Air quality across much of the U.S. has improved dramatically in recent decades in response to emissions reductions. Summer photochemical ozone, for example, has shown strong decreasing trends in most regions. Winter particulate matter, by contrast, has responded less robustly despite reductions in many of the precursors responsible for both summer and winter air pollution. In the eastern U.S., winter air quality generally no longer exceeds regulatory standards but has not responded linearly to emissions reductions. In the western U.S., winter particulate matter exceeds standards in a number of mountain basins subject to stagnant winter meteorology, such as California’s San Joaquin Valley and Salt Lake City, Utah. Winter particulate matter arises from a complex interaction between emissions, boundary layer meteorology and atmospheric chemistry. Aircraft measurements provide detailed understanding of these interactions by probing the vertical structure and composition of shallow, stratified boundary layers that are common in winter. Coupled to this winter meteorology are chemical cycles involving heterogeneous and multiphase reactions that are prevalent during cold and dark conditions and that regulate the source of oxidants responsible for chemical transformation and production of secondary pollutants. The 2015 WINTER campaign (Wintertime INvestigation of Transport, Emissions and Reactivity) surveyed the eastern U.S. with the NSF C-130 aircraft. The 2017 UWFPS campaign (Utah Winter Fine Particulate Matter Study) focused on the mountain basins of Northern Utah using the NOAA Twin Otter aircraft. Results from these studies serve to define the rates and variability of key heterogenous chemical processes and may point to new control strategies for wintertime air pollution based on insights into wintertime oxidants. These results have broad significance to areas impacted by winter air pollution in Asia, Europe and North America.