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 (NOx = 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, NOx 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?
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 NOx, VOCs, or both to lower surface O3 levels? Abundant natural sources of VOCs, such as isoprene, typically make controlling anthropogenic NOx emissions the only viable way to reduce surface O3 in the Eastern U.S. during summer. That is, NOx is the limiting reagent for O3 formation (i.e., NOx-limited), except in urban areas with very high NOx 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 NOx 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 NOx Emissions Reductions & Human Health
Benefits for O3: NOx 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 NOx 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 NOx 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 NOx 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.