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  • View in gallery

    Schematic of various coupled meteorological and chemical processes within wintertime PCAPs. Figure is not to scale. Importantly, stable nocturnal inversions can be extremely shallow (tens of meters) and elevated inversions can also be present depending on the large-scale synoptic flow.

  • View in gallery

    (bottom left) Elevation map of the western United States showing selected air quality monitoring sites (black dots) that exhibit winter maxima in PM2.5. Colored markers show three sites that are highlighted in the top panel. Rings show the approximate ranges of a large research aircraft (P-3 or C-130) based out of Salt Lake City or smaller aircraft (Twin Otter) based out of either Salt Lake City, UT, or Fresno, CA. (top) Daily PM2.5 at Salt Lake City, UT, and Bakersfield, CA, from 1999 to 2020. The dashed lines are the 24-h U.S. National Ambient Air Quality Standards (NAAQS) for PM2.5 of 35 µg m−3. Gray shaded areas indicate November–February. (bottom right) Median, 25th and 75th percentile, and 10th and 90th percentile PM2.5 at Salt Lake City, UT, and Bakersfield, CA, for each day of year from the record in the top panel. Dashed lines indicate the NAAQS, as in the top panel.

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Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study

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  • 1 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 2 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 3 Department of Life, Earth, and Environmental Sciences, West Texas A&M University, Canyon, Texas
  • | 4 Department of Chemical and Environmental Engineering, Center for Environmental Research and Technology, University of California, Riverside, Riverside, California
  • | 5 Department of Civil and Environmental Engineering, University of California, Davis, Davis, California
  • | 6 Department of Land, Air and Water Resources, University of California, Davis, Davis, California
  • | 7 Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington
  • | 8 Department of Chemical Engineering, University of Utah, Salt Lake City, Utah
  • | 9 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 10 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 11 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 12 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
  • | 13 Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, and NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 14 Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina
  • | 15 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 16 Department of Environmental Sciences, University of California, Riverside, Riverside, California
  • | 17 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 18 Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina
  • | 19 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 20 Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming
  • | 21 Cooperative Institute for Research in Environmental Sciences, and Department of Chemistry, University of Colorado Boulder, Boulder, Colorado
  • | 22 Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia
  • | 23 Department of Chemistry, Colorado State University, Fort Collins, Colorado
  • | 24 Department of Atmospheric Science, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida
  • | 25 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 26 Department of Environmental Sciences, University of California, Riverside, Riverside, California
  • | 27 Environmental Science and Engineering, The University of Texas at El Paso, El Paso, Texas
  • | 28 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina
  • | 29 Department of Chemical Engineering, University of Utah, Salt Lake City, Utah
  • | 30 Atmospheric Sciences and Environmental Sciences and Health, University of Nevada, Reno, Reno, Nevada
  • | 31 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Science and Engineering, Peking University, Beijing, China
  • | 32 Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado
  • | 33 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 34 Civil and Environmental Engineering, and Utah Water Research Laboratory, Utah State University, Logan, Utah
  • | 35 Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • | 36 Department of Civil and Environmental Engineering, University of California, Davis, Davis, California
  • | 37 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 38 Department of Chemistry, University of Michigan, Ann Arbor, Michigan
  • | 39 Department of Atmospheric and Oceanic Sciences, and Institute of the Environment and Sustainability, University of California, Los Angeles, Los Angeles, California
  • | 40 Food Animal Environmental Systems Research Unit, USDA-ARS, Bowling Green, Kentucky
  • | 41 Department of Chemistry and Biochemistry, and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska
  • | 42 Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, Colorado
  • | 43 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California
  • | 44 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
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Abstract

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical–meteorological interactions that drive high-pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in western U.S. basins. Approximately 120 people participated, representing 50 institutions and five countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological–chemical linkages outlined here, nor to validate complex processes within coupled atmosphere–chemistry models.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: A. Gannet Hallar, gannet.hallar@utah.edu

