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). Ozone is not directly emitted into the atmosphere by anthropogenic activities but is a secondary product of photochemical reactions between nitrogen oxides (NOx = nitric oxide (NO) + nitrogen dioxide (NO2)) and carbon monoxide (CO), methane (CH4), or volatile organic compounds (VOCs). Thus, 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 annual mean daily maximum 1 h NO2, the benchmark used to ascertain NAAQS attainment, across the US between 2000 and 2019 (https://www.epa.gov/air-trends/nitrogen-dioxide-trends, last access: 16 August 2021). In response, the US mean fourth-highest annual maximum daily 8 h average (4MDA8) O3, the metric by which compliance with the O3 NAAQS is determined, declined from 82 to 65 parts per billion by volume (ppbv), or 21 % over the same period (https://www.epa.gov/air-trends/ozone-trends, last access: 16 August 2021).

The air quality improvements in the US over the last 2 decades represent a major success, but the gains have not been uniform, with large decreases in the southeastern US (SEUS: Alabama, Florida, Georgia, North Carolina, South Carolina, and Virginia), where NO2 and O3 declined by 39 % and 25 %, respectively, but much smaller changes in the southwestern US (SWUS: Utah, Arizona, Colorado, and New Mexico), where a 38 % reduction in NO2 led to an O3 decline of just 11 %. The 4MDA8 O3 decreased from 83 to 63 ppbv in the SEUS but only decreased from 77 to 69 ppbv in the SWUS. 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 particularly vulnerable to both stratospheric intrusions (Lin et al., 2012a) and pollution transported across the Pacific Ocean from East Asia (Lin et al., 2012b).

Background O3 cannot be directly measured, but an upper limit can be estimated from measurements at remote “baseline” locations sufficiently removed from anthropogenic sources. Jaffe et al. (2018) estimated that the monthly mean MDA8 O3 in the higher elevations of the IMW is around 50 ppbv in late spring, with the 4MDA8 O3 concentrations exceeding 60 ppbv in some locations. These high baseline concentrations constitute more than 70 % of the 70 ppbv NAAQS set in 2015 (EPA, 2014) and limit the ability of western air quality managers to maintain surface O3 concentrations below this standard through local control strategies (Cooper et al., 2015; Uhl and Moore, 2018; Faloona et al., 2020).

Concern about the potential impacts of stratospheric intrusions, Asian pollution, and other NCOSs on NAAQS attainment in the greater Las Vegas area motivated the first Las Vegas Ozone Study (LVOS) conducted by the NOAA Chemical Sciences Laboratory (CSL) in late May and June of 2013 (Langford et al., 2015b) in partnership with the Clark County (Nevada) Department of Air Quality (CCDAQ). The LVOS 2013 field campaign was organized around the truck-mounted TOPAZ (Tunable Optical Profiler for Aerosol and oZone) differential absorption lidar (Alvarez et al., 2011), which is part of the NASA-sponsored Tropospheric Ozone Lidar Network (TOLNet). The lidar was deployed for 6 weeks to a decommissioned US Air Force radar base on the summit of Angel Peak (AP: 36.32∘ N, -115.57∘ E; 2680 m above mean sea level, a.s.l.), about 45 km west of Las Vegas, in the Spring Mountains (Fig. 1c). The lidar measurements were augmented by in situ measurements of O3 and CO and basic meteorological parameters.

The LVOS 2013 campaign documented several episodes in which the appearance of stratospheric intrusions, Asian pollution, or wildfire plumes at AP was followed by MDA8 O3 concentrations greater than 70 ppbv at multiple regulatory monitors in Clark County and in some cases by exceedances of the 2008 O3 NAAQS of 75 ppbv then in effect. Since stratospheric intrusions do not typically reach the relatively low elevations of Las Vegas (620 m a.s.l.), it was hypothesized that some of the high-O3 episodes resulted from entrainment of mid-tropospheric O3 layers (Langford et al., 2017) by the deep-convective boundary layers that form over the Mojave Desert (Seidel et al., 2012). This hypothesis could not be confirmed, however, because of the placement of the lidar above the valley floor, and a more extensive follow-up study, the Fires, Asian, and Stratospheric Transport–Las Vegas Ozone Study (FAST-LVOS), was conducted over the same time period in 2017 to (i) test the entrainment hypothesis, (ii) determine the representativeness of the LVOS 2013 results, and (iii) better characterize the different sources of surface O3 in the Las Vegas Valley (LVV). In this paper, we present an overview of the FAST-LVOS campaign with brief examples highlighting the influence of stratospheric intrusions, Asian pollution, biomass burning, and both local and regional pollution on surface O3 in Clark County and the greater IMW. The companion paper by Zhang et al. (2020) uses the GFDL-AM4 and GEOS-Chem global models to simulate these measurements and quantify the impacts of these processes on high-O3 events in southern Nevada and the greater SWUS and IMW.

