Abstract
Abstract. The Fires, Asian, and Stratospheric Transport–Las Vegas Ozone Study (FAST-LVOS) was conducted in May and June of 2017 to study the transport of ozone (O3) to Clark County, Nevada, a marginal non-attainment area in the southwestern United States (SWUS). This 6-week (20 May–30 June 2017) field campaign used lidar, ozonesonde, aircraft, and in situ measurements in conjunction with a variety of models to characterize the distribution of O3 and related species above southern Nevada and neighboring California and to probe the influence of stratospheric intrusions and wildfires as well as local, regional, and Asian pollution on surface O3 concentrations in the Las Vegas Valley (≈ 900 m above sea level, a.s.l.). In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 in Clark County and southern Nevada. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to simulate the measurements and estimate the impacts of transported O3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O3 layers above Las Vegas on more than 75 % (35 of 45) of the sample days and show that entrainment of these layers contributed to mean 8 h average regional background O3 concentrations of 50–55 parts per billion by volume (ppbv), or about 85–95 µg m−3. These high background concentrations constitute 70 %–80 % of the current US National Ambient Air Quality Standard (NAAQS) of 70 ppbv (≈ 120 µg m−3 at 900 m a.s.l.) for the daily maximum 8 h average (MDA8) and will make attainment of the more stringent standards of 60 or 65 ppbv currently being considered extremely difficult in the interior SWUS.
Highlights
Ground-level ozone (O3) is one of six “criteria” air pollutants identified as serious threats to human health and welfare and made subject to National Ambient Air Quality Standards (NAAQS) by the US Clean Air Act (CAA) (Karstadt et al, 1993)
Springtime (March–May) measurements from 2012 and 2013 at Cathedral Gorge State Park (CGSP) (Fine et al, 2015a) found very similar mean MDA8 O3 concentrations of 51 ± 6 and 50 ± 8 ppbv, respectively. These measurements are comparable to the background monthly mean springtime MDA8 O3 concentration of 50 ppbv estimated by Lin et al (2012a, b) for higher elevations in the Intermountain West (IMW) using the GFDL AM3 model for the year 2010 but are significantly higher than the mean MDA8 O3 of 40 ± 7 ppbv derived by Zhang et al (2011) using GEOS-Chem model runs for the years 2006– 2008 and the upper limit of 40–45 ppbv for the seasonal mean MDA8 estimated by Dolwick et al (2015) using Community Multiscale Air Quality (CMAQ) and Comprehensive Air Quality Model with Extensions (CAMx) runs for the year 2007
The 6-week-long FAST-Las Vegas Ozone Study (LVOS) field campaign collected a wealth of lidar, surface, aircraft, and ozonesonde measurements that greatly improve our understanding of O3 transport in Clark County, Nevada, and the greater southwestern US and Intermountain West in late spring and early summer
Summary
Ground-level ozone (O3) is one of six “criteria” air pollutants identified as serious threats to human health and welfare and made subject to National Ambient Air Quality Standards (NAAQS) by the US Clean Air Act (CAA) (Karstadt et al, 1993). The weaker response to NO2 reductions in the SWUS is attributed in part to increased oil and gas development (Pozzer et al, 2020) and in part to the much higher background O3 in this region (EPA, 2013; Lefohn et al, 2014; Cooper et al, 2015) This background is derived from a variety of non-controllable ozone sources (NCOSs) including O3 produced by photochemical reactions of anthropogenic emissions outside the US borders or by soils, vegetation, lightning, or wildfires and by naturally occurring O3 transported downward from the stratosphere (Jaffe et al, 2018). The high mean elevations of Colorado (2078 m a.s.l.), Wyoming (2047 m a.s.l.), Utah (1864 m a.s.l.), Nevada (1681 m a.s.l.), and Idaho (1528 m a.s.l.), which make up the heart of the Intermountain West (IMW), i.e., the area of the US bounded by the Cascade (≤ 4392 m a.s.l.) and Sierra Nevada (≤ 4421 m a.s.l.) mountains to the west and the Front Range of the Rocky Mountains (≤ 4401 m a.s.l.) to the east (Fig. 1a), make this entire region vulnerable to both stratospheric intrusions (Lin et al, 2012a) and pollution transported across the Pacific Ocean from East Asia (Lin et al, 2012b)
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