Despite sustained estimated reductions in anthropogenic precursor emissions, ozone design values remain above the 2015 8-hour National Ambient Air Quality Standard in four large, urban nonattainment areas (NAAs) in the Intermountain Western U.S.: Phoenix-Mesa, Arizona; Las Vegas, Nevada; Wasatch Front, Utah; and Denver Metro/North Front Range, Colorado. We synthesize recent field campaigns, regulatory analyses, and observationally constrained modeling to summarize shared drivers of stalled progress and the region-specific mechanisms that modulate ozone accumulation. Across these areas, elevated background ozone, terrain-driven recirculation, and meteorological conditions favoring stagnation and vertical coupling all reduce the effectiveness of local emission controls. However, many important factors also differ by airshed: Phoenix is strongly influenced by monsoon-modulated photochemistry and boundary-layer dynamics; Las Vegas by frequent entrainment of ozone-rich layers from aloft and a high ozone "floor"; the Wasatch Front by corridor confinement, elevated background concentrations, and multi-day recirculation; and the Front Range by complex terrain flows interacting with spatially heterogeneous precursor sources, including strong within-NAA oil and gas emissions. Together, each of these case studies underscores the need for ozone management strategies tailored to each airshed's structural and dynamical constraints, supported by coordinated collaborative science.Implications: Despite decades of declining anthropogenic precursor emissions, four major Intermountain Western U.S. metropolitan areas (Phoenix, Las Vegas, Salt Lake City, and Denver) continue to exceed the 2015 ozone standard. This synthesis identifies factors, both shared and specific to each airshed, driving persistent nonattainment. Elevated background ozone, wildfire smoke, terrain driven recirculation, and vertical entrainment all limit the effectiveness of local controls, though dominant mechanisms differ by area. For lawmakers and regulators, these findings underscore the need for region specific strategies, better vertical and precursor monitoring, and improved attribution of controllable versus transported ozone to support sound, technically defensible regulatory decisions.

The U.S. Intermountain West frequently experiences high O-3 events that exceed the National Ambient Air Quality Standard (NAAQS). This study examines 2009-2024 by combining EPA mean daily maximum 8-h average (MDA8 O-3) data with NOAA Hazard Mapping System smoke products, satellite-based tropospheric column retrievals, and meteorology and trajectory data to characterize the frequency, patterns, and potential drivers of common O-3 exceedances, defined as days with MDA8 O-3 > 70 ppb. Four non-attainment areas (NAAs) are evaluated: Phoenix-Mesa-Scottsdale (Arizona); Las Vegas, Clark County (Nevada); Denver Metro/North Front Range (Colorado); Northern and Southern Wasatch Front (Utah). Across all months, daily MDA8 O-3 is moderate (peaking near 45-55 ppb, with similar to 2-3% of days >70 ppb), and 37 days are identified as common exceedances when all four NAAs exceed the threshold simultaneously, occurring in 12 years (2010-2013, 2015-2018, 2020-2021, and 2023-2024). Exceedances typically begin in April-June and end in August-September, peaking in mid-summer (June-August). Common exceedances exhibit substantial heterogeneity within individual NAAs in terms of O-3 levels across monitoring sites, occurring more often on weekdays (Tuesday-Friday) and least frequently on Sunday, suggestive of anthropogenic influence superimposed on potential regional factors. Wildfire smoke coincides with common exceedances, especially in 2018, 2020, and 2024 when smoke coverage simultaneously affected all four NAAs. Exceedances are characterized by enhanced temperatures and solar radiation, seasonally varying boundary layer and pressure anomalies, and generally lower formaldehyde-to-NO2 ratios. Trajectory analyses suggest potential inter-basin transport, motivating continued research into this factor.

TiO2-based photocatalytic coatings applied to glass substrates, such as solar panels, demonstrated the ability to reduce NOx concentrations by up to 36% in laboratory settings and generate nitrate flux values of up to 33 mg per m2 per day in field tests.

