A methodology for estimating indoor sources contributing to PM.

Quantifying source contributions to indoor PM levels by indoor PM sources has been limited by the costs associated with chemical speciation analyses of indoor PM samples. Here, we propose a new methodology to estimate this contribution. We applied FUzzy SpatioTemporal Apportionment (FUSTA) to a database of indoor and outdoor PM concentrations in school classrooms plus surface meteorological data to determine the main spatiotemporal patterns (STPs) of PM. We found four dominant STPs in outdoor PM, and we denoted them as regional, overnight mix, traffic, and secondary PM. For indoor PM we found the same four outdoor STPs plus another STP with a distinctive temporal evolution characteristic of indoor-generated PM. Concentration peaks were evident for this indoor STP due to children's activities and classroom housekeeping, and there were minimum contributions on sundays when schools were closed. The average indoor-generated estimated contribution to PM was 5.7 μg m, which contributed to 17% of the total PM, and if we consider only school hours, the respective figures are 8.1 μg m and 22%. A cluster-wise indoor-outdoor PM regression was applied to estimate STP-specific infiltration factors () per school. The median and interquartile range (IQR) values for are 0.83 [0.7-0.89], 0.76 [0.68-0.84], 0.72 [0.64-0.81], and 0.7 [0.62-0.9], for overnight mix, secondary, traffic, and regional sources, respectively. This cost-effective methodology can identify the indoor-generated contributions to indoor PM, including their temporal variability.