State of the Planet

News from the Columbia Climate School


Bad Ozone Days in Western U.S. Linked to Pacific Weather

A new study shows that ozone pollution in the western United States can be increased by La Niña, a natural weather cycle at the surface of the Pacific Ocean. Scientists have come to recognize that La Niña and its opposite phase, El Niño, affect various kinds of weather around the world; the ozone finding is the first to show that it also directly affects pollution. The study appears in the current issue of the journal Nature Communications.

La Nina, a weather cycle over the Pacific Ocean, can cause changes in air circulation that drive stratospheric ozone to ground level in the western United States (courtesy NOAA)
La Nina, a weather cycle over the Pacific Ocean, can cause changes in air circulation that drive stratospheric ozone to ground level in the western United States (courtesy NOAA)

High ozone is harmful to human health, causing breathing difficulty, coughing, sore throats, and asthma attacks; it can also damage plants. The researchers say that recognition of its link to La Niña offers a way to forecast levels several months in advance, which would help air-quality managers prepare for events. “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’s Lamont-Doherty Earth Observatory and a coauthor of the research.

High ozone typically occurs on muggy summer days, when pollution from cars and power plants grips the ground. But the stratosphere also contains large amounts of ozone, 6 to 30 miles above the ground—so-called “good” ozone, which protects surface dwellers from harmful ultraviolet solar radiation.

During La Niña periods, the surface of the tropical Pacific cools. La Niña winters tend to be followed in late spring by strong downward gusts of cold, dry air from the stratosphere, and this sends the ozone downward. The western United States, much of which sits at high elevations, is particularly vulnerable. “The polar jet stream meanders southward over the western U.S. and facilitates intrusions of stratospheric ozone to where people live,” said lead author Meiyun Lin of the National Oceanic and Atmospheric Administration (NOAA). The study “looks at the factors that cause good ozone to go bad,” said Andrew Langford, a NOAA atmospheric scientist who also coauthored the paper.

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 two to three days at a time during late spring in high-altitude locations of the U.S. West.

The researchers found that these intrusions of stratospheric ozone could add 20 to 40 parts per billion of ozone to existing ground-level concentrations—enough to provide more than half the ozone needed to exceed the standard set by the U.S. Environmental Protection Agency. The current standard is 75 parts per billion, but the EPA has proposed tightening the eight-hour average to 65-70 parts per billion.

Predicting where and when stratospheric ozone intrusions may occur would provide authorities time to deploy air sensors to figure out how much ground-level ozone can be attributed to naturally occurring intrusions, versus human-caused emissions. Such determinations will become more important as EPA decides how to regulate allowable levels, say the authors.

Lin and Fiore conducted the research with Larry Horowitz, also of NOAA; Samuel Oltmans of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder; David Tarasick of Environment Canada; and Harald Rieder of the University of Graz in Austria.


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