A side-by-side comparison of filter-based PM2.5 measurements at a suburban site: A closure study

Assessing the effects of air quality on public health and the environment requires reliable measurement of PM2.5 mass and its chemical components. This study seeks to evaluate PM2.5 measurements that are part of a newly established national network by comparing them with more versatile sampling systems. Experiments were carried out during 2002 at a suburban site in Maryland, United States, where two samplers from the US Environmental Protection Agency (US EPA) Speciation Trends Network: Met One Speciation Air Sampling System—STNS and Thermo Scientific Reference Ambient Air Sampler—STNR, two Desert Research Institute Sequential Filter Samplers—DRIF, and a continuous TEOM monitor (Thermo Scientific Tapered Element Oscillating Microbalance, 1400a) sampled air in parallel. These monitors differ not only in sampling configuration but also in protocol-specific laboratory analysis procedures. Measurements of PM2.5 mass and major contributing species (i.e., sulfate, ammonium, organic carbon, and total carbon) were well correlated among the different methods with r-values >0.8. Despite the good correlations, daily concentrations of PM2.5 mass and major contributing species were significantly different at the 95% confidence level from 5% to 100% of the time. Larger values of PM2.5 mass and individual species were generally reported from STNR and STNS. These differences can only be partially accounted for by known random errors. Variations in flow design, face velocity, and sampling artifacts possibly influenced the measurement of PM2.5 speciation and mass closure. Statistical tests indicate that the current uncertainty estimates used in the STN and DRI network may underestimate the actual uncertainty.