• Measure the effectiveness of emission-control programs.

  • Serve as a warning system to alert individuals that are sensitive to poor air quality and to aid in the development of air quality forecasts.

Each one of the objectives places different demands on a network in terms of the species measured, instrument sensitivity, time resolution and frequency of the measurements, and location of the monitoring stations. Even though more than $200 million per year is spent on routine air quality monitoring in the United States (EPA 2002p), monitoring networks have limited resources. As a result, they are able to address only some of the above objectives adequately. At least two major issues arise as a result of resource limitations (NARSTO 2000):

  • Identifying the problem versus finding the solution and assessing program effectiveness. Most of the existing networks have been designed only to measure compliance with the existing NAAQS and reveal little about the appropriate management strategies needed to solve the problems or measure the success of various emission-control strategies (see earlier discussion on emission trends). Network design should be evaluated and expanded to make air quality networks in the United States more relevant to other important objectives of monitoring. Some specific changes are discussed below.

  • Measuring critical species in a regular monitoring mode. Because of the variety of criteria pollutants, their precursors, and HAPs and their large range of concentrations, monitoring is technically challenging as well as expensive. In some cases, these challenges are beyond the capabilities of state and local regulatory agencies. Creative mechanisms for fostering collaboration and technology transfer among regulatory agencies, research and academic institutions, and small businesses need to be devised to meet the challenges (see discussion in Box 7-5 in Chapter 7).

Siting of Air Quality Monitoring Stations

Following enactment of the CAA, the AQM system in the United States emphasized urban-scale air quality problems and the use of controls on local emissions to solve air pollution. Moreover, major urban areas were the only areas specifically identified in the congressional mandate that initially directed the EPA administrator to develop a national monitoring program. As a result, the nation’s ambient air quality monitoring networks have been dominated by urban sites. The intervening years have seen a growing concern for large-scale air quality problems that extend over multistate airsheds and are affected by long-range transport. However, a

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