Short-Term Temporal Variation in PM_2.5 Mass and Chemical Composition during the Atlanta Supersite Experiment, 1999

Abstract

Measurements in urban Atlanta of transient aerosol events in which PM_2.5 mass concentrations rapidly rise and fall over a period of 3–6 hr are reported. The data are based on new measurement techniques demonstrated at the U.S. Environmental Protection Agency (EPA) Atlanta Supersite Experiment in August 1999. These independent instruments for aerosol chemical speciation of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, and organic and elemental carbon (OC and EC), reconstructed the observed hourly dry PM_2.5 mass to within 20% or better. Data from the experiment indicated that transient PM_2.5 events were ubiquitous in Atlanta and were typically characterized by a sudden increase of EC (soot) and OC in the early morning or SO₄ ²⁻ in the late afternoon. The frequent temporal decoupling of these events provides insights into their origins, suggesting mobile sources in metro Atlanta as the main contributor to early morning PM_2.5 and more regionally located point SO₂ sources for afternoon PM_2.5 events. The transient events may also have health implications. New data suggest that short-term PM_2.5 exposures may lead to adverse health effects.¹ Standard integrated filter-based techniques used in PM_2.5 compliance monitoring networks and in most past PM_2.5 epidemiologic studies collect samples over 24-hr periods and thus are unable to capture these transient events. Moreover, health-effects studies that focus on daily PM_2.5 mass alone cannot evaluate the health implications of the unique and variable chemical properties of these episodes.