NARSTO_EPA_SS_FRESNO_EC_PM25_FRACTION is the North American Research Strategy for Tropospheric Ozone (NARSTO) Environmental Protection Agency (EPA) Supersite (SS) Fresno, Elemental Carbon in 2.5 um Aerosol Fraction Data product. This data set contains the measurements taken with a single and dual wavelength aethalometer. The single wavelength aethalometer (model AE14) was operated at the Fresno supersite from December 17, 1999 to September 27, 2002. This instrument used a broad spectrum incandescent lamp to illuminate the collected aerosol. Aerosol samples were collected for five minute periods. The air sample was collected through a sharp cut size-selective cyclone to limit the size of particles to aerodynamic diameters of 2.5 m or less. A single concentration of black carbon was determined for each five minute period. A dual-wavelength aethalometer (model AE21) operated at the Fresno supersite from February 25, 2003 to December 31, 2006. The collected aerosol sample is illuminated with light from two light emitting diodes at wavelengths of 370 and 880 nm. Aerosol samples are collected for five minute periods. The air sample is collected through a sharp cut size-selective cyclone to limit the size of particles to aerodynamic diameters of 2.5 m or less. The concentration of black carbon corresponds to the 880 nm measurement. The black carbon equivalent at the ultraviolet wavelength was also determined.The Fresno Supersite is one of several Supersites established in urban areas within the United States by the EPA to better understand the measurement, sources, and health effects of suspended particulate matter (PM). The site is located at 3425 First Street, approximately 1 km north of the downtown commercial district. First Street was a four-lane artery with moderate traffic levels. Commercial establishments, office buildings, churches, and schools were located north and south of the monitor. Medium-density single-family homes and some apartments were located in the blocks to the east and west of First Street. The Fresno Supersite began operation in May of 1999.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 the following 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.