Previous airborne and ground-based lidar measurements have shown that elevated O3 layers are common features of the lower free troposphere between ≈ 3 and 6 km a.s.l. above California (Langford et al., 2012; Ryerson et al., 2013; Faloona et al., 2020) and southern Nevada (Langford et al., 2015b, 2017) in late spring and early summer, and it is well established that the US west coast is one of the global hotspots for deep stratosphere-to-troposphere transport (STT) of O3 in springtime (Wernli and Bourqui, 2002; Sprenger and Wernli, 2003; James et al., 2003; Skerlak et al., 2014, 2015; Breeden et al., 2021). This occurs primarily through the formation of tropopause folds, tongues of lower stratospheric air that descend isentropically beneath the jet stream circulating around upper-level lows. These intrusions of dry, O3-rich air often form near the end of the North Pacific storm track and descend behind the mixture of clean or polluted tropospheric air in the dry airstream (Wernli, 1997; Cooper et al., 2004; Trickl et al., 2014). They are thus an important mechanism for transport of East Asian pollution from the upper troposphere down to the boundary layer above the western US (Brown-Steiner and Hess, 2011; Lin et al., 2012a, b).

Tropopause folds typically follow one of two pathways as they reach the middle troposphere. Many intrusions, particularly those formed by deep closed lows, continue to curve cyclonically as they descend and wrap up above the surface low in the lower troposphere (Danielsen, 1964). These deep LC2 (lifecycle 2) intrusions (Thorncroft et al., 1993; Polvani and Esler, 2007) often reach the top of the boundary layer and sometimes even the surface at higher elevations (Schuepbach et al., 1999; Stohl et al., 2000; Bonasoni et al., 2000; Trickl et al., 2020). These intrusions were first described by Reed and Danielsen (1958) and have long been observed as steeply sloping tongues in ozone lidar curtains (Browell et al., 1987; Ancellet et al., 1994; Vaughan et al., 1994; Langford et al., 1996; Eisele et al., 1999).

Much less attention has been paid to the so-called LC1 (lifecycle 1) intrusions that are sheared anticyclonically from elongated troughs to form quasi-horizontal filaments in the middle and upper troposphere (Appenzeller and Davies, 1992; Appenzeller et al., 1996; Vaughan et al., 2001; Albers et al., 2021). These streamers are well-known features of satellite water vapor imagery (Manney and Stanford, 1987) and can stretch for thousands of kilometers. They usually roll up horizontally into a series of diminishing vortices to be irreversibly mixed into the free troposphere (Wirth et al., 1997; Vaughan and Worthington, 2000; Vaughan et al., 2001; Colette and Ancellet, 2006), but they can also be dissipated by moist convection (Langford and Reid, 1998). They are a significant source of background O3 in the free troposphere (Albers et al., 2021) but usually remain well above the boundary layer and thus do not influence surface O3 directly.

Both types of intrusions are most common in winter, when cyclonic activity is at a maximum, but stratospheric intrusions that form in late May and early June are more likely to affect compliance with the ozone NAAQS since (i) more O3 is available for transport from the lower stratospheric reservoir in springtime (Albers et al., 2018), (ii) surface concentrations are higher because of increased photochemical production, and (iii) deeper afternoon mixed layers (Seidel et al., 2012) can potentially entrain some intrusions that would otherwise pass overhead. Stratospheric influence plays an important role in driving the observed year-to-year variability in springtime ozone air quality over the western US (Lin et al., 2015). The potential surface impacts of stratospheric intrusions and co-transported pollution decrease in summer after the jet stream migrates poleward into Canada, and deep convection lifts pollution higher into the upper troposphere over East Asia (Hudman et al., 2004; Brown-Steiner and Hess, 2011).