Valley fever, a disease caused by Coccidioides spp., is a fungal respiratory disease with an expanding range. Methods to culture the pathogen from soil, especially Coccidioides posadasii, are very challenging, limiting the genomics knowledge of environmental strains. In this study, we designed and tested a targeted DNA capture and enrichment system for the characterization of Coccidioides genomes without the need to culture. In this system, RNA probes are hybridized to Coccidioides DNA in a complex sample, followed by DNA amplification, sequencing, and analysis. Our enrichment system was targeted toward coding region sequences in C. posadasii str. Silveira and tested on control DNA spiked into soil; DNA hybridized to probes was then sequenced and correctly placed into a reference phylogeny, based on the known placement of the whole-genome sequence. We then applied the enrichment system to a range of sample types (soil, air filters, rodent tissue) from a site in Mesa, Arizona, USA. The enriched samples were sequenced and placed into the C. posadasii phylogeny to understand the phylogenetic diversity within the Mesa site over time. The results demonstrate that low DNA signal in most sample types was boosted after enrichment. Enriched sequences from air filters collected at multiple time points from the Mesa site linked two different isolates collected from fatal cases of Coccidioidomycosis in a pig-tailed macaque colony housed at the Mesa site. This represents the first time that environmental C. posadasii DNA was directly linked to Coccidioidomycosis and demonstrates the power of this approach for genomic epidemiology.IMPORTANCEAll human cases of Valley fever are acquired through environmental exposure, so surveillance and characterization of the pathogen in soil are critical for risk mitigation efforts. Current databases are biased toward human clinical isolates, and little is known about the genomics of environmental strains of Coccidioides posadasii. In this study, we designed, tested, and validated a probe enrichment system that amplifies trace DNA in a complex sample. Sequenced DNA can be used to link environmental exposure with human cases, directing public health agencies to interventions that limit human exposure. This use case was demonstrated in this study, as trace DNA trapped on air filters was linked to a fatal case of primate Coccidioidomycosis at a site in Arizona. The probe enrichment system described in this study represents a powerful tool to better understand the genomic composition of environmental C. posadasii strains, which can aid in public health investigations.

The impact of wildfire smoke on surface air quality is examined for one of the largest metro areas in the United States (Phoenix, Arizona) as a case study of urban areas in arid regions. While many studies have examined direct smoke impacts at surface level, the present work seeks to also identify the composite impact of smoke throughout the atmospheric column on surface air quality. O3 and PM2.5 concentrations increased on smoke days compared to no smoke days, although increases in these pollutants were not distributed equally across the study region. O3 and PM2.5 showed concurrent increases on smoke days, while no relationship between the degrees degrees two was found for no smoke days. Smoke days tended to have higher mean daily air temperatures than no smoke days (34.1 C vs. 32.9 C, respectively). Given that the National Ambient Air Quality Standard (NAAQS) for O3 was exceeded on 51 % of smoke days compared to 28 % of no smoke days examined, results of this work highlight the need to understand the mechanisms by which wildfire smoke throughout the atmospheric column impacts surface air quality to develop effective strategies for protecting vulnerable populations and improving air quality.

Valley fever is a lung infection caused by the inhalation of infectious spores from the fungus Coccidioides spp. Coccidioides is a genus of soil dwelling fungi endemic to the arid regions of the southwestern United States, Mexico, and Central and South America. Few Valley fever studies have focused on detecting Coccidioides spores in airborne respirable particles, which is the primary infection vector. This study looks at the presence of Coccidioides in air at a highly soil positive site in Mesa, Arizona. Aerosol samples were collected for 24 h every 6 days, following the Environmental Protection Agency sampling schedule. Meteorological data were collected from a nearby weather station. Coccidioides were detected in similar to 68% of the aerosol samples. Bulk PM10 did not have a statistically significant relationship with presence of Coccidioides; however, there was a significant relationship between the amount of crustal material in the aerosols and presence of Coccidioides. Previous studies link the presence of Coccidioides in air with bulk PM10 concentrations; however, we found that bulk PM10 concentrations give an incomplete story. Additionally, there were statistically significant relationships with the presence of Coccidioides and meteorological parameters, including relative humidity, temperature, and wind speed. This study emphasizes the importance of dust entrainment in the transmission of Coccidioides.