Abstract

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical–meteorological interactions that drive high-pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in western U.S. basins. Approximately 120 people participated, representing 50 institutions and five countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological–chemical linkages outlined here, nor to validate complex processes within coupled atmosphere–chemistry models.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: A. Gannet Hallar, gannet.hallar@utah.edu

Winter episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys across the globe [e.g., Yamuna Basin, India (Tiwari and Kulshrestha 2019); Tokyo Basin, Japan (Osada et al. 2019); Taiyuan Basin, China (Miao et al. 2018); and the San Joaquin and Salt Lake Basins, United States (Whiteman et al. 2014; Zhang et al. 2020)]. These episodes may last from several days to several weeks and often arise due to the development of persistent cold-air pools (PCAPs), within which lateral and vertical mixing are inhibited due to sheltering by surrounding topography and a stable temperature profile (Dorninger et al. 2011; Reeves et al. 2011; Lareau et al. 2013; Sheridan et al. 2014; Holmes et al. 2015; Sun and Holmes 2019; Ivey et al. 2019; Sun et al. 2020). While a number of field campaigns targeting either wintertime basin meteorology (e.g., Lareau et al. 2013; McCaffrey et al. 2019) or wintertime pollution chemistry (e.g., Brown et al. 2013; Franchin et al. 2018; Young et al. 2016) have been conducted in the United States, only a few of these campaigns have explicitly considered coupling between interconnected chemical and meteorological processes (e.g., Baasandorj et al. 2017; Prabhakar et al. 2017; Salvador et al. 2021). The upcoming Alaskan Pollution and Chemical Analysis (ALPACA) is specifically targeting this knowledge gap in cold and dark conditions.

Current gaps in our understanding of the coupled chemical–meteorological interactions that result in high-pollution events in many basins worldwide make identification of the most effective air-basin specific emission control strategies challenging. Meteorological processes (thermodynamic, radiative, and dynamical) influence both pollution accumulation, dispersion, and transport and aerosol pollution chemistry, while chemical processes in turn influence radiative transfer, cloud formation, and mixing processes. Figure 1 presents a graphical illustration of some of the coupled chemical–meteorological processes that occur in basins. Some key meteorological processes that control the formation, duration, and breakdown of PCAPs include synoptic drivers such as high pressure and associated subsidence, which can precipitate elevated thermal inversions; warm air advection aloft and large-scale winds and turbulent mixing, alongside local drivers such as the surface energy and radiation budget, which strongly influence formation and dissipation of surface-based thermal inversions; characteristics of the underlying surface (e.g., snow cover, water, urban or non-urban landscape); low clouds and fog; and local boundary layer flows near the surface. In turn, the location and types of urban emissions, aerosol formation and growth processes, and chemical cycling processes influence and are influenced by the ambient meteorology (Fig. 1). The unique basin topography (e.g., slope, how enclosed the basin is, and the size of basin) also play an important role in modulating the rate of pollutant buildup and vertical profiles of temperature and moisture. The interactions between these numerous meteorological processes regulating the frequency, location, and speed of chemical processes in PCAPs, and complex wintertime chemistry, has not yet been observed with sufficient detail to provide satisfactory understanding of these complex pollution episodes and their evolution in time and space

Fig. 1.
Fig. 1.

Schematic of various coupled meteorological and chemical processes within wintertime PCAPs. Figure is not to scale. Importantly, stable nocturnal inversions can be extremely shallow (tens of meters) and elevated inversions can also be present depending on the large-scale synoptic flow.

Citation: Bulletin of the American Meteorological Society 102, 10; 10.1175/BAMS-D-20-0017.1

