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When Good Ozone Goes Bad

22 Feb 2017, 4:12 am

Late spring and early summer is when the air quality is generally good across most of the United States. But for the desert southwest, newly published NOAA research details how a common springtime weather pattern and pollution transported from Asia often conspire to create unhealthy ozone levels.

Ozone in the stratosphere, located 6 to 30 miles above the ground, typically stays in the stratosphere. But not on some days in late spring when the polar jet stream meanders southward over the western U.S., bringing large cyclonic storm systems that cause stratospheric ozone to sink down into the troposphere closer to where people live.

The study analyzed high ozone readings near Las Vegas, Nevada in May 2013, adding to a growing body of work that explores how ozone transported down from the lower stratosphere can occasionally push some areas of the desert southwest above federal air quality standards.  The research, conducted as part of the Las Vegas Ozone Study or LVOS, (pronounced “Elvis”) at the request of the Clark County Department of Air Quality,  was published in the Journal of Geophysical Research – Atmospheres.

Christoph Senff, a NOAA/CIRES researcher, examines equipment used in NOAA’s Las Vegas Ozone Study in June, 2013. Credit: NOAA, Andrew Langford.

“These are examples of what we like to call good ozone going bad,” says NOAA research chemist and lead author Andrew Langford. ““Good” ozone in the stratosphere protects us from the harmful ultraviolet rays of the sun, but ground level ozone can cause respiratory problems and crop damage.”

The U.S. Environmental Protection Agency’s proposal to tighten the federal standard for ozone from 75 parts per billion to 70 ppb for an eight-hour period has added urgency to the need to understand regional ozone sources and transport mechanisms. More than 50 metropolitan areas are designated as nonattainment areas for the 75 ppb standard, most because of high summer ozone levels.

The study focused on the week of May 19-26, 2013 when ozone concentrations in the Mojave Desert exceeded the federal standard on several days. Working from a site in the Spring Mountains, Langford’s team found that naturally occurring ozone that descended from the stratosphere can be captured by the exceptionally deep mixed layers of the lower atmosphere that form above the Mojave Desert, and brought down to the ground in areas like Las Vegas, Phoenix, and even Death Valley National Park.

The research team includes scientists from CIRES, NOAA’s Geophysical Fluid Dynamics Laboratory, the Center for Satellite Applications and Research, Princeton University, and the University of Nevada-Reno.

Stratospheric intrusions can also bring pollution originating from Asia down to ground levels, though  this contribution was usually much smaller. Combined, they added between 20 and 40 ppb of ozone to background levels, so the exceedances would not have occurred without their contribution.

The EPA’s “Exceptional Events Rule” allows states to exclude monitoring data affected by these intrusions, but most lack the resources to identify these events and quantify their impacts on surface ozone. NOAA plans to return to Clark County this spring to learn more about these processes and help these agencies and the EPA better understand these episodes.

The Angel Peak radar site used by the Federal Aviation Administration was the site selected for NOAA’s lidar during the LVOS study in June of 2013. Credit: NOAA, Andrew Langford.

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