In this report, defined (a) the size of NCAP city airsheds (b) the recommended number of ambient air quality monitoring sites in an airshed (c) the operational sampling frequency to support receptor-based source apportionment studies. These resources are necessary for strengthening the monitoring needs of an airshed to track pollution levels, to conduct receptor-model-based source apportionment studies, and to support long-term air quality management plans.