The FAST-LVOS field experiment was also organized around the TOPAZ lidar, which was upgraded with a new data acquisition system in 2015. This upgrade more than doubled the useful operating range of the lidar from ≈ 3 to 8 km, which allowed it to reach higher altitudes than was possible during LVOS 2013, even while operating from a much lower-elevation site at the North Las Vegas Airport (NLVA; 36.2∘ N, -115.2∘ E; 680 m a.s.l.), located about 8 km NW of downtown Las Vegas. NOAA CSL also brought a vertically staring Doppler lidar to the NLVA to characterize mixing between the boundary layer and free troposphere and a mobile laboratory with a more extensive suite of in situ measurements to the summit of AP. These primary measurements were supplemented by measurements from a single-engine Mooney TLS Bravo aircraft operated by Scientific Aviation, Inc. (SA) and by ozonesondes launched by the NOAA Global Monitoring Laboratory (GML) during four 2 to 4 d intensive operating periods (IOPs) initiated when the synoptic conditions showed that stratospheric intrusions or Asian pollution transport events were likely.

The FAST-LVOS campaign can be divided into two meteorologically distinct periods, a late spring period from mid-May to mid-June and an early summer period from mid-June through the end of June. The Synoptic Discussion for May 2017 from the National Centers for Environmental Information (NCEI) (https://www.ncdc.noaa.gov/sotc/synoptic/201705, last access: 16 August 2021) describes the jet stream as being very active in the late spring period, with a series of closed lows and upper-level troughs crossing the contiguous US every few days. These cyclonic systems spawned a series of stratospheric intrusions that appear as potential vorticity (PV) enhancements (Duncan et al., 2021) in the 300 hPa (≈ 9.5 km a.s.l.) NASA MERRA-2 Reanalysis plots in Fig. 3a–d. GOES water vapor images coinciding with the PV analyses are displayed in Fig. S2. The first extratropical cyclone, designated L1 in Fig. 3a, passed through Nevada before the official start of the campaign on 20 May, but the next three low-pressure systems, labeled L2, L3, and L4 in Fig. 3b–d, were targeted by IOPs on 23–25 May, 31 May–2 June, and 11–14 June. The solar radiation data in Fig. S1 show the appearance of cirrus ahead of the cold fronts as well as perturbations in the normal diurnal variations in pressure, temperature, dew point, and winds created by thermally driven regional (e.g., plains–mountain) and mesoscale (e.g., valley and slope) circulations (Stewart et al., 2002). There was no measurable precipitation, but the temperatures on AP dropped below freezing on the nights of 17–18 May and 11–12 June, and the cold fronts decreased the depth of the afternoon boundary layers above the NLVA to ≈ 2 km, or roughly the elevation of AP above the valley.

The jet stream retreated north into Canada in the wake of L4, and the NCEI Synoptic Discussion for June 2017 confirms that the expanding subtropical ridge seen in Fig. 3e dominated the weather across the SWUS for the rest of the campaign (https://www.ncdc.noaa.gov/sotc/synoptic/201706, last access: 16 August 2021). The ridge brought clear skies, dry conditions, and record high temperatures to Clark County, and June 2017 was the third-hottest in Las Vegas since official record keeping began in 1948, and the extreme temperatures and dry conditions exacerbated multiple wildfires across the SWUS. The high temperatures at the NLVA exceeded 41 ∘C (106 ∘F) each day during the last 2 weeks of the study, and the daily records for Las Vegas were tied or exceeded on 5 straight days from 20 to 24 June. The official high of 47.2 ∘C (117 ∘F) at Harry Reid International Airport on 20 June, the last day of astronomical spring, tied the all-time Las Vegas record. Coincidentally, this record was also tied on the final day of the first LVOS campaign (30 June 2013). The daily high temperatures abated slightly to 42–43 ∘C during the last few days of the campaign and IOP4, when a weak cold front associated with the shallow trough (L8) in Fig. 3f passed through the LVV. The temperatures on AP were typically 10–15 ∘C lower due to its higher elevation.

