Characterizing indoor and outdoor 15 minute average PM2.5 concentrations in urban neighborhoods

Abstract

While a number of studies have looked at the relationship be tween 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 minute to several hours might be of health significance. This article report variability in measurements of daily (24 h) average PM2.5 concentrations and short-term (15 min average) PM2.5 concentrations in outdoor and indoor microenvironments. Daily average PM2.5 con centrations were measured using gravimetry, while measurement of 15 min average PM2.5 mass concentrations were made using a light scattering photometer whose readings were normalized us ing 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 PM2.5 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 PM2.5 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 μg/m3 (GSD = 2.1), 6.7 μg/m3 (GSD = 1.6), and 20.8 μg/m3 (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 t 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 μg/m3 (GSD = 1.66) and 35.9 μg/m3 (GSD = 1.8), respectively. The correlation between the matched outdoor and indoor 15 min average PM2.5 concentrations showed a strong seasonal effect, with higher values observed in the spring and summer (Radj2 = a 0.49 ± 0.33) and lower values in the fall (Radj2 = 0.13 ± 0.13).