Bi et al.: Characterizing outdoor infiltration and indoor contribution of PM2.5 with PurpleAir data

Bi, J., Wallace, L. A., Sarnat, J. A., & Liu, Y. (2021). Characterizing outdoor infiltration and indoor contribution of PM2.5 with citizen-based low-cost monitoring data. Environmental Pollution, 276, 116763.

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Research in quantifying infiltration factors (Finf), the fraction of outdoor PM2.5 that infiltrates indoors, has been historically limited in space and time due to the high costs of monitor deployment and maintenance. Recently, the growth of openly accessible PM2.5 measurements provides an unprecedented opportunity to characterize Finf at large spatiotemporal scales. In this analysis, 91 consumer-grade PurpleAir indoor/outdoor monitor pairs were identified in California (41 residential houses and 50 public/commercial buildings) during a 20-month period in 2019 – 2020 with around 650k hours of paired PM2.5 measurements. An empirical method was developed based on local polynomial regression to estimate site-specific Finf. The estimated Finf had a mean of 0.26 (25th, 75th percentiles: [0.15, 0.34]) with a mean bootstrap standard deviation of 0.04. The resulting exposure errors (differences between total indoor exposure and exposure due to particles of ambient origin) were plotted as a function of the ambient concentration for the first time. The peak of the exposure errors occurred at low outdoor concentrations < 5 µg/m3 and dropped to nearly zero near 30 µg/m3.