With the increasing number of electric vehicles taking to the roads, the impact of tailpipe emissions on air quality will decrease, while resuspended road dust and brake/tire wear will become more significant. This study quantified PM10 emissions from tire wear under a range of real highway conditions with measurements across different seasons and roadway surface types in Phoenix, Arizona. Tire wear was quantified in the sampled PM10 using benzothiazoles (vulcanization accelerators) as tire markers. The measured emission factors had a range of 0.005–0.22 mg km−1 veh−1 and are consistent with an earlier experimental study conducted in Phoenix. However, these results are lower than values typically found in the literature and values calculated from emissions models, such as MOVES (MOtor Vehicle Emission Simulator). We found no significant difference in tire wear PM10 emission factors for different surface types (asphalt vs. diamond grind concrete) but saw a significant decrease in the winter compared to the summer.

As part of the Desert Southwest Coarse Particulate Matter Study which characterized the composition of fine and coarse particulate matter in Pinal County, AZ during 2010-2011, several source samples were collected from several different soil types to assist in source apportionment analysis of the study results. Soil types included native desert soils, agricultural soils (crop farming), dirt-road material adjacent to agricultural areas, paved road dusts, dirt road material from within and adjacent to a cattle feedlot, and material from an active cattle feedlot. Following laboratory resuspension of the soil, size-segregated PM2.5 and PM10 fractions for each source type were collected on filters and characterized for mass, ions, OC, EC, and trace elements. While there are unique chemical compositions of soils in the region (e.g., high As and Sb) that reiterate the importance of using local source profiles (e.g., native soils) as compared to Upper Continental Crust or soil profiles from other regions in receptor modeling studies. The study also provides new insights into the impact of land-use modification on source emission profiles. Specifically, high OC and PO43- are found in material representative of local cattle feedlot activities while elevated Cu, Sb and Zn are found from sources impacted by motor vehicle traffic. Results of the study indicate that the local native soil composition is only slightly modified by agricultural activities and this study provides the chemical composition of both native and agricultural soil for source apportionment studies in the Desert Southwest.

Microplastics (MPs) have been extensively studied in the marine environment in recent years, but their occurrence in recreational waters, and recreational activities as a source of MPs, have been less explored. In this study, we investigate the temporal variation of MPs in the Salt River, a natural surface waterway heavily used for recreation, and in community swimming pools in nearby Tempe, Arizona. Samples were processed using established methodologies and MP shapes and number concentrations were obtained by optical microscopy. The MP concentrations in samples of surface water collected during recreational activity ranged from 27,798 to 222,391 MPs/m3, with the highest concentrations occurring at 16:00 and lowest at 8:00, consistent with recreational activities. Fibers were the dominant shape (≥ 71%) of MPs overall in the Salt River, accounting for as much as 96% of all MPs at peak activity time (16:00). MP concentrations in water samples from apartment community swimming pools ranged from 59,160 to 254,574 MPs/m3. In terms of shape, fibers were again dominant (sometimes as high as 87%) in these water samples. Raman spectroscopic characterization of the MPs revealed the presence of polyethylene (PE), polyvinyl chloride (PVC), polyester (PES), polyamide (PA), and polypropylene (PP), showing a larger variety of polymers in the pool samples, while more MP pieces remained chemically unidentifiable. The prevalence of PES and PA fibers indicates that release from synthetic fabrics such as swimwear is a substantial source of MPs in the environment.

An oxidizing and harmful pollutant gas, tropospheric ozone is a product of a complex set of photochemical reactions that can make it difficult to enact effective control measures. A better understanding of its precursors including volatile organic compounds (VOCs) and nitrogen oxides (NOx) and their spatial distribution can enable policymakers to focus their control efforts. In this study we used low-cost sensors (LCSs) to increase the spatial resolution of an existing NO2 monitoring network in addition to VOC sampling to better understand summer ozone formation in Maricopa County, Arizona, and observed that afternoon O3 values at the downwind sites were significantly correlated, ~0.27, to the morning NO2 × rate values at the urban sites. Additionally, we looked at the impact of wildfire smoke on ozone exceedances and compared non-smoke days to smoke days. The average O3 on smoke days was approximately 20% higher than on non-smoke days, however, the average NO2 concentration multiplied by estimated photolysis rate (NO2 × rate) values were only 2% higher on smoke days. Finally, we evaluated the ozone sensitivity of the region by calculating HCHO/NO2 ratios using three different datasets: ground, satellite, and model. Although the satellite dataset produced higher HCHO/NO2 ratios than the other datasets, when the proper regime thresholds are applied the three datasets consistently show transition and VOC-limited O3 production regimes over the Phoenix metro area. This suggests a need to implement more VOC emission controls in order to reach O3 attainment in the county.

