The U.S. EPA is currently implementing the National Ambient Air Monitoring Strategy ( NAAMS). The NAAMS goals include improvement of the scientific and technical competency of the nation's air monitoring networks and increased value in protecting public health and the environment. Monitoring of ambient air pollution is a critical part of the nation's program infrastructure. Monitoring data are used to characterize air quality and associated health and ecosystem impacts, develop emission strategies to reduce impacts, and account for progress over time. While the obvious problems of widespread elevated concentrations have been largely solved for some criteria pollutants, problems related to particulate matter (PM), ozone (O3), and toxic air pollutants remain. It is now clear that even very low air pollution levels can be associated with adverse environmental and human health effects. As a result, new approaches monitoring are needed to measure these low levels and to incorporate these measurements other data into comprehensive assessments of human and environmental health. One of the major areas of investment in the NAAMS is the use of highly sensitive commercial air pollutant monitors for the characterization of the precursor gases CO, SO2, and total reactive oxides of nitrogen (NOy) in a new national core monitoring network (NCore).