CSD News & Events:

News & Events - 2015

New Study Links Stratosphere, La Niña Climate Conditions, and Surface Air Quality

12 May 2015
NOAA Twin Otter research aircraft

Photo: R. Marchbanks, NOAA / CIRES

The NOAA Twin Otter sits on the tarmac at Mammoth Lakes, California, awaiting refueling. The aircraft had just arrived from Sacramento carrying a downward looking ozone lidar used to track stratospheric intrusions above California during a field study in June 2010.

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

New research reveals a strong connection between high ozone days in the western U.S. during late spring, the stratosphere, and La Niña, an ocean-atmosphere phenomenon that affects global weather patterns. The study was published May 12 in Nature Communications, and was led by Meiyun Lin (of NOAA's Geophysical Fluid Dynamics Laboratory and NOAA's cooperative institute at Princeton University) and co-authored by CSD's Andy Langford and others.

Following a La Niiña winter, ozone-rich air is more likely to descend from the stratosphere and reach the surface in western U.S. communities at higher elevations, the study finds.

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 would 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 U.S. and facilitates intrusions of stratospheric ozone to ground level where people live."

stratospheric contribution map

The stratospheric contribution to US surface ozone peaks in the western Rockies during late spring. This map shows mean contribution in parts per billion by volume (ppbv) for May to June.

Over 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 familiar to hear about high ozone occurring 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% 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 U.S. Environmental Protection Agency. 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. As 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 U.S. 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.

La Nina jetstream map

Researchers found that when the jet stream meanders over the western US, as occurs after a strong La Niña winter, more frequent high ozone days are expected at the western US surface in the following spring. Using lidar and balloon observations alongside a global chemistry-climate model, the researchers trace the cause of these poor air quality days to ozone plunging down to the ground from high in the upper atmosphere in dynamical events called "tropopause folds." The tropopause is the boundary between the lowest layer of the atmosphere (the troposphere, from the surface to about 10-15 kilometers) and the ozone-rich stratosphere (which extends from the tropopause to about 50 kilometers high).

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 U.S., 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.

Citation: Meiyun Lin, Arlene M. Fiore, Larry W. Horowitz, Andrew O. Langford, Samuel J. Oltmans, David Tarasick, and Harald E. Rieder, Climate variability modulates western U.S. ozone air quality in spring via deep stratospheric intrusions, Nature Communications, doi:10.1038/ncomms8105, 2015.


Evidence suggests deep stratospheric intrusions can elevate western US surface ozone to unhealthy levels during spring. These intrusions can be classified as 'exceptional events', which are not counted towards non-attainment determinations. Understanding the factors driving the year-to-year variability of these intrusions is thus relevant for effective implementation of the US ozone air quality standard. Here we use observations and model simulations to link these events to modes of climate variability. We show more frequent late spring stratospheric intrusions when the polar jet meanders towards the western United States, such as occurs following strong La Niña winters (Niño3.4<−1.0 °C). While El Niño leads to enhancements of upper tropospheric ozone, we find this influence does not reach surface air. Fewer and weaker intrusion events follow in the two springs after the 1991 volcanic eruption of Mt. Pinatubo. The linkage between La Niña and western US stratospheric intrusions can be exploited to provide a few months of lead time during which preparations could be made to deploy targeted measurements aimed at identifying these exceptional events.

More information:

High Country News: EPA releases a stricter, health-based smog standard

State of the Planet, The Earth Institute, Columbia University: Bad Ozone Days in Western U.S. Linked to Pacific Weather 12 May 2015

Summit County [Colorado] Citizens Voice: Climate- When good ozone goes bad 12 May 2015

CIRES Press Release: New study links stratosphere, La Niña and surface air quality

NOAA Research: New research will help forecast bad ozone days over the western U.S.