EPA is amending its regulations to define the appropriate ambient air monitoring criteria for open path (long-path) analyzers. These revisions to the Ambient Air Quality Surveillance regulations define the siting requirements for open path analyzers used as State and Local Air Monitoring Stations (SLAMS), National Air Monitoring Stations (NAMS) and Photochemical Assessment Monitoring Stations (PAMS), as well as general quality assurance procedures for this technology. These changes provide the ambient air monitoring community with criteria needed to effectively use open path analyzers and associated data for regulatory purposes.