O₃ exceedence days due to the lower NO_x emissions in 2011 versus 2002. Analysis and figure credit: Dr. Chris Loughner, NOAA.

Benefits for Human Health: In unpublished work, we estimate, over the study region (most of the Eastern US), that these emission reductions prevented 569-801 deaths (95% CI: 319-1011), 950 hospital admissions due to respiratory symptoms (95% CI: 91-2346), 573 Emergency Room visits for Asthma (95% CI: 0-1645), and asthma exacerbation symptoms for more than 430,000 people (95% CI: 0-960,000+). We used EPA's Environmental Benefits Mapping and Analysis Program - Community Edition (BenMAP-CE) program for these estimates.

Health benefits of emission reductions for asthma. Analysis and figure credit: Dr. Melanie Follette-Cook, NASA.

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