Ammonium nitrate (NH4NO3) is a major component of aerosol mass in many polluted boundary layers during wintertime episodes (e.g., Womack et al. 2019; Fu et al. 2020; Kelly et al. 2018; Kim et al. 2014; Aksoyoglu et al. 2017; Schaap et al. 2004). Organic aerosol (OA) can also contribute substantially (Chen et al. 2018). In some basins, the NH4NO3 contribution increases with the total aerosol loading while in others OA dominates at high loadings, often with differences between day and night. Understanding the factors that govern the evolution of pollution within a basin is critical for implementing effective emission control policies. Precursor emissions and chemical transformations leading to NH4NO3 formation are an area of ongoing research, as are a number of processes controlling OA production and loss. While OA formation has been extensively studied in warm seasons with large emissions of volatile organic compounds (VOCs, precursors to secondary organic aerosol) often from biogenic sources, characterization for winter urban environments is lacking. Slower photochemistry occurs in the winter, and biogenic and evaporative emissions tend to decrease with temperature, suggesting that these emissions play a smaller role in wintertime OA formation. However, recent evidence of rapid and widespread OA formation (Shah et al. 2019; Schroder et al. 2018) indicate that urban wintertime OA formation is an important contributor to PM2.5 mass. Sulfate is a small fraction of PM2.5 mass during winter in basins such as the Salt Lake Valley but is more important elsewhere (Wang et al. 2016). Mechanisms leading to winter sulfate oxidation are a topic of current interest for which detailed studies in the western United States may serve as a test bed. Consequently, it is imperative to better understand the mechanisms that drive local nitrate, organic and sulfate formation, alongside the emission sources of key precursor gases—e.g., nitrogen oxides, ammonia, SO2, and VOCs. This is particularly important given recent findings showing the importance of urban VOC emissions from evaporative sources relative to those from fuel use (McDonald et al. 2018). Process-level understanding also requires investigation of radical cycling involving VOCs and NOx in the winter and its relationship to high pollutant levels in stagnant boundary layers.

In recent decades, overall aerosol concentrations have declined in the United States owing to emissions changes driven by regulatory policies (Bennett et al. 2019). However, a recent study has conclusively demonstrated that particulate levels across the United States are associated with mortality impacts and loss of life expectancy, with the highest rates observed within the San Joaquin Valley in California (Bennett et al. 2019). Furthermore, particulate matter with diameters less than 2.5 microns (PM2.5) within major urban areas across the United States has declined more slowly in winter than in summer, and average winter levels are now higher in winter than in summer (Chan et al. 1994). These trends are particularly pronounced in basins across the western United States, where wintertime aerosol concentrations regularly approach or exceed regulatory standards (Green et al. 2015). Gaps in understanding the coupled chemical–meteorological interactions that result in high-pollution events may preclude identifying the most effective emission control strategy for a given air basin. With this challenge at the forefront, a workshop took place in September 2019 at the University of Utah with the goal of planning a future winter research campaign to investigate mountain basins of the western United States. With funding from the National Science Foundation (NSF) Atmospheric Chemistry Program and the National Oceanic and Atmospheric Administration (NOAA) Atmospheric Chemistry, Climate and Carbon Cycle Program, the workshop brought together ∼120 air quality experts and meteorologists from across the globe, representing 50 institutions and five countries.

As summarized in this article, the workshop outlined the rationale and design for a comprehensive study that couples atmospheric chemistry and meteorology for wintertime poor air quality episodes in mountain basins across the western United States. The campaign framework is laid out in the next section. The existing uncertainties and opportunities for this campaign are summarized in the following sections, organized by science subthemes. Finally, an integrated perspective on measurements and modeling is presented, along with next steps, in the final section.

Design of the research study

The two western U.S. regions with the most severe winter aerosol pollution are northern Utah (comprised of the Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley (SJV) in California. The workshop concluded that while winter meteorology and a mix of urban and rural emissions affect basins across the western United States (see Fig. 2), a comprehensive study should focus on these two regions, which exhibit contrasting aerosol chemistry, basin topography, climate, agricultural practices, and meteorological–chemical interactions. In the Salt Lake City area, ammonium nitrate is the major contributor to wintertime PM, whereas in the SJV primary and secondary organic aerosol (SOA) are also major contributors (Baasandorj et al. 2017; Lurmann et al. 2006; McDuffie et al. 2019; Chow and Watson 2002). Furthermore, historical databases and previous studies provide context for an air quality study in both of these regions, as PM regulatory standard exceedances in wintertime are common to both. Finally, strong science capacity is available in these areas to conduct this research. Figure 2 shows a long-term record of PM2.5 at the Hawthorne monitoring site in Salt Lake City, the SJV (Bakersfield), and, for comparison, a site in California’s South Coast basin (Riverside–Rubidoux). The decreasing trend in PM2.5 in the South Coast basin is visually apparent from Fig. 2, in contrast to the SJV and Salt Lake City sites, which still regularly experience regulatory exceedances during winter, despite modestly decreasing PM2.5 trends (Green et al. 2015).

