Characterizing Indoor and Outdoor 15 Minute Average PM 2.5 Concentrations in Urban Neighborhoods

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Abstract
While a number of studies have looked at the relationship between outdoor and indoor particulate levels based on daily (24 h) average concentrations, little is known about the within-day variability of indoor and outdoor PM levels. It has been hypothesized that brief airborne particle excursions on a time scale of a few minutes to several hours might be of health significance. This article reports variability in measurements of daily (24 h) average PM 2.5 concentrations and short-term (15 min average) PM 2.5 concentrations in outdoor and indoor microenvironments. Daily average PM 2.5 concentrations were measured using gravimetry, while measurements of 15 min average PM 2.5 mass concentrations were made using a light scattering photometer whose readings were normalized using the gravimetric measurements. The measurements were made in 3 urban residential neighborhoods in the Minneapolis-St. Paul metropolitan area over 3 seasons: spring, summer, and fall of 1999. Outdoor measurements were made at a central monitoring site in each of the 3 communities, and indoor measurements were made in 9-10 residences (with nonsmoking occupants) in each community. Residential participants completed a baseline questionnaire to determine smoking status, sociodemographics, and housing characteristics. Outdoor PM 2.5 concentrations across the Minneapolis-St. Paul metropolitan area appear to be spatially homogeneous on a 24 h time scale as well as on a 15 min time scale. Short-term average outdoor PM 2.5 concentrations can vary by as much as an order of magnitude within a day. The frequency distribution of outdoor 15 min averages can be described by a trimodal lognormal distribution, with the 3 modes having geometric means of 1.1 w g/m 3 (GSD = 2.1), 6.7 w g/m 3 (GSD = 1.6), and 20.8 w g/m 3 (GSD = 1.3). There is much greater variability in the within-day 15 min indoor concentrations than outdoor concentrations (as much as ∼40-fold). This is most likely due to the influence of indoor sources and activities that cause high short-term peaks in concentrations. The indoor 15 min averages have a bimodal lognormal frequency distribution, with the 2 modes having geometric means of 8.3 w g/m 3 (GSD = 1.66) and 35.9 w g/m 3 (GSD = 1.8), respectively. The correlation between the matched outdoor and indoor 15 min average PM 2.5 concentrations showed a strong seasonal effect, with higher values observed in the spring and summer ( R 2 adj = 0.49 - 0.33) and lower values in the fall ( R 2 adj = 0.13 - 0.13).