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
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).