Fig. 2.
Fig. 2.

(bottom left) Elevation map of the western United States showing selected air quality monitoring sites (black dots) that exhibit winter maxima in PM2.5. Colored markers show three sites that are highlighted in the top panel. Rings show the approximate ranges of a large research aircraft (P-3 or C-130) based out of Salt Lake City or smaller aircraft (Twin Otter) based out of either Salt Lake City, UT, or Fresno, CA. (top) Daily PM2.5 at Salt Lake City, UT, and Bakersfield, CA, from 1999 to 2020. The dashed lines are the 24-h U.S. National Ambient Air Quality Standards (NAAQS) for PM2.5 of 35 µg m−3. Gray shaded areas indicate November–February. (bottom right) Median, 25th and 75th percentile, and 10th and 90th percentile PM2.5 at Salt Lake City, UT, and Bakersfield, CA, for each day of year from the record in the top panel. Dashed lines indicate the NAAQS, as in the top panel.

Citation: Bulletin of the American Meteorological Society 102, 10; 10.1175/BAMS-D-20-0017.1

The onset and evolution of wintertime pollution episodes in the SJV of California differs in many respects from those in the colder, smaller and frequently snow-covered Intermountain West basins. Additionally, western U.S. mountain valleys show significant variability in emissions. Emissions of nitrogen oxides (NOx), VOCs, and ammonia (NH3) are highly dependent on the nature of the agricultural and industrial sectors in each valley (Kelly et al. 2013; Wang et al. 2015), and their chemical transformations are dependent on many meteorological and topographical factors (Green et al. 2015; Wang et al. 2015; Kleeman et al. 2005; Pusede et al. 2016).

The proposed field program focuses on understanding how variations in coupled meteorological and chemical processes contribute to the production, transformation, cycling, and destruction of chemical species in each locale. This paper highlights the need for acquisition of chemically and meteorologically comprehensive datasets using airborne platforms with sufficient range, coupled to extensive surface-based measurements that provide continuous, chemically detailed data at ground level where human exposure occurs. Sampling of the boundary layer at high vertical resolution will provide representative profiles that reflect regional topographical, meteorological, and emissions variability. The chemical measurements will be combined with comprehensive meteorological measurements to characterize the influences of a wide range of meteorological and land surface processes and parameters (e.g., transport and mixing, surface albedo) and topography on chemical processes during the PCAP episode.

Understanding the coupling between “cold-air pool” meteorology and air quality

The complex cold-air pool basin meteorology that impacts pollutant dispersion and air pollution chemistry remains an active area of research worldwide (Giovannini et al. 2021). Many of the meteorological processes and their effects on air pollution transport and chemistry are still not well documented, understood, or adequately modeled despite having contributed to poor air quality in western U.S. basins for over a century (Fig. 1: see Lareau et al. 2013; Giovannini et al. 2021; Lighthall and Capitman 2007). Figure 1 illustrates the coupling of chemical and meteorological processes wherein the atmospheric state impacts the atmospheric chemistry and, in some cases, results in feedbacks between the two (e.g., aerosols affect cloud properties and lifetime and can absorb shortwave solar radiation, which leads to alteration of the vertical temperature profile and hence pollutant vertical transport). Transport and mixing processes, insolation, and microphysical cloud processes all affect the type and extent of different chemical processes. In turn, weaker feedbacks between aerosol loading and cloud chemistry can have significant impact—via radiative feedbacks (e.g., longwave energy transfer back to the surface from low clouds, net decreases in incoming shortwave radiation into the PCAP due to reflection of the shortwave solar energy back to space at cloud top)—on the vertical stratification and thus vertical mixing processes within the PCAPs.