The aircraft and ozonesonde profiles acquired during the four IOPs provided spatial context for the lidar measurements and helped distinguish stratospheric intrusions from Asian pollution. The planning and successful execution of these intensives relied on the ability of the RAQMS (96 h) and RAP-Chem (48 h) models to predict stratospheric intrusion and pollution transport events more than 48 h in advance so that the aircraft and ozonesonde teams could return to Las Vegas from their home stations. Examples of the model forecasts are shown in Fig. 8, which displays the O3 and CO forecasts for 12:00 UT 12 June (cf. Fig. 3d). The left panels of Fig. 8 show the 96 h total O3 (top) and CO (bottom) at 320 K (≈ 500 hPa or 5.8 km a.s.l. above Las Vegas) from RAQMS initialized at 12:00 UT on 7 June, and the right panels show the 36 h RAP-Chem O3 and CO 500 hPa forecasts initialized at 00:00 UT on 11 June. Note that the higher-resolution RAP-Chem model gets its boundary conditions from the RAQMS model. This example shows that the RAQMS model captured both the timing and location of the stratospheric intrusion–Asian pollution event 4 d out or 3 d before the start of IOP3 on 10 June. The retrospective MERRA-2 PV analyses in Fig. 3 and FLEXPART stratospheric ozone (STTO3) and Asian CO (ASIACO) tracer distributions (https://csl.noaa.gov/projects/fastlvos/FAST-LVOSfinalreport604318-16.pdf, last access: 7 December 2021) in Figs. 9 and 10, respectively, show that stratospheric air and/or Asian pollution was present in the middle and upper troposphere above Nevada during each of the IOPs. The correspondence between the 96 h RAQMS forecasts in Fig. 8 and the retrospective FLEXPART distributions in the third columns of Figs. 9 and 10 is particularly impressive.

The curtain plots in Fig. 4 show that the TOPAZ lidar measured high O3 in the middle and upper troposphere during each of the IOPs; the expanded 3 d curtain plots in Figs. 11–14 show these measurements in more detail. The superimposed horizontal ribbons represent the NLVA and AP surface measurements and the nearly vertical ribbons the ascending profiles from the ozonesondes. Those portions of the descending profiles within 20 km of the NLVA are also plotted. The NLVA and AP surface wind measurements and continuous Doppler lidar boundary layer heights are also overlaid on the curtains, with the boundary layer heights derived from the ozonesonde potential temperature profiles (open black circles) plotted for comparison. The heavy arrows at the top of each plot correspond to the MERRA-2 analyses in Fig. 3. The labels A, B, D, and G match the corresponding peaks in Fig. 6.

The SA aircraft profiles overlap with the midday ozonesonde profiles and are omitted from the curtain plots for clarity, but Fig. 15 shows longitudinal transects of the sections of the outbound (left) and inbound (right) flight legs between the NLVA and AP on 25 May (IOP1), 2 June (IOP2), 12 June (IOP3), and 28 June (IOP4). The flight tracks are colorized by the O3 measurements, and the plots also show the mean lidar and ozonesonde profiles and surface O3 measurements from the roughly 1 h interval bracketing the NLVA and AP profiles. Ozonesondes were only launched between 09:00 and 15:00 PDT and thus did not overlap with the return flight legs. These plots underscore the good agreement between the different FAST-LVOS O3 measurements and support the assumption that the air above AP was usually representative of the air above the LVV. Similar plots colorized by H2O and CH4 are displayed in Figs. S2 and S3. Figure 16 shows the measured O3 mixing ratios along the outbound flight legs from the NLVA to Big Bear and the inbound flight legs back to Jean. The AP profiles and TOPAZ measurements are omitted from these plots as are the afternoon profiles above the LVV. In the following sections, we briefly describe each of the IOPs and compare the upper-air measurements with the FLEXPART STTO3 and ASIACO tracer distributions. More complete descriptions of these measurements are planned for future publications.

