Characterizing outdoor infiltration and indoor contribution of PM with citizen-based low-cost monitoring data.

Epidemiological research on the adverse health outcomes due to PM exposure frequently relies on measurements from regulatory air quality monitors to provide ambient exposure estimates, whereas personal PM exposure may deviate from ambient concentrations due to outdoor infiltration and contributions from indoor sources. Research in quantifying infiltration factors (F), the fraction of outdoor PM 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, citizen-based PM measurements provides an unprecedented opportunity to characterize F 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 with around 650000 h of paired PM measurements. An empirical method was developed based on local polynomial regression to estimate site-specific F. The estimated site-specific F had a mean of 0.26 (25, 75 percentiles: [0.15, 0.34]) with a mean bootstrap standard deviation of 0.04. The F estimates were toward the lower end of those reported previously. A threshold of ambient PM concentration, approximately 30 μg/m, below which indoor sources contributed substantially to personal exposures, was also identified. The quantified relationship between indoor source contributions and ambient PM concentrations could serve as a metric of exposure errors when using outdoor monitors as an exposure proxy (without considering indoor-generated PM), which may be of interest to epidemiological research. The proposed method can be generalized to larger geographical areas to better quantify PM outdoor infiltration and personal exposure.