Abstract: Urban air quality in the U.S. has generally been improving for many decades. Decreasing emissions of nitrogen oxides (NOx) and anthropogenic volatile organic compounds (VOCs) have caused decreases in ozone concentrations, albeit in a highly non-linear fashion. In this seminar, I will present results from a summer 2022 study in the LA basin. We observed lower VOC emissions than during previous studies. However, concentrations of secondary pollutants had not decreased, indicative of changes in the NOx-VOC regime. The role of ambient temperature will also be described. Wildfires have become a strong driver for poor air quality in much of the U.S. I will present results from our work on the environmental impacts of the Marshall Fire at the wildland-urban interface (WUI) in Boulder County, which took place on December 30, 2021 and destroyed over 1,000 structures. In particular, I will focus on the surprising effects of the smoke on indoor environments, which lasted weeks beyond the actual fire. Implications for the clean-up from the LA Fires in January will be discussed.