Not all of the interesting transport episodes occurred during one of the four IOPs, and the labels C, E, and F in Figs. 6, 17, and 18 highlight three other examples. Figure 6a shows that the highest 1 min O3 concentrations (≈ 95 ppbv) measured on AP during FAST-LVOS 2017 were sampled during episode C around 02:00 PDT on the morning of 9 June. The high O3 lasted for about an hour but occurred several hours after TOPAZ had ceased operations for the night. However, the 8 June curtain plot in Fig. 4d shows a 1 km deep layer of high O3 just above the top of the boundary layer throughout the observing session that appears to have persisted through the night and into the next morning, when the lidar operations resumed. The unusual dryness of the air sampled on AP and relationships between O3 and the other measured species (Fig. 17) suggest that this layer was also Asian pollution that recently descended from the upper troposphere. The episode C measurements completely overlap with the measurements from 28 June (episode G) in many of the scatterplots, suggesting a common origin. The HYSPLIT trajectories in Fig. 18 are also similar, and the peak concentrations measured by TOPAZ, ≈ 105 ppbv on 8 June and ≈ 115 ppbv on 28 June, are also comparable.

The 1 min mobile laboratory measurements from the morning of 18 June (episode E) include both the highest H2O (12 g kg-1) and lowest O3 concentrations (29 ppbv) measured on AP during FAST-LVOS 2017. Figure 4f shows that TOPAZ also measured unusually low O3 throughout the tropospheric column. As noted above, this episode is attributed to transport of clean marine air deep into the IMW by the anticyclonic flow around the subtropical ridge (Fig. 3e), and the HYSPLIT back trajectory (Fig. 18, dashed red line) suggests that this moist background air was lifted into the lower free troposphere above the Pacific Ocean more than 4 d earlier. Figures 6 and 17 show that the mobile laboratory also measured very low O3, CO, CO2, CH4, and NO2 concentrations on AP (the NOy converter was offline). The filled black circles in Fig. 17 show that the O3, N2O, CO2, and CH4 concentrations measured at AP on the morning of 18 June were comparable to the mean (±1σ) June 2017 values measured at the high-elevation NOAA GML Mauna Loa Observatory (https://gml.noaa.gov/dv/data/, last access: 16 August 2021). These measurements are thought to represent the background concentrations in the free-tropospheric air reaching the US west coast.

The final example (episode F) coincides with the highest backscatter (Fig. 5f) measured above the NLVA during FAST-LVOS 2017. These measurements were made about 24 h after the start of the Holcomb Fire near Big Bear on the afternoon of 19 June (cf. Fig. 1). This fire grew rapidly but was battled aggressively, and the expansion was checked at ≈ 600 ha (≈ 1500 acres) by the evening of 21 June. The anticyclonic path followed by the HYSPLIT trajectory (dashed green line) in Fig. 18 passed directly over the Holcomb Fire and possibly over older fires in Arizona and northern Mexico. The curtain plots in Figs. 4f and 5f show that there was ≈ 85 ppbv of O3 in the smoke that appeared at ≈ 3 km a.g.l. on the morning of 20 June but only ≈ 50 ppbv of O3 in the denser smoke that appeared between 4 and 6 km a.g.l. in the afternoon and evening. Some of the smoke from the second plume was entrained by the convective boundary layer, and the CO time series from the mobile laboratory on AP shows a corresponding rise to more than 140 ppbv. However, Fig. 17e shows that there was little NO2 in the second plume, which is consistent with the absence of any O3 enhancement in either the sampled air or the lidar measurements.

The top five panels of Fig. 19 show the hourly averaged and MDA8 O3 measurements from the remote sampling sites operated by the WMRC and the NVDEP during FAST-LVOS. All of these sites are located well away from populated areas, with the WMRC located ≈ 310 km WNW of the LVV and the NVDEP monitors located between 180 and 250 km to the north (cf. Fig. 1). The bottom panel shows the measurements from the monitor maintained by the CCDAQ in the town of Mesquite ≈ 120 km NE of Las Vegas, near the Nevada–Utah border. MDA8 O3 concentrations exceeding 65 and 70 ppbv are highlighted in orange and red, respectively. The mean MDA8 O3 mixing ratio (±1σ, the standard deviation of the mean) is shown in each plot. These measurements show a pronounced elevation trend consistent with descent of O3 from the free troposphere and destruction at the surface. The former is reflected in the higher mean values and episodic increases at the higher elevations and the latter in the low nighttime values at the lower elevations.