A region often neglected in the grander scale of general atmospheric chemistry studies and model evaluation for gas-phase chemistry is the desert southwest of the U.S. Despite regulatory progress, challenges in meeting the National Ambient Air Quality Standard for ozone motivate a re-examination of the unique meteorological conditions, interactions between the desert, agricultural, and built environmental landscapes, emissions across natural and anthropogenic sources, and regional transport of precursors that govern ozone formation in the desert Southwest. Arizona includes multiple nonattainment counties with a unique situation in terms of its environment (e.g., vegetation, meteorology, fire prone areas), complex terrain, urban growth, transport vulnerability, and limited knowledge base. Here we summarize past works investigating the ozone over Arizona, including 61 peer-reviewed publications found since the first one in 1996, and determine significant knowledge gaps to guide future research with the aim of improving regulatory policy. A more in-depth focus is placed here on Maricopa County, which includes the Phoenix Metropolitan area, where significant population growth in recent decades coupled with the extreme high temperatures and surrounding complex terrain creates a poorly understood airshed in terms of ozone chemistry, thereby complicating regulatory decisions. We suggest paths forward, including improved monitoring, assessment, and modeling tools for the region, better leveraging of archived data, and engagement with the public, government, and policy. This Review is highly relevant as well to other semiarid and arid regions, which represent the most common land type globally, warranting more attention.

With the goal of corroborating existing emissions inventories of volatile organic compounds (VOCs), a statistical analysis was undertaken on measured ambient VOC concentrations in Maricopa County, Arizona. The Chemical Mass Balance (CMB) model was used to generate emissions source contribution estimates based on ambient VOC concentrations collected at the JLG Supersite in Phoenix, Arizona, and emissions source profiles obtained from EPA's SPECIATE database. With trial-and-error, optimal model performance using a combination of emissions source profiles yielded source contribution estimates which could be compared to existing regulatory engineering-based emissions inventories. The ultimate objective of this study is to offer a comparison to the "top-down" emissions modeling via CMB and the "bottom-up" modeling traditionally used in preparing emission inventories to identify possible discrepancies and help direct future investigations to better understand local air quality. The methods used to develop the "bottom-up" inventory rely upon sound modeling developed to accurately capture emissions from various source categories. The results show discrepancies between the "bottom-up" and "top-down" emission inventory for VOC emissions from biogenic and natural gas combustion sources, suggesting that the emission strength from these source categories should be further investigated.Implications: The following implication statement has been prepared for the manuscript titled Source Apportionment of Measured Volatile Organic Compounds in Maricopa County, Arizona. The purpose of preparing such a study was to independently corroborate the findings of Maricopa County Air Quality Department (MCAQD) on source contribution estimates of VOC emissions as published in their 2020 Periodic Emissions Inventory for Ozone Precursors. The goal of preparing the findings in the study was to provide additional commentary on the significance of various VOC emissions sources to tropospheric ozone formation in Maricopa County through an alternate air quality modeling approach. The findings from this study are significant to the environment and health of Maricopa County as they offer additional insights into the pathways by which tropospheric ozone may form.

