emissions are not routinely monitored in the United States. One notable exception is the congressional requirement for continuous emissions monitoring (CEM) of sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (PM) from any source regulated under the acid rain provisions of the 1990 Amendments to CAA. As discussed in Chapter 5, the inclusion of such monitoring is viewed as being essential to ensuring the success of the cap-and-trade mechanism incorporated into the legislation. Moreover, the application of CEM has provided direct evidence of substantial reductions in SO2 emissions from utilities since the implementation of the acid rain controls (see Figure 5-1 in Chapter 5). Inspection and maintenance (I/M) programs for motor vehicles, which were mandated in the CAA, could serve, in principle, as a check on the effectiveness of mobile-source emission controls. However, as discussed in Chapter 4, the effectiveness of I/M programs has been limited because of shortcomings in program design and effectiveness, and public resistance to such programs in some areas of the country.

There are a number of reasons why emissions are not routinely monitored. There are myriad stationary and area sources that contribute to pollution, and technologies are not available to monitor their emissions routinely and reliably. Given the resources and measurement technologies available to the AQM system, a program that attempted to monitor emissions comprehensively through direct measurement would be unrealistically expensive and complex. In addition, efforts by the government to monitor certain types of emissions on a continuous basis (for example, mobile emissions) might be viewed by some as an unacceptable invasion of privacy. On the other hand, the application of new technologies and creative measurement strategies could help to make the task more tractable and less invasive. For example, a number of emerging technologies and methods could be deployed to augment I/M for mobile emissions. Remote sensors have been used to identify high-emitting vehicles without inconveniencing motorists or interfering with traffic (Stedman et al. 1997; Bishop et al. 2000); on-board diagnostic systems are being developed to automatically monitor and document problems that lead to increased emissions from individual motor vehicles; and standard air quality monitors could be deployed inside tunnels and along roadways to help characterize in-use emissions from fleets of vehicles (Kean et al. 2001). As discussed in Chapter 5, CEM technologies are very valuable in tracking stationary-source emissions and could be used more widely, but the development of a broader range of CEM systems has been slow.

Using Ambient Concentrations to Confirm Emission Trends

EPA estimates that nationwide emissions of volatile organic compounds (VOCs), SO2, PM, carbon monoxide (CO), and lead (Pb) have decreased

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