The report summarizes the data analysis of two extensive field studies on urban particulate matter, the 1974-77 St. Louis (RAPS) and the July/August 1982 Philadelphia (PAFS) studies. The major conclusion of the study is that in both cities the majority (more than 50%) of the total mass collected by dichotomous samplers is of regional, not local, origin. The regional share is about equally large for long-term (yearly, seasonal, monthly) and short-term (24-hours) averaging of concentrations. In the fine fraction, the regional component is even larger, 60% in St. Louis and 83% in Philadelphia. This conclusion is supported primarily by the observation that with the exception of a single site in both St. Louis and Philadelphia, all other sites within the extensive monitoring network, including remote rural sites, show a very low gradient of concentrations, indicating that the majority of collected aerosols does not originate from local sources, but must come from distant, regional sources that impact all monitors equally.