The WMRC operates the highest-elevation long-term O3 monitors in the continental US, with both the WMS (4.34 km a.s.l.) and BCO (3.88 km a.s.l.) located roughly 1 km higher than the CASTNET site in Gothic, CO (2.93 km a.s.l.), which is the highest regulatory O3 monitor in the US. The MDA8 O3 measured at the WMS exceeded the 2015 NAAQS of 70 ppbv on 17 of the 46 FAST-LVOS sampling days, with a mean MDA8 O3 of 68 ± 12 ppbv between 15 May and 30 June, when there were persistent O3 layers aloft, and 73 ± 11 ppbv between 23 May and 14 June, the 22 d interval bracketing the beginning of the first and end of the third IOPs. High O3 was measured during each of the four IOPs, with the 1 h peak concentrations exceeding 100 ppbv on 16 May, 25–26 May, and 9–10 June. The MDA8 O3 was greater than 65 ppbv on 24 d, including 18 of the 22 d between 23 May and 14 June, and the 4MDA8 O3 for the 6-week period was 86 ppbv. The MDA8 O3 concentrations were similar but about 8 ppbv lower on average at the BCO.

The mean MDA8 O3 at WSSU, the highest-elevation NVDEP site at 2.3 km a.s.l., was about 17 ppbv lower than that at WMS, with a 4MDA8 O3 of 61 ppbv. The large episodic increases measured by the WMRC monitors during the IOPs did not reach the lower-lying NVDEP sites, and there were no NAAQS exceedances by any of these non-regulatory monitors. Nevertheless, the mean MDA8 O3 at these three remote sites ranged from ≈ 50 to 55 ppbv, or 70 %–80 % of the NAAQS, between mid-May and the end of June, with 4MDA8 O3 concentrations ranging from 59 to 62 ppbv (84 %–88 % of the NAAQS). An earlier analysis of springtime (March–May) measurements from 2012 and 2013 at 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 (i.e., formed from natural sources plus anthropogenic sources in countries outside the US) monthly mean springtime MDA8 O3 concentration of 50 ppbv estimated by Lin et al. (2012a, b) for higher elevations in the 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. These differences should be viewed with caution since they do not account for changes in Asian precursor emissions or differences in trans-Pacific pollution transport and STT caused by the position of the jet stream (Lin et al., 2015; Albers et al., 2018; Breeden et al., 2021).

The EPA has designated Las Vegas, Nevada, a marginal non-attainment area for O3 (https://www3.epa.gov/airquality/greenbook/jnc.html, last access: 16 August 2021). The warm and sunny conditions that lead to rapid photochemical production of O3 also drive the upslope flow into the Spring Mountains that causes O3 and other pollutants to accumulate in the northwestern LVV. The highest O3 concentrations are usually measured at the Joe Neal (C75) and Walter Johnson (C71) monitors (cf. Fig. 1), which had 2017 ozone design values, i.e., the 3-year running average of the fourth-highest MDA8 O3, of 74 and 72 ppbv, respectively. Figure 20 plots the hourly averaged and MDA8 O3 measurements from these two sites during FAST-LVOS, along with the corresponding measurements from the Apex (C22), Jean (1019), and Indian Springs (C7772) monitors (cf. Fig. 1). The thin blue line in each plot represents the average of the RRVA, CGSP, and WSSU MDA8 O3 measurements from Fig. 19, which we use as an estimate for the southern Nevada mean baseline MDA8 O3. The Joe Neal and Walter Johnson monitors also recorded the highest O3 concentrations in Clark County during FAST-LVOS and exceeded the 2015 NAAQS on 7 and 6 d, respectively. Most of these exceedances coincided with the higher temperatures that followed the poleward expansion of the subtropical ridge in mid-June (cf. Fig. S1). Several of the other monitors located within a 15 km radius of the NLVA also exceeded the NAAQS on at least 1 of these days. There was only one exceedance each at the slightly more distant Apex (32 km NE) and Green Valley (22 km SE) monitors and no exceedances at the Boulder City (40 km SE), Jean (49 km SSE), or Indian Springs (58 km NW) monitors (cf. Fig. 1c). Note that the Jean monitor was offline from 9–12 June. All of the CCDAQ monitors measured unusually low O3 on 18–19 June.

