The PM2.5 mass and species concentration data were obtained online from the EPA Technology Transfer Network Air Quality System [29 ].
We selected the same cities studied by Franklin et al [30 (link)], but also included Chicago, IL, as we had MEDICARE and sufficient speciation data between 2000 and 2003. These cities were originally chosen due to availability of daily PM2.5 data. For most of these cities, the metropolitan county encompassed the city and much of its suburbs, but we used multiple counties for Boston (Suffolk, Norfolk, and Middlesex), and Minneapolis-St. Paul (Ramsey and Hennepin). Henceforth we refer to the analyzed geographical areas as communities.
The STN monitors operate on a 24 hour schedule and collect particles on Teflon, nylon or quartz filters which are analyzed for trace elements using X-ray fluorescence, for ions using ion chromatography and for organic and elemental carbon using thermal-optical analysis.
The EPA maintains multiple PM2.5 mass sites, but typically only one PM2.5 speciation site within a county. In order to use all the available PM2.5 monitoring sites, the 24-hour integrated mass concentrations were averaged over the county using a method previously described [31 (link)]. Briefly, we computed local daily mean concentrations using an algorithm that accounts for the different monitor-specific means and variances. However, before averaging, any monitor that was not well correlated with the others (r < 0.8 for two or more monitor pairs within a community) was excluded as it likely measured a local pollution source and would not represent the general population exposure over the entire county. The number of monitors across the counties varied between 1 and 4.
Based on results from previous epidemiological studies [26 (link)-33 (link)] we focused on the species with different sources and toxicological background. In particular we focus on the paper of Franklin et al [30 (link)] who also screened the STN data for inconsistencies based on the percentage of data below the minimum detection limit and with quality control flags. We therefore examined the following species: Arsenic (As), Aluminium (Al), Bromine (Br), Chromium (Cr), Iron (Fe), Lead (Pb), Manganese (Mn), Nickel (Ni), Potassium (K), Silicon (Si), Vanadium (V), Zinc (Zn), ions nitrate (NO3-), Sulfate (SO42-), ammonium (NH4+), sodium (Na+), elemental carbon (EC) and organic carbon (OC).
For all available observations we computed the ratio between each species and PM2.5 mass and then took averages by season across all years to obtain season- and community-specific long-term mean seasonal concentration ratios.
Meteorological data including daily mean temperature and dew point temperature from the predominant weather station in each community were acquired from the National Climatic Data Center [34 ].
We selected the same cities studied by Franklin et al [30 (link)], but also included Chicago, IL, as we had MEDICARE and sufficient speciation data between 2000 and 2003. These cities were originally chosen due to availability of daily PM2.5 data. For most of these cities, the metropolitan county encompassed the city and much of its suburbs, but we used multiple counties for Boston (Suffolk, Norfolk, and Middlesex), and Minneapolis-St. Paul (Ramsey and Hennepin). Henceforth we refer to the analyzed geographical areas as communities.
The STN monitors operate on a 24 hour schedule and collect particles on Teflon, nylon or quartz filters which are analyzed for trace elements using X-ray fluorescence, for ions using ion chromatography and for organic and elemental carbon using thermal-optical analysis.
The EPA maintains multiple PM2.5 mass sites, but typically only one PM2.5 speciation site within a county. In order to use all the available PM2.5 monitoring sites, the 24-hour integrated mass concentrations were averaged over the county using a method previously described [31 (link)]. Briefly, we computed local daily mean concentrations using an algorithm that accounts for the different monitor-specific means and variances. However, before averaging, any monitor that was not well correlated with the others (r < 0.8 for two or more monitor pairs within a community) was excluded as it likely measured a local pollution source and would not represent the general population exposure over the entire county. The number of monitors across the counties varied between 1 and 4.
Based on results from previous epidemiological studies [26 (link)-33 (link)] we focused on the species with different sources and toxicological background. In particular we focus on the paper of Franklin et al [30 (link)] who also screened the STN data for inconsistencies based on the percentage of data below the minimum detection limit and with quality control flags. We therefore examined the following species: Arsenic (As), Aluminium (Al), Bromine (Br), Chromium (Cr), Iron (Fe), Lead (Pb), Manganese (Mn), Nickel (Ni), Potassium (K), Silicon (Si), Vanadium (V), Zinc (Zn), ions nitrate (NO3-), Sulfate (SO42-), ammonium (NH4+), sodium (Na+), elemental carbon (EC) and organic carbon (OC).
For all available observations we computed the ratio between each species and PM2.5 mass and then took averages by season across all years to obtain season- and community-specific long-term mean seasonal concentration ratios.
Meteorological data including daily mean temperature and dew point temperature from the predominant weather station in each community were acquired from the National Climatic Data Center [34 ].
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