Cooperative Institute for Research in Environmental Sciences

New study links stratosphere, La Niña and surface air quality

New study links stratosphere, La Niña and surface air quality

Findings will help experts forecast bad ozone days over the U.S. West


New research reveals a strong connection between high ozone days in the U.S. West during late spring, the stratosphere, and La Niña, an ocean-atmosphere phenomenon that affects global weather patterns.

Following a La Nina winter, ozone-rich air is more likely to descend from the stratosphere and reach the surface in western U.S. communities at higher elevations, according to the new study led by Meiyun Lin (of NOAA’s Geophysical Fluid Dynamics Laboratory and NOAA’s cooperative institute at Princeton University). Co-authors of the study, published May 12 in Nature Communications, include Andy Langford of NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colorado and CIRES’s Samuel Oltmans, who also works at ESRL.

Recognizing this link offers an opportunity to forecast high surface ozone pollution several months in advance, which could improve public education to reduce health effects. It could also help western U.S. air quality managers prepare to track these events, which can have implications for attaining the national standard for ozone, a regulated pollutant that has harmful effects on human health, crops and ecosystems.   

“Ozone in the stratosphere, located 6 to 30 miles (10 to 48 kilometers) above the ground, typically stays in the stratosphere,” said Lin. “But not on some days in late spring following a strong La Niña winter. That’s when the polar jet stream meanders southward over the western United States and facilitates intrusions of stratospheric ozone to ground level where people live.”

During the last two decades, there have been three La Niña events: 1998-1999, 2007-2008 and 2010-2011. After these events, scientists saw spikes in ground level ozone for periods of two to three days at a time during late spring in high-altitude locations of the U.S. West, including the Denver-Boulder area.

It’s more common to hear about high ozone levels on muggy summer days when pollution from cars and power plants fuels the formation of regional ozone pollution. But in springtime in high-altitude regions of the U.S. West, the stratosphere—which contains 90 percent of the ozone in Earth’s atmosphere—can be a source of the ozone at ground level. High-elevation areas are more vulnerable to the intrusions of air from above, owing to their closer proximity to the stratosphere.

Lin and her colleagues found that these deep intrusions of stratospheric ozone could add 20 to 40 parts per billion of ozone to the ground-level ozone concentration, which can push the ozone levels closer to, or even over, the standard set by the Environmental Protection Agency (EPA). The EPA has proposed tightening that standard currently set at 75 parts per billion for an eight-hour average to between 65 and 70 parts per billion.

Under the Clean Air Act, these deep stratospheric ozone intrusions can be classified as “exceptional events” that are not counted towards EPA attainment determinations. If our national ozone standard becomes more stringent, the relative importance of these stratospheric intrusions grows, leaving less room for human-caused emissions to add to the surface ozone levels without triggering an exceedance of the standard set by the  EPA.  

“Regardless of whether these events count towards non-attainment, people are living in these regions and the possibility of predicting a high-ozone season might allow for public education to minimize adverse health effects,” said Arlene Fiore, an atmospheric scientist at Columbia University and a co-author of the research.

Though stratospheric intrusions have been recognized and studied for many years, the link to La Niña is a new finding that opens up the possibility of longer-term predictions of the intrusions. Predicting where and when stratospheric ozone intrusions may occur would also provide time to deploy air sensors to obtain evidence as to how much of ground-level ozone can be attributed to these naturally occurring intrusions and how much is due to human-caused emissions.

The study involved collaboration across two NOAA laboratories, NOAA’s cooperative institutes at Princeton and the University of Colorado Boulder, and scientists at partner institutions in the United States, Canada and Austria.

“This study brings together observations and chemistry-climate modeling to help understand the processes that contribute to springtime high-ozone events in the western U.S.,” said Langford, an atmospheric scientist who measures ozone concentrations using lidars.

“You’ve heard about good ozone, the kind found high in the stratosphere that protects the earth from harmful ultraviolet radiation,” said Langford. “And you’ve heard about bad ozone at ground level. This study looks at the factors that cause good ozone to go bad.”

Lin, Fiore, and Langford conducted the research with Larry Horowitz of GFDL; Samuel Oltmans of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, who works in NOAA's Earth System Research Laboratory; David Tarasick of Environment Canada; and Harald Rieder of the University of Graz in Austria. 

CIRES is a partnership of NOAA and the University of Colorado Boulder

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