25 January 2017
adapted from the story by NOAA Communications
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, which analyzed a spate of high ozone readings near Las Vegas in May in 2013, adds 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 work was conducted as part of the Las Vegas Ozone Study (LVOS) (pronounced "Elvis") conducted by NOAA ESRL at the request of the Clark County, Nevada Department of Air Quality. The paper was published in the Journal of Geophysical Research Atmospheres.
"These events 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 at 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 US metropolitan areas are designated as nonattainment areas for the 75 ppb standard, most because of high summer ozone levels. Summer ozone pollution is primarily caused by volatile organic compounds and nitrogen oxides produced here in the US that are trapped in the lower atmosphere and cooked by the sun. The late spring ozone maximum in the desert southwest also has other sources.
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 high-elevation site in the Spring Mountains west of Las Vegas, Langford's team used a unique mobile lidar (light detection and ranging) system built by NOAA to examine how large-scale storms moving inland from the Pacific Ocean can cause air from the stratosphere to descend deep into the free troposphere above the southwestern U.S.
They found that the naturally occurring ozone in these intrusions can be captured by the exceptionally deep mixed layers in the lower atmosphere that form above the Mojave Desert and areas like Las Vegas, Phoenix, and even Death Valley National Park. Intense surface heating and dry air in the Southwest during late spring and early summer can cause these mixed layers to grow to more than 2.5 miles above the surface, or more than twice as deep as typically occurs in the eastern US. This phenomenon occurs less frequently later in the summer when large-scale storm systems are less common, and has less impact on surface ozone earlier in the winter when storms are more frequent, but there is less ozone in the lower stratosphere.
The research team includes scientists from CIRES, NOAA's Geophysical Fluid Dynamics Laboratory and Center for Satellite Applications and Research, Princeton, the University of Nevada-Reno and the California Air Resources Board.
Stratospheric intrusions can also bring down pollution originating from Asia. In this study the researchers showed that some of this imported ozone was also able to enter the convective boundary layers above the Las Vegas Valley and contribute to the higher surface ozone levels observed during the study. The contribution from this source was usually much smaller than that from the stratosphere, however. Although stratosphere-to-troposphere transport and associated Asian pollution weren't the only sources of surface ozone during these days, they added between 20 and 40 ppb of ozone to background levels, so the exceedances would not have occurred without their contribution.
The EPA has a provision called the "Exceptional Events Rule" to exclude monitoring data affected by these intrusions, but most state, tribal, and local regulatory agencies lack the resources to identify these events and quantify their impacts on surface ozone. NOAA ESRL plans to return to Clark County this spring for a more extensive campaign called FAST-LVOS (Fires, Asian, and Stratospheric Transport-Las Vegas Ozone Study) to learn more about how these processes affect surface ozone in the southwest. The results from both LVOS and FAST-LVOS should help these agencies better understand these events.
"We learned a lot from the first LVOS campaign, but the data have also left us with lots of new questions, so we're all looking forward to the second coming of LVOS!" added Langford.
A. O. Langford, R. J. Alvarez II, J. Brioude, R. Fine, M. Gustin, M. Y. Lin, R. D. Marchbanks, R. B. Pierce, S. P. Sandberg, C. J. Senff, A. M. Weickmann, and E. J. Williams, Entrainment of stratospheric air and Asian pollution by the convective boundary layer in the Southwestern U.S., Journal of Geophysical Research Atmospheres, doi:10.1002/2016JD025987, 2017.
A series of deep stratospheric intrusions in late May 2013 increased the daily maximum 8-h surface ozone (O3) concentrations to more than 70 parts-per-billion by volume (ppbv) at rural and urban surface monitors in California and Nevada. This influx of ozone-rich lower stratospheric air and entrained Asian pollution persisted for more than 5 days and contributed to exceedances of the 2008 8-h National Ambient Air Quality Standard (NAAQS) of 75 ppbv on May 21 and 25 in Clark County, NV. Exceedances would also have occurred on May 22 and 23 had the new standard of 70 ppbv been in effect. In this paper, we examine this episode using lidar measurements from a high-elevation site on Angel Peak, NV and surface measurements from NOAA, the Clark County, Nevada Department of Air Quality, the EPA Air Quality System (AQS), and the Nevada Rural Ozone Initiative (NVROI). These measurements, together with analyses from the National Centers for Environmental Prediction/North American Regional Reanalysis (NCEP/NARR), NOAA Geophysical Fluid Dynamics Laboratory (GFDL) AM3, NOAA National Environmental Satellite, Data, and Information Service (NESDIS) Real-time Air Quality Modeling System (RAQMS), and FLEXPART models, show that the exceedances followed entrainment of ~20 to 40 ppbv of lower stratospheric ozone mingled with another 0 to 10 ppbv of ozone transported from Asia, by the unusually deep convective boundary layers above the Mojave Desert. Our analysis suggests that this vigorous mixing can affect both high and low elevations and help explain the springtime ozone maximum in the Southwestern U.S.