This research examines changes in airborne fine particulate matter (PM2.5) concentrations following the COVID-19 lockdown measures established in California in 2020. These lockdown measures involved temporary closures of local economies, resulting in reduced travel and vehicle usage. To study trends in ambient PM2.5 pollution, the project used daily 24hr PM2.5 data from the baseline years (defined as 2017-2019) to 2020 obtained from the Environmental Protection Agency (EPA) Air Now website to determine differences in mass concentration of PM2.5between those two time periods in the San Diego County and San Francisco and Alameda Counties of the Bay area pre-and post-COVID-19. Time periods were defined and divided according to each major lockdown phase of the pandemic year: Lockdown Phase 1 was defined from March 16 to June 30 in the Bay Area, and March 19 to June 30 in San Diego. To both regions, Lockdown Phase 2 was defined from July 1 to October 3; and Lockdown Phase 3 from November 1 to December 31. March was divided between the pre-lockdown (March A) and Lockdown 1 (March B). The first lockdown period showed a significant reduction in PM2.5 in the Bay Area (median=-5.2 μg/m3, p <.000), but not in San Diego. However, the period of March B was statistically significantly lower in both San Diego (p=.005) and the Bay Area (p=.005). Lockdown 2 analysis was eliminated due to wildfire effects on PM2.5 which significantly increased the cumulative median PM2.5 in both San Diego (p>.000) and the Bay Area (p=.008) by September. Lockdown 3 had no significant effect on PM2.5 in either region. Ultimately, this work observed a decline in outdoor air pollution immediately following the first COVID-19 lockdowns. This demonstrates the potential environmental benefits of reducing traffic volume and congestion in cities highly impacted by traffic pollution, which may help mitigate ambient pollution exposure and reduce associated health risks amongst vulnerable communities.