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 U.S. (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 neighbouring California, and to probe the influence of stratospheric intrusions, wildfires, and local, regional, and Asian pollution on surface O3 concentrations in Las Vegas and the surrounding area. In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 and air quality attainment in the SWUS. The 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 background O3 concentrations of 50–55 parts-per-billion by volume (ppbv) across southern Nevada. These background concentrations constitute 70–80 % of the current U.S. National Ambient Air Quality Standard (NAAQS) of 70 ppbv, and illustrate some of the challenges facing air quality managers tasked with O3 attainment in the SWUS during late spring and early summer. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to estimate the impacts of transported O3 on surface air quality in the Southwestern U.S. and Intermountain West during the FAST-LVOS campaign.
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 U.S Clean Air Act (CAA) (Karstadt et al, 1993). 50 Ozone is not directly emitted into the atmosphere by anthropogenic activities, but is a secondary product of photochemical reactions between nitrogen oxides (NOx = NO + NO2) and CO, CH4, or volatile organic compounds (VOCs)
The high elevations of the Intermountain West (IMW), i.e., the region of the U.S bounded by the Cascade and Sierra Nevada Mountains to the west and the Front Range of the Rocky Mountains to the east (Fig. 1a) which includes most of the Southwest U.S (SWUS), make it vulnerable to both stratospheric intrusions (Lin et al, 2012a) and pollution transported across the Pacific 70 Ocean from East Asia (Lin et al, 2012b)
Summary and conclusions The 6-week long FAST-Las Vegas Ozone Study (LVOS) field campaign collected a wealth of lidar, surface, aircraft, and ozonesonde measurements 850 that greatly improve our understanding of O3 transport in Clark County, Nevada and the greater Southwestern U.S and Intermountain West in late spring and early summer
Summary
Efforts to control ambient O3 concentrations have sought to reduce anthropogenic emissions of these precursors and more stringent NOx emission controls have contributed to a 36% decrease in the mean daily maximum 1-h average NO2 across the U.S between 2000 and 2019 (https://www.epa.gov/air-trends/nitrogen-dioxide-trends, last access 16 August 2021). 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 (NCOS) including 65 O3 produced by photochemical reactions of anthropogenic emissions outside the U.S borders, or by soils, vegetation, lightning, or wildfires (Jaffe et al, 2018), and by naturally-occurring O3 transported downward from the stratosphere. The high elevations of the Intermountain West (IMW), i.e., the region of the U.S bounded by the Cascade and Sierra Nevada Mountains to the west and the Front Range of the Rocky Mountains to the east (Fig. 1a) which includes most of the SWUS, make it vulnerable to both stratospheric intrusions (Lin et al, 2012a) and pollution transported across the Pacific 70 Ocean from East Asia (Lin et al, 2012b)
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