High background levels of ozone pollution make it hard for Las Vegas and other southwestern cities to meet US air quality standards in spring, two NOAA studies find. But local pollution sources drive summer’s high ozone days. It’s an annoying rite of summer – hot, smoggy days when spiking levels of ozone pollution make it hard – or even unhealthy – to breathe. But for years, NOAA scientists have been investigating why some regions of the southwestern U.S. experience episodes of high ozone during spring, weeks before the arrival of summer’s dog days. A pair of field studies led by Andy Langford, a research chemist with NOAA’s Chemical Sciences Laboratory, in 2013 and 2017 have now demonstrated there are two separate atmospheric mechanisms that swamp Las Vegas and the desert southwest with high background levels of imported ozone during spring.
“These are things that can’t be controlled,” Langford said. “Looking at the bigger picture, it doesn’t leave Las Vegas a lot of wiggle room when it comes to meeting federal air quality standards.” In 2013, Langford’s team of scientists showed how ozone-rich air descending from the stratosphere – where the protective ozone layer is found – during spring storms can sometimes reach the surface to create short episodes of high ozone that by themselves exceed federal air quality standards. These so-called “stratospheric intrusions” can also capture ozone created by pollution from Asia as they descend and transport it to the surface as well. Now, results from an intensive field study in 2017, published this month in the journal Atmospheric Chemistry and Physics, have helped reveal how even those intrusions that don’t directly reach the surface or occur hundreds or even thousands of miles upwind over the Pacific Ocean also contribute to chronically elevated surface ozone in the region during spring.
During a 45-day field study in May and June of that year, scientists from three NOAA labs and the NOAA cooperative institute CIRES collected data from different elevations in the atmosphere using an array of instruments on airplanes, weather balloons, and ground stations. The data were fed into sophisticated atmospheric transport models to distinguish incoming ozone pollution generated as far away as Asia from regional sources like wildfires and stratospheric intrusions. Scientists also measured local ozone production from the growing Las Vegas metro area, which like much of the far southwest is currently designated by the EPA as a nonattainment area for ozone. In the new paper, the scientists demonstrate how layers composed of naturally occurring ozone from the stratosphere and Asian pollution can be mixed down to the surface above the Desert Southwest and mingle with ozone produced by local pollution. These chronic, but less acute, events can still raise surface ozone concentrations to 70 to 80% of the current U.S. National Ambient Air Quality Standard of 70 parts per billion ppb averaged over 8 hours – even before one whiff of local pollution is factored in. “We were seeing background ozone levels averaging 50 to 55 ppb over five weeks out in the desert, as far away as Great Basin and Grand Canyon national parks,” he said. “If you bring that air into Las Vegas, you don’t have a lot of room before you violate the national standard. Should the national standard be lowered to 60 ppb as some are proposing, he said, “during spring Las Vegas will have virtually no wiggle room at all.” The results of the two campaigns have largely validated the complaints of Nevada officials, who have long claimed that their high springtime ozone levels are not a home-grown problem, but are caused by non-local ozone sources. The national standard, however, doesn’t account for regional differences in background sources. In 2016, the EPA updated its Exceptional Events rule, which provides a workaround allowing data that would otherwise factor into air quality violations to be disregarded if created by an event that was beyond control. But the exceptional event rule applies to single events, not chronic episodes, and the burden of proof falls on states, tribes, local governments or regulated agencies, which have varying degrees of technical expertise and financial resources. Indeed, both NOAA field studies were supported by Clark County, whose county seat is Las Vegas. “During our first study, we saw lots examples of big stratospheric intrusions that created exceedences all by themselves,” he said. “We didn’t see any of these big events in 2017, but we saw a lot of smaller transport events. There was high ozone just a few kilometers above the ground almost all the time.” In other parts of the west, some regulated entities tried to blame this background ozone for their high summer ozone concentrations. Langford said that it’s important to note these springtime events in the southwest don’t extend into summer, when most of the western U.S. sees the highest ozone days of the year. “Background ozone is a real issue in terms of meeting standards in May and June in the southwest, but it is not responsible for high ozone in western cities in July and August,” he said. Work in the Las Vegas airshed continues for NOAA scientists. This summer, CSL scientists in an instrumented mobile laboratory conducted sampling inside the metro area to help better understand the influence of local pollution on the seasonal ozone cycle. Support for the new paper was provided by Clark County, Nevada, the NOAA Climate Program Office and NASA.
For more information, contact Theo Stein, NOAA Communications, at email@example.com, or Chelsea Thompson, NOAA Chemical Sciences Laboratory, at firstname.lastname@example.org.