The NARSTO_EPA_SS_LOS_ANGELES_APS_DATA were collected between December 2000 and September 2001. At several locations in Los Angeles County, California, a TSI Aerodynamic Particle Sizer (APS) was used in a mobile trailer to collect size characteristics of particles ranging from about 0.5 to 20 mm. Based on the time-of-flight principle, the APS measured particle count concentrations for 52 channels that cover sizes from 0.5 to 20 mm in every 15 minutes. Note that the first channel reports particle count concentrations for sizes < 0.523 mm.The overall objective of the Los Angeles Supersite in Southern California Particle Center and Supersite (SCPCS) is 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 U.S. EPA Particulate Matter (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 (formerly North American Research Strategy for Tropospheric Ozone) is a public/private partnership, whose membership spans government, the utilities, industry, and academe throughout Mexico, the United States, and Canada. The primary mission is 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 available.