The complexity of interactions between topographical and other physical characteristics of basins, meteorological, and chemical processes introduced in Fig. 1 leads to pollution with characteristics, severity, and duration differing both in time and space and across different basins. For example, variations in the vertical temperature profile and magnitude of turbulence and mixing from solar heating or wind shear can dramatically impact the amount of entrainment/dilution of pollutants and vertical layering of pollutant precursors within the basin atmosphere.

The workshop identified four critical couplings processes between meteorology and chemistry requiring investigation as part of the proposed field program: 1) surface fluxes of energy and momentum and chemistry; 2) moisture and fog, and heterogeneous, multiphase, and aqueous-phase chemistry; 3) PCAP vertical thermodynamic profiles and vertical chemical profiles, and 4) meteorology and chemistry associated with thermally and dynamically forced exchange processes. No prior wintertime field campaign has provided the breadth of observations necessary to analyze the meteorological–chemical linkages outlined here or to validate complex processes within coupled atmosphere–chemistry models.

A critical tool for both basic research and air pollution control strategies are coupled meteorological and chemical models. Development of useful guidance for the research and regulatory communities requires improvements to both meteorological and chemical components of these models (Giovanni et al. 2021). To improve meteorological model simulations of PCAPs, considerable work is underway or has been conducted in recent years (e.g., Saide et al. 2011; Lareau and Horel 2015; Ahmadov et al. 2015; Saide et al. 2016; Tran et al. 2018; Sun and Holmes 2019; Kelly et al. 2018; Sun et al. 2020). Some of the key meteorological processes that are very difficult to model in stable wintertime boundary layers include vertical temperature and humidity structure, cloudiness, turbulent mixing, and boundary layer flows (Baklanov et al. 2011; Holmes et al. 2015). Improvements in and testing of the representations of heterogeneous chemical processes (Holmes et al. 2019; S. S. Brown et al. 2006), links between aerosol phase and gas–particle equilibration and partitioning (Shiraiwa et al. 2013b; Zaveri et al. 2018), secondary organic aerosol life cycles (Cappa et al. 2016), and organic compound oxidation pathways (Bianchi et al. 2019), especially as they occur within colder wintertime environments, is needed to accurately understand and predict both aerosol and gas-phase abundance and composition, discussed further later on.

The coupled meteorological and chemical models parameterize vertical mixing using relationships between turbulent fluxes and mean thermodynamic profiles that have significant impacts on predicted atmospheric composition. It is therefore paramount that meteorological (e.g., temperature and wind speeds to quantify mixing and transport, relative humidity and downwelling radiance to estimate liquid water path and boundary layer moisture) and chemical vertical profile measurements be collocated with observations of the radiative and turbulent components of the surface energy balance. This approach will allow a detailed quantification of mass, moisture, heat, and chemical budgets within basins. It is also important that variations in the surface energy balance across the basins be well captured using flux sites and satellite data. Important land surface parameters to observe include the depth and age of snow (which can impact the amount of reflected solar radiation and rates of photochemical reactions), and soil temperature and moisture (which can impact fog formation and vertical mixing processes, both which impact chemistry).

Sufficient data, both temporally and spatially, are needed to ensure that spatial gradients are captured across the basins of interest to form a three-dimensional representation of the atmospheric state for both chemical and meteorological properties. Chemical processes driven by mixing also need to be resolved alongside the meteorological measurements for turbulence, mixing, and transport, to allow linking of these two processes. A holistic, interdisciplinary, and multiagency approach will be used in this study where existing infrastructure such as National Weather Service daily rawinsonde launches, weather stations from public and private sectors available from MesoWest (Horel et al. 2002), wind sodars, lidar profilers, and ceilometers will be supplemented with instrumentation dedicated to this field study.

One study design approach is a process-focused deployment, targeting regions of interest within a basin to investigate and quantify coupled chemical–meteorological processes. Three examples of targeted meteorological processes and their impacts on the chemistry include 1) interbas