Microplastics are rapidly emerging anthropogenic stressors that pose a potential threat to ecosystems and human health. While the ubiquitous nature of microplastics in water has been well documented, studies on their distribution in the air are limited. Measuring microplastics, as a component of atmospheric particulate matter, is important in assessing air pollution impacts within a breathing zone. Here we investigate and present results for the occurrence of microplastics in suspended particulate matter in Tempe, a suburban location in Arizona. Samples were collected from Oct 28th, 2020, to Nov 1st, 2021, on quartz fiber filters using a high-volume air sampler, and processed with microplastics counted under an optical microscope to obtain quantitative information of their presence and distribution in the atmosphere. Microplastics were present in all collected suspended particulate samples ranging from between 0.02 and 1.1 microplastics/m3 (average concentration of 0.2 microplastics/m3) with the size range of (5–5000) μm. Fibrous microplastics were the most prevalent accounting for a large majority (≥82%) of the microplastics suspended in air in all samples. To characterize the type of microplastics present, micro-Raman spectroscopy was used to identify the chemical composition of microplastics. Chemical characterization results revealed an array of polymers for the airborne microplastics. The most abundant identified polymer was polyvinyl chloride (19%). However, many micro-Raman spectra lacked characteristic peaks, making the chemical identification process more challenging. Laboratory experiments simulating weathering of microplastics were performed to understand how microplastics change under weathering processes; these experiments revealed that Raman spectra of microplastics change over time due to weathering processes.

Microplastics are environmental contaminants that have been extensively studied in marine and aquatic environments; terrestrial ecosystems, where most microplastics originate and have the potential to accumulate, typically receive less attention. This study aims to investigate the spatial and temporal soil concentrations of microplastics in a large desert metropolitan area, the Central Arizona-Phoenix Long-Term Ecological Research (CAP-LTER) area. Soil samples from the Ecological Survey of Central Arizona (ESCA) surveys (2005 and 2015) were leveraged to study spatial distributions and the temporal change of microplastic abundances. The temporal soil microplastics data were supplemented by microplastics deposition fluxes in a central location within the area (Tempe, AZ) for a period of one year (Oct 5th, 2020 to Sept 22nd, 2021). Samples were processed and microplastics were counted under an optical microscope to obtain quantitative information of their distribution in soil. Results for the spatial variation of the microplastic abundances in soil samples in Phoenix and the surrounding areas of the Sonoran Desert from 2015 depict microplastics as ubiquitous and abundant in soils (122 to 1299 microplastics/kg) with no clear trends between different locations. Microplastics deposition fluxes show substantial deposition in the local area (71 to 389 microplastics/m2/day with an average deposition flux of 178 microplastics/m2/day) but the role of resuspension and redistribution by dust storms to deposition may contribute to the unclear spatial trends. Comparison between the 2005 and 2015 surveys show a systematic increase in the abundance of microplastics and a decrease in microplastics size. Micro-Raman spectroscopy identified a variety of plastics including PE, PS, PVC, PA, PES and PP. However, a majority of microplastics remained chemically unidentifiable. Polyethylene was present in 75 % of the sampling sites and was the most abundant polymer on average in all soil samples.

Frequent collocation and calibration paired with temperature, relative humidity, and ozone correction factors improved the performance of a low-cost NO2 sensor network in Maricopa County, Arizona.

Shark Bay Marine Park is a UNESCO World Heritage Property located in a region of marginal tropical cyclone influence. Sustainable management of this unique environment as the climate changes requires a quantified understanding of its vulnerability to natural hazards. Here, we outline a structured analysis of novel historical archive information that has uncovered reports of an extreme storm surge associated with a Tropical Cyclone in 1921 that generated remarkable overland flow which left fish and sharks stranded up to 9.66 km (6 miles) inland. Weighted information from historical archives is placed in a new framework and provide inputs to modelling of this event which improves the understanding of its magnitude and furnishes records of the impacts of what occurred on that day and notably also in the years following. The suite of plausible tracks that reproduce the historical data contextualise the storm as a marginal Category 4 or 5 storm and its return interval as equivalent or slightly greater than the current local planning level for coastal flooding in the region. The outcome underscores the global importance of examining the probable maximum event for risk management in areas of marginal cyclone influence where vulnerable ecosystems or vital regional infrastructure of key economic importance are located, and the need to factor in TC risk in marine conservation and planning in the Shark Bay World Heritage Property.

