NARSTO_EPA_SS_FRESNO_BAM_PM_MASS FRACTION is the North American Research Strategy for Tropospheric Ozone (NARSTO) Environmental Protection Agency (EPA) Supersite (SS) Fresno, Beta Attenuation Monitors (BAM), Particulate Mass Concentration Data product. This data set contains measurements taken from two BAMs, PM10, and PM2.5, operated at the Fresno Supersite. The MetOne BAM Monitor measured the attenuation of a beam of beta particles (electrons) generated by a 14ºC source transmitted through an aerosol sample collected on a glass fiber filter tape. Before sample collection, the beta attenuation was measured through a clean part of the tape to obtain a baseline. A sample was collected on the same location on the tape. After sample collection, the beta attenuation was measured through the exposed part of the tape. The net attenuation is proportional to the amount of mass collected on the filter. A mass flow controller controls the flow rate during sample collection at a flow rate of approximately 16.7 l/min. The mass concentration of the collected aerosol was determined from the net attenuation, the sample air flow, the sample time, and the attenuation coefficient for the instrument. 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.