The MDA8 O3 measured by the Indian Springs monitor was very similar to the NVDEP baseline values, with the largest differences arising on 16–17 June, when the anticyclonic marine incursion had reached the more northerly NVDEP sites but had not yet reached Clark County. The mean NVDEP and Indian Springs time series agree to within 3 % on average (R2 = 0.69) if the measurements from these 2 d are excluded. This suggests that the Indian Springs measurements may be a good proxy for the LVV baseline O3 during the FAST-LVOS campaign, and the solid black line in Fig. 20f represents the difference between the Joe Neal MDA8 O3 concentrations and the Indian Springs monitor. The dotted line is similar but shows the difference for the Walter Johnson monitor. The measurements from days where the MDA8 O3 concentrations exceeded 65 or 70 ppbv are highlighted as before. The MDA8 O3 concentrations at Joe Neal and Walter Johnson were significantly higher than the baseline concentrations on 23 May, with the Walter Johnson and Joe Neal monitors reaching 71 and 69 ppbv, respectively. Although the lidar, aircraft, and ozonesonde observations showed high O3 aloft throughout the first IOP, there was no obvious mixing of the high O3 into the boundary layer. There were multiple exceedances in the San Joaquin Valley and South Coast air basins on 22–24 May, however, and the elevated O3 in the LVV on 23 May is most likely due to regional transport and local production (cf. Sect. 6.1). Ozone was also elevated in the LVV on 2–3 June, and the measurements in Fig. 12 do appear to show entrainment of stratospheric O3 and Asian pollution into the boundary layer, although this supposition could not be confirmed by the AP measurements. The MDA8 O3 reached 68 and 65 ppbv, respectively, at Walter Johnson and Joe Neal on 2 June and 70 and 69 ppbv on 3 June. The descending O3 may also have contributed to an isolated cluster of NAAQS exceedances at state and tribal monitors in southwestern Colorado and northwestern New Mexico and to the MDA8 O3 of 70 ppbv recorded at Mesa Verde National Park on 3 June.

The high winds (> 20 m s-1 at the surface) preceding the major intrusion event of 11–13 June (IOP3) cleared local pollution out of the LVV ahead of the descending stratospheric air, and the MDA8 O3 concentrations at Walter Johnson and Joe Neal reached only 59 and 57 ppbv on 11 June. Interestingly, the highest reported MDA8 O3 concentrations in Clark County on that day were 63 and 60 ppbv at Indian Springs and Apex, respectively. There were multiple NAAQS exceedances in Arizona on 13–15 June, however, including an MDA8 O3 concentration of 91 ppbv measured on 14 June about 60 km northeast of Phoenix at the Tonto National Monument. The GFDL-AM4 model estimated 20–30 ppbv MDA8 O3 enhancements from stratospheric influence around this site and other areas in Arizona on 14 June (see Fig. 9 in Zhang et al., 2020). The next highest values recorded at this site in 2017 were 78 ppbv on 15 June and 77 ppbv on 13 June.

