The NARSTO_EPA_SS_LOS_ANGELES_MOUDI_DATA is the North American Research Strategy for Tropospheric Ozone (NARSTO) Environmental Protection Agency (EPA) Supersite (SS) Los Angeles Size-Fractionated Particulate Matter (PM) Composition - micro-orifice uniform deposit impactors (MOUDI) Data product. Data for this collection was collected between late 2000 and late 2003 from sites at Downey, Claremont, Riverside, Rubidoux, and the University of Southern California (USC). Samples were typically collected for a one-day period, but in some cases, duration was less than or more than one day. Element/metals, carbon, nitrate/sulfate ion, and mass concentration data were obtained. The MOUDI is a multiple stage inertial cascade impactor. At each stage, particles larger than the cut point of the stage are collected on the impaction plate while smaller particles pass through to the next stage. This continues through the cascade impactor until the smallest particles are collected on the after filter. At Downey, a size range of 10um to 0um was collected (10.0-2.5um, 2.5-1.0um, 1.0-0.32um, 0.32-0um). Most of the 10.0-2.5um size range samples were eliminated at Claremont, Riverside, Rubidoux, and USC because this size range was collected using the Partisol sampler. All samples were analyzed using X-ray florescence and mass concentration analysis at an independent laboratory. The overall objective of the Los Angeles Supersite in Southern California Particle Center and Supersite (SCPCS) was to conduct monitoring and research that contributes to a better understanding of the measurement, sources, size distribution, chemical composition and physical state, spatial and temporal variability, and linkages to health effects of airborne particulate matter in the Los Angeles Basin. The EPA PM Supersites Program was an ambient air monitoring research program designed to provide information of value to the atmospheric sciences, and human health and exposure research communities. Eight geographically diverse projects were chosen to specifically address these EPA research priorities: (1) to characterize PM, its constituents, precursors, co-pollutants, atmospheric transport, and its source categories that affect the PM in any region; (2) to address the research questions and scientific uncertainties about PM source-receptor and exposure-health effects relationships; and (3) to compare and evaluate different methods of characterizing PM including testing new and emerging measurement methods.NARSTO, which has since disbanded, was a public/private partnership, whose membership spanned across government, utilities, industry, and academe throughout Mexico, the United States, and Canada. The primary mission was to coordinate and enhance policy-relevant scientific research and assessment of tropospheric pollution behavior; activities provide input for science-based decision-making and determination of workable, efficient, and effective strategies for local and regional air-pollution management. Data products from local, regional, and international monitoring and research programs are still available.