FIGURE S-3 Plot of the estimated relative trends in emissions versus ambient concentrations of various primary pollutants (PM10, NOx, SO2, Pb, and CO). Emission trends, which were derived from emission inventories, are shown along the xaxis, and the trends in average concentrations, which were derived from air quality monitoring networks, are shown along the y-axis. The squares are the relative trends in emissions and ambient concentrations for the 20-year period spanning 1983–2002 (except for PM10 emissions, which are for the trend period 1985–2002), and the circles are the relative trends for the 10-year period of 1993–2002. If the emission inventory trends were accurate and the nation’s air quality monitoring networks were able to accurately measure the average concentration of primary pollutants in the air overlying the United States, all the points on the graph would fall on the 1:1 (diagonal) line. However, the fact that most of the points on the graph do not fall on the 1:1 line indicates that the emission inventory trends are inaccurate and/or that the nation’s air quality network, which was initially designed to monitor urban pollution and compliance with NAAQS, has not been able to track trends in pollutant concentrations quantitatively across urban, suburban, and rural settings. Despite such uncertainty, it is important to note that the downward trend in ambient pollutant concentrations provides qualitative confirmation that pollutant emissions have been decreasing. SOURCE: Data from EPA 2003.

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