By 2050, some parts of the US, and especially California, the Southwest and the Northeast, could be experiencing up to 9 extra days a year of unhealthy ozone levels, unless emissions are brought under control. This is the conclusion of research published in Geophysical Research Letters.
Other regions are likely to see an additional 2.3 days on average of dangerous ozone levels, with accompanying increases in respiratory illness, exacerbating the health risks for children, seniors and those prone to asthma.
The US Environmental Protection Agency (EPA) explain that there are two types of ozone.
Stratospheric, or “good,” ozone occurs naturally in the upper atmosphere and protects the earth from harmful ultraviolet (UV) rays.
Tropospheric, or “bad,” ozone results when pollutants emitted by cars and industry react in the presence of sunlight. Tropospheric ozone is the main ingredient in smog.
Short-term exposure to ozone is associated with adverse health effects, and high ozone levels can worsen chronic lung disease, potentially leading to fatalities.
Scientists know that higher temperatures can trigger ozone episodes, but they do not know how severely rising global temperatures will impact surface level ozone.
Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) in Cambridge, MA, and the National Center for Atmospheric Research (NCAR), collaborated to create a model, based on observed relationships between temperature and ozone, that can predict future ozone episodes.
This is the first time existing observations have been used to this extent to predict trends. The team believes that its technique offers more reliable output than previous approaches.
The authors analyzed ozone-temperature relationships at measurement sites across the US, and they found that they were more complex than expected.
Normally, surface ozone increases as the temperature rises, explains coauthor Loretta J. Mickley, of SEAS.
“Ozone production accelerates at high temperatures, and emissions of the natural components of ozone increase,” she says. “High temperatures are also accompanied by weak winds, causing the atmosphere to stagnate. So the air just cooks and ozone levels can build up.”
However, in extremely high temperatures, at or above the mid-90s ºF, ozone suppression may occur, where ozone levels stop rising, despite the higher temperature.
Only in California has ozone suppression previously been registered, so in the current study, the team decided to investigate its occurrence in other regions, and whether or not chemical factors are responsible.
Findings revealed that ozone suppression occurs at ons in five measurement sites in the US at extreme temperatures and suggest that ozone suppression is caused by meteorology rather than chemistry.
First author Lu Shen, of SEAS, explains that ozone correlates closely with temperature, while temperature correlates with other meteorological factors, such as solar radiation, circulation and atmospheric stagnation. However, she says, these relationships break down at extreme temperatures.
Regarding predictions for the future, Shen remarks:
“In the coming decades, global climate change will likely cause more heat waves during the summer, which in turn could cause a 70-100% increase in ozone episodes, depending on the region.”
The authors note that the study provides a better insight into the relationship between ozone and temperature, and the effect this might have on air quality in the future.
They call for “ambitious emissions controls to offset the potential of more than a week of additional days with unhealthy ozone levels.”
The research was supported by the NASA Air Quality Applied Sciences Team and the National Institute of Environmental Health Sciences.