The burden of fine particle pollution (PM 2.5 ) and gaseous air pollutants (NOx, NO 2 , O 3 , and SO 2 ) disproportionately affects communities with higher poverty rates and minority-majority populations. Understanding the spatiotemporal dynamics of indoor and outdoor air pollution in these communities is crucial to defining health risk, identifying hotspots, and making changes through future policies. Using low-cost PurpleAir sensors, this study characterized PM 2.5 with high temporal resolution at 15 sites for 16 months from July 2024 to October 2025 in West Philadelphia, a minority-majority community. Ogawa passive samplers were deployed alongside PurpleAir monitors for summer (July 2024) and spring (Mar – Apr 2025) observations. Mean daily PM 2.5 ranged from 7.07 to 9.41 μg/m 3 , with small difference between each site. PM 2.5 displayed strong temporal trends, including a diurnal pattern seemingly unrelated to commuter rush hour, elevated weekend concentrations, and substantial increases in summer (Jun – Aug) and winter (Dec – Feb) concentrations. Collectively, West Philadelphia displayed similar PM 2.5 variation compared to Philadelphia and was lower than five of six central monitoring stations during the 16-month period. NOx (10.29 ppb) and O 3 (12.94 ppb) were found at low mean ambient concentrations. Indoor PM 2.5 and NOx were higher than outdoor concentrations with mean indoor to outdoor ratios of 1.46 and 4.0, respectively. PM 2.5 indoor to outdoor ratios were driven by changes in indoor concentrations rather than outdoor. This study shows the need for high temporal and wide spatial resolution data in communities with higher pre-existing risk factors. With such monitoring, communities can better understand their peak exposure times and locations, as we demonstrate herein. • PM2.5 and NOx had indoor to outdoor ratios one of 1.6 and 4, respectively. • PM2.5 was found to be higher on weekends than weekdays. • PM2.5 and NOx had strong seasonal trends, while O3 did not. • Mean daily PM2.5 in West Philadelphia ranged from 7.07 to 9.41 μg/m3.
Sylvester et al. (Fri,) studied this question.