La Niña could be driving spring ozone levels in the West, according to new research that has major implications for the Obama administration’s proposed tightening of the federal ozone standard.
Analyzing air quality and meteorological data from 1990 to 2012, researchers found the frequency of high-ozone events increased after strong La Niña winters. The polar jet stream at that time creates upper-atmosphere intrusions that funnel ozone toward the ground in the western United States.
The finding is significant in light of U.S. EPA’s proposal to toughen the national ozone standard, the researchers said, because it may allow regulatory entities to better predict and document when high-ozone days occur as a result of background concentrations.
The EPA proposal would tighten the standard from 75 parts per billion to between 65 and 70 ppb — a range that approaches background levels of ozone in some high-elevation areas in the Intermountain West.
"The linkage between La Niña and western U.S. stratospheric intrusions can be exploited" the scientists wrote, "to provide a few months of lead time during which preparations could be made to deploy targeted measurements aimed at identifying these exceptional events."
The study was published last week in the journal Nature Communications.
Meyun Lin, a research scientist at the National Oceanic and Atmospheric Administration and Princeton University’s Cooperative Institute for Climate Science, led the study. Researchers from NOAA, Columbia University, the Meteorological Service of Canada and the University of Graz in Austria collaborated.
Background ozone is defined as the ground-level ozone pollution that would occur if all man-made sources of pollution in North America were switched off. In some high-elevation areas of the United States, a substantial proportion of total ozone is background.
Previous studies have found stratospheric intrusions — tongues of ozone that make their way from the upper atmosphere to ground level — are a key component of background levels in the West. Although ozone is beneficial in the upper atmosphere, shielding the Earth from most of the sun’s ultraviolet rays, at the Earth’s surface it helps create smog and can damage human health (Greenwire, Nov. 17, 2014).
But until recently, scientists understood little about the factors that drive stratospheric intrusions and their frequency.
In the current study, the researchers said they analyzed hourly ozone data from 1990 to 2012 at 22 sites in the high-elevation Intermountain West. They found that both the level and frequency of high ozone measurements increased in the springs that followed the strong La Niña winters of 1998-99, 2007-08 and 2010-11.
La Niña appeared to produce spikes of high ozone in late spring lasting two to three days at high-elevation sites in the West, according to the results. The intrusions of ozone from the stratosphere increase ground-level ozone concentrations by between 20 and 40 ppb, they found.
"Ozone in the stratosphere, located 6 to 30 miles [10 to 48 kilometers] above the ground, typically stays in the stratosphere," Lin said in a statement on the study. "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."
On the other hand, the researchers found that the level and frequency of high-ozone events in the West fell in the two springs after the massive eruption of Mount Pinatubo, a volcano in the Philippines, in 1991. The volcano, they said, led to fewer and weaker upper-atmospheric intrusions.
The researchers also found that El Niño had very little effect on the frequency and strength of stratospheric intrusions.
In the spring following the strong 1998-99 La Niña event, 3 percent of measurements exceeded the 75 ppb EPA ozone limit. The spring following the extreme El Niño event of the winter of 1997-98, on the other hand, produced 1 percent of measurements above the 75-ppb standard.
"While El Niño leads to enhancements of upper tropospheric ozone, we find this influence does not reach surface air," the researchers wrote.
Industry and GOP critics of EPA have raised the issue of background as one of their main arguments against tightening the ozone standard. They’ve argued that a tighter standard would be difficult to achieve for Western states because a high proportion of background means there’s only a small margin for emissions controls to work.
Aside for stratospheric intrusions, other documented causes of high background ozone levels in the Intermountain West include pollution from Asia and wildfires. EPA has a process to excuse "exceptional" pollution events that occur out of states’ control, but some experts believe that the process is not up to the task of dealing with frequent high-ozone events caused by background under a tighter standard.
In their study, the researchers said that their study could make it easier to predict and prove that high-ozone days occur as a result of background, helping states avoid costly "nonattainment" designations under a tighter standard.
The knowledge that high ozone days occur after strong La Niña winters could also benefit public health, said Arlene Fiore, a study author and an atmospheric scientist at Columbia University.
"Regardless of whether these events count toward nonattainment," she said, "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."