The Walter Johnson and Joe Neal measurements in Fig. 20 appear qualitatively very different from the White Mountain Summit and Barcroft Observatory measurements in Fig. 19. Nearly all of the NAAQS exceedances at the high elevation sites occurred before the middle of June, when stratospheric intrusions and Asian pollution plumes appeared frequently in the middle and lower troposphere, but nearly all of the exceedances in the LVV occurred after the middle of June, when higher temperatures and stagnant conditions increased photochemical production of O3 in the boundary layer. The high temperatures also exacerbated regional wildfires, including the Holcomb and Brian Head fires, whose plumes reached the LVV in the third week of June (cf. Figs. 4f and 5f). Note that these fires did not increase surface O3 at the Jean, Indian Springs, or Mesquite monitors, suggesting that the smoke plumes were NOx-limited, and significant O3 production required additional NOx from urban sources in the LVV (Singh et al., 2012). High O3 was measured both in and above the boundary layer on the last 3 d of June during IOP4, with a clear example showing the entrainment of a fine-scale Asian pollution plume by the growing convective boundary layer on 28 June. This event contributed to MDA8 O3 of 69 and 74 ppbv at Walter Johnson and Joe Neal as well as the only exceedance by the Apex monitor which measured 71 ppbv. The high-elevation WMRC monitors also measured elevated O3 in the lower free troposphere on 29 June, and some of this O3 may have contributed to the exceedances in the LVV on that day.

The 2013 LVOS and 2017 FAST-LVOS campaigns were conducted over the same late spring/early summer time period, but the distribution and sources of the high O3 measured in Clark County during 2013 and 2017 were very different. The MDA8 O3 exceeded 70 ppbv at one or more of the Clark County monitors on 17 d in May and June of 2013 but only 8 d in May and June of 2017. In 2013, 12 of the 17 exceedance days occurred before 13 June, and 9 were in May. In 2017, 7 of the 8 exceedance days occurred after 13 June, with only 1 in May. Only one of the high-O3 events in 2017 affected more than 4 of the 12 active monitors, but 8 of the 17 high-O3 events in 2013 affected 4 of the 11 active monitors, and 4 affected 9 or more of the CAMSs. These were the stratospheric intrusion and wildfire event of 4–5 May (Langford et al., 2015a), stratospheric intrusion events of 21 and 25 May (Langford et al., 2018), and Asian pollution event of 21 June (Langford et al., 2017). These major events affected monitors outside of Las Vegas as well as those in the valley, and the Jean monitor exceeded 70 ppbv on 8 d in 2013 but not once in 2017 (the Indian Springs monitor was not installed until 2015).

The 6-week-long FAST-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. Daily lidar observations found high-O3 layers in the lower and middle free troposphere above Las Vegas on more than 75 % (35 of 45) of the measurement days. The highest tropospheric concentrations were measured on 11–12 June, when both the lidar and an ozonesonde launched nearby found up to 250 ppbv within 5.5 km of the surface. Several of these elevated O3 layers were also sampled by a high-elevation research monitor located more than 300 km to the west-northwest, in the White Mountains of California (Burley and Bytnerowicz, 2011), and the MDA8 O3 concentrations at this site averaged more than 68 ppbv over the course of the study. Simulations from the FLEXPART, GEOS-Chem, and AM4 models (Zhang et al., 2020) show that the elevated layers were created by stratospheric intrusions or long-range transport from Asia and, in most cases, a mixture of the two. Aircraft measurements made during the four IOPs support this conclusion, and the aircraft mapped the spatial distribution of these plumes above the Mojave Desert in southern Nevada and California. FAST-LVOS also documented several fine-scale anticyclonic streamers in addition to the deep cyclonic intrusions that are the focus of most studies.

The FAST-LVOS measurements also captured several examples of O3 being entrained from aloft by the convective boundary layer, and correlations between the different tracers measured on AP (2.7 km a.s.l.) showed the distinctive signatures of both stratospheric air and Asian pollution on multiple occasions. The AP measurements also found wildfire influences and evidence for regional transport of anthropogenic pollution from the San Joaquin Valley in addition to local upslope transport of pollution from the LVV. Although none of the stratospheric intrusions or Asian pollution events directly caused any of the NAAQS exceedances in the LVV during FAST-LVOS, they clearly added to the surface concentrations on at least 1 (14 June) of the exceedance days and contributed to the mean MDA8 O3 of 50–55 ppbv measured at the remote sites in rural Nevada during the study. These mean baseline concentrations represent 70 %–80 % of the 2015 NAAQS of 70 ppbv, making it more difficult for western air quality managers to maintain surface O3 concentrations below the NAAQS in the IMW during late spring and early summer (Cooper et al., 2015; Uhl and Moore, 2018).