<p><strong>Shark Bay Marine Park is a UNESCO World Heritage Property in a region of marginal tropical cyclone influence and its sustainability requires a deep consideration of cyclone hazards. Here, we analyse historical records of a large storm surge from a Tropical Cyclone in 1921 that generated remarkable overland flow leaving fish and sharks stranded over 9 km inland. We weight information from the historical archives in a new framework and model event scenarios to reconstruct its magnitude. The plausible event scenarios imply that the cyclone was a marginal Category 4 or 5 storm with a return interval equivalent or slightly greater than the regional planning level. The outcome underscores the importance of examining the pre-instrumental events in areas of marginal cyclone influence as they are commonly of key economic importance.  Our work also implies that TC risk affects marine conservation in the Shark Bay World Heritage Property and requires attention. </strong></p>

One commonly proposed strategy for reducing urban air pollution is transitioning from single-occupancy vehicle (SOV) travel to alternative transportation (AT) modes, such as walking, biking, and using public transportation. While many studies have addressed the benefits of switching from SOV to AT, fewer studies have examined the potential for negative outcomes due to increased exposure to heat when using AT modes. This work uses Maricopa County, Arizona, home to the metropolitan Phoenix area, as a test case to examine the potential impacts of heat on commuters who utilize AT. First, regions of the county with the most candidates for switching from SOV to AT were identified and used to develop an AT candidate index. This index was based on both the current rates of AT use and the number of SOV commuters with the shortest commuting times in the dataset (<10 min). Next, typical weather conditions during warnings for high ozone (O-3) pollution were examined. From 2017 to 2020, over one-quarter of all days with an O-3 warning also were subject to an excessive heat warning. Last, land surface temperature data were used to determine the potential for increased heat exposure during AT commuting at both the ZIP code and AT infrastructure (public transit stops and bikeways) scales. Although this work focuses on Maricopa County, the issues presented here are increasingly relevant for cities across the world that are subject to poor air quality, hotter temperatures, and heat waves.

The Nigerian city of Lagos experiences severe air pollution as a result of emissions and subsequent atmospheric photochemistry and aerosol chemistry. A year-long study, between August 2020 and July 2021, included measurements of gas-phase and aerosol processes, with surface meteorology at six urban sites. The sites were selected to represent near seacoast conditions, urban sites, and inland locations near agricultural and grassland ecosystems. The observations included continuous concentrations for CO, SO2, NOx, O3, PM2.5, and PM10. Samples were collected and analyzed for speciated volatile organic compounds (VOCs) and particulate chemical composition including inorganic and organic chemical species. The average diel variations in concentrations indicated well-known local photochemistry resulting from the presence of combustion sources, including motor vehicles, petroleum production and use, and open burning. The annual diel characteristics were emission-dependent and were modulated by meteorological variability, including the sea breeze and the seasonal changes associated with monsoons and Harmattan winds. Gases and particulate matter varied daily, consistent with the onset of source activities during the day. Fine particles less than 2.5 μm in diameter (PM2.5) included both primary particles from emission sources and secondary particles produced in the atmosphere by photochemical reactions. Importantly, particle sources included a large component of dust and carbonaceous material. For the latter, there was evidence that particle concentrations were dominated by primary sources, with little secondary material formed in the atmosphere. From complementary measurements, there were occasions when regional chemical processes affected the local conditions, including transportation, industry, commercial activity, and open waste burning.

The troubling trend of rising heat-associated mortalities in an urban desert region (Maricopa County, AZ, USA) has motivated us to explore the extent to which environmental factors may contribute to increased heat-health risks Summertime data from 2010 to 2019 were used to construct a suite of models for daily heat-associated mortalities. The best-performing full model included the following predictors, ordered from strongest to weakest influence: daily average air temperature, average of previous 5 days daily average air temperature, year, day of year, average of previous 5 days daily average dew point temperature, average of previous 5 days daily average PM2.5, and daily average PM10. This full model exhibited a 5.39% reduction in mean absolute error in daily heat-associated mortalities as compared to the best-performing model that included only air temperature as an environmental predictor. The extent to which issued and modeled excessive heat warnings (from both the temperature only and full models) corresponded with heat-associated mortalities was also examined. Model hindcasts for 2020 and 2021 showed that the models were able to capture the high number of heat-associated mortalities in 2020, but greatly undercounted the highest yet observed number of heat-associated mortalities in 2021. Results from this study lend insights into environmental factors corresponding to an increased number of heat-associated mortalities and can be used for informing strategies towards reducing heat-health risks. However, as the best-performing model was unable to fully capture the observed number of heat-associated mortalities, continued scrutiny of both environmental and non-environmental factors affecting these observations is needed.