2014 News & Events

Air from stratosphere makes it tough for Las Vegas to meet surface ozone pollution standards

25 August 2014

U.S. map
Intrusive ozone. Air from the ozone-rich stratosphere sometimes "intrudes" down to the surface in the springtime in the Intermountain West, making it harder for some regions to meet health-based standards for ozone pollution. This map shows results from an atmospheric model of May and June 2010. In dark orange areas, stratospheric intrusions contribute about 20 parts per billion of ozone at the surface, where the chemical is a lung-damaging pollutant. The federal, health-based limit for ozone is 75 parts per billion (averaged over 8 hours), so at times, stratospheric intrusions push regions such as Las Vegas over regulatory limits. Image: Meiyun Lin, NOAA GFDL / Princeton University

In Las Vegas, air from the naturally ozone-rich stratosphere is sometimes an unwelcome intruder, making it difficult for the region to meet the national ground-level ozone standards in the springtime, according to a new NOAA-led study published online this month in the journal Atmospheric Environment.

"We found that the ozone coming down from the stratosphere is pushing the Clark County area up to and even over the limit," said Andy Langford, a research chemist at CSD and lead author of the new study. "Our finding means that the region would be especially hard pressed to meet the even tougher ozone pollution limits now being considered," said Langford.

The stratosphere is a region 8-30 miles above Earth that contains over 90 percent of the atmosphere's ozone. It's this ozone layer that's the "good ozone," filtering harmful ultraviolet radiation from the Sun. But lower down, at Earth's surface, ozone is a pollutant that's harmful to human health and other living things.

Areas at higher elevations, particularly in the intermountain western United States, are especially prone to atmospheric events called stratospheric intrusions that bring air from the stratosphere down to Earth's surface. With background levels of surface ozone gradually rising over the last few decades due to increases in human emissions of pollutants, the "extra" ozone that comes occasionally from the stratosphere now pushes some areas over the ozone standard.

"For air quality managers in Clark County, Nevada, these results explain why they've frequently seen ozone values that exceed the national air quality standard in springtime – well before the midsummer peak expected for ozone formed as a result of local pollution. This new study will help them identify when they are being affected by natural events beyond their control."

In different regions of the United States, different factors contribute to elevated ozone levels. In some places, oil and gas activities can release very high levels of chemicals that react to form ozone. In other places, the precursors of ozone sweep in on winds from distant locations, even Asia. This study shows that in the Las Vegas region, at least in the springtime, the stratosphere is the source of some of the ozone at ground level. "Pollution from Los Angeles or Asia, wildfires, and local production are smaller factors," said Langford.

TOPAZ instrument installation
TOPAZ lidar scanner installed atop Angel Peak, NV during the 2013 Las Vegas Ozone Study. Photo: A. Langford, NOAA

Researchers used models to analyze the observations they gathered during a roughly 6-week period from May to June 2013, in NOAA's Las Vegas Ozone Study (LVOS). A lidar (light detection and ranging) instrument, a remote sensing method, measured ozone from the Earth's surface to about 1.5 miles overhead, and other instruments measured gases and meteorological parameters at a mountain site about 30 miles northwest of Las Vegas.

"The stratospheric intrusions added at least 30 ppb of ozone in some of the high-ozone events we observed," said Christoph Senff, a scientist with CSD & CIRES, who analyzed the lidar observations from the study. "With the normal 'background' level of ozone usually at 50-60 ppb, this meant that ozone frequently pushed above 80 ppb."

If the current 8-hour ozone standard of 75 parts per billion (ppb) were lowered to 65 parts per billion, as the Environmental Protection Agency (EPA) is considering, the researchers estimate that the Las Vegas region and other high-elevation sites in the Intermountain West would likely exceed the air quality standard over half of springtime days.

The study's findings have implications for air quality management in the U.S. Previous studies have shown that several areas of the western U.S. receive pollution from sources that are beyond local control strategies, with some sources – such as transport from Asia and wildfires – on the rise and likely to increase further in the future.

"Our work shows that not only do air quality managers have to contend with pollution coming from across the continent or across the ocean; they also have to consider what's coming down from far above their heads," said Langford.

Authors of the study include 12 scientists from NOAA's Earth System Research Laboratory, CIRES, the Laboratoire de l'Atmosphere et des Cyclones, Princeton University, NOAA's Geophysical Fluid Dynamics Laboratory, and the NOAA/NESDIS Center for Satellite Applications and Research, Cooperative Institute for Meteorological Satellite Studies.

A.O. Langford, C.J. Senff, R.J. Alvarez II, J. Brioude, O.R. Cooper, J.S. Holloway, M.Y. Lin, R.D. Marchbanks, R.B. Pierce, S.P. Sandberg, A.M. Weickmann, E.J. Williams, An overview of the 2013 Las Vegas Transport Study (LVOS): Impact of stratospheric intrusions and long-range transport on surface air quality, Atmospheric Environmment, doi:10.1016/j.atmosenv.2014.08.040, 2014.

Abstract

The 2013 Las Vegas Ozone Study (LVOS) was conducted in the late spring and early summer of 2013 to assess the seasonal contribution of stratosphere-to-troposphere transport (STT) and long-range transport to surface ozone in Clark County, Nevada and determine if these processes directly contribute to exceedances of the National Ambient Air Quality Standard (NAAQS) in this area. Secondary goals included the characterization of local ozone production, regional transport from the Los Angeles Basin, and impacts from wildfires. The LVOS measurement campaign took place at a former U.S. Air Force radar station ∼45 km northwest of Las Vegas on Angel Peak (∼2.7 km above mean sea level, asl) in the Spring Mountains. The study consisted of two extended periods (May 19-June 4 and June 22–28, 2013) with near daily 5-min averaged lidar measurements of ozone and backscatter profiles from the surface to ∼2.5 km above ground level (∼5.2 km asl), and continuous in situ measurements (May 20–June 28) of O3, CO, (1-min) and meteorological parameters (5-min) at the surface. These activities were guided by forecasts and analyses from the FLEXPART (FLEXible PARTticle) dispersion model and the Real Time Air Quality Modeling System (RAQMS), and the NOAA Geophysical Research Laboratory (NOAA GFDL) AM3 chemistry-climate model. In this paper, we describe the LVOS measurements and present an overview of the results. The combined measurements and model analyses show that STT directly contributed to each of the three O3 exceedances that occurred in Clark County during LVOS, with contributions to 8-h surface concentrations in excess of 30 ppbv on each of these days. The analyses show that long-range transport from Asia made smaller contributions (<10 ppbv) to surface O3 during two of those exceedances. The contribution of regional wildfires to surface O3 during the three LVOS exceedance events was found to be negligible, but wildfires were found to be a major factor during exceedance events that occurred before and after the LVOS campaign. Our analyses also shows that ozone exceedances would have occurred on more than 50% of the days during the six-week LVOS campaign if the 8-h ozone NAAQS had been 65 ppbv instead of 75 ppbv.