PROJECT SUMMARY Emerging data indicate disproportionate COVID-19 death rates among members of racial/ethnic minority groups in the United States. The contribution of demographic factors, socioeconomic status (SES), population mobility, and environmental exposures is unclear. Using California death certificate data, we will study potential determinants of probable COVID-19 deaths and excess all-cause mortality during the pandemic. In Aim 1 we will evaluate the independent contribution of acute (prior 1-8 weeks) and chronic (prior 1-2 years) air pollution exposure to COVID-19 mortality and excess all-cause mortality that has been observed during the pandemic. In Aim 2 we will estimate the joint association of individual level demographic determinants (e.g., sex, age, race/ethnicity) and contextual factors (e.g., neighborhood demographics/SES, time-varying cell-phone based mobility, air pollution). The expected outcome of this investigation is an improved understanding of the effect of ambient air pollution on COVID-19 mortality risk and the impact of individual and contextual factors on COVID- 19 mortality, with a goal of discerning factors that may have led to the troubling higher COVID-19 mortality being observed in racial/ethnic minority populations. Study findings will have an important positive impact by identifying characteristics of high-risk, vulnerable communities that can guide more targeted and effective public health interventions. Clarifying the role of air pollution in COVID-19 deaths could be highly relevant to air quality regulations that potentially could reduce mortality.