Despite decades of emissions reductions, the South Coast Air Basin, which includes Los Angeles, CA and surrounding areas, experiences some of the worst air quality in the country; with violations of the National Ambient Air Quality Standard (NAAQS) for 8-hr ozone (O3) on one in three days. Further, while the past decades have also seen steadily decreasing levels of O3 and fine particulate matter (PM2.5), of which secondary organic aerosol (SOA) is a significant fraction, recent years have seen a leveling of these trends. Thus there are many unanswered questions about what drives production of O3 and SOA formation in Southern California, which are relevant in the broader context of improving the process-level understanding of atmospheric chemistry under relatively low nitrogen oxide (NOx) conditions, and as influenced by a changing mix of emissions; and the ability to represent these processes and sources in predictive models. This project will build on a collaborative effort, initiated in March 2020, to collect in-situ data in Pasadena, CA within the South Coast Air Basin. A suite of gas- and particle-phase instrumentation was co-located on the Caltech campus and samples were collected from April-August 2020. The data collected and analyzed during this unprecedented time of reduced human activity will be of great utility to the atmospheric chemistry, observational, modeling, and regulatory communities in developing the understanding and model representation of the sources and processes that affect atmospheric composition and urban air quality. Analysis of trends in atmospheric composition, during and after the COVID-19 shelter-in-place orders, and comparison with historical data sets will provide insight regarding the relative roles of chemistry and climate in controlling observed air pollution levels.