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