2.57 In the United States, the Environmental Protection Agency is charged with the job of monitoring the nation’s environment, that is, air, water, and soil. Some of the major indicators of air quality are levels of carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone, and sulfur dioxide (SO2). Table below gives the national air quality data for years 1980–2000 (National Air Quality and Emissions Report, 2003).

a Are these different measures of air quality associated with each other?
b If yes, model relationships among them over these two decades.
c Plot each measure over time and discuss trends in different air quality measures over the two decades.

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CO Pb NO2 Ozone SO Year (ppm) (ppm) (ppm) (ppm) (ppm) 1981 8.4 0.58 0.024 0.126 0.0102 1982 8.1 0.58 0.023 0.125 0.0095 1983 7.9 0.47 0.023 0.137 0.0093 1984 7.8 0.45 0.023 0.125 0.0095 1985 7.1 0.28 0.023 0.123 0.0090 1986 7.2 0.18 0.023 0.118 0.0088 1987 6.7 0.13 0.023 0.125 0.0086 1988 6.4 0.12 0.023 0.136 0.0087 1989 6.4 0.10 0.023 0.116 0.0085 1990 5.9 0.08 0.022 0.114 0.0079 1991 5.6 0.08 0.019 0.111 0.0081 1992 5.3 0.07 0.019 0.105 0.0076 1993 5.0 0.06 0.019 0.107 0.0074 1994 5.1 0.05 0.020 0.106 0.0072 1995 4.6 0.05 0.019 0.112 0.0057 1996 4.3 0.05 0.019 0.105 0.0057 1997 4.1 0.04 0.018 0.104 0.0056 1998 3.9 0.04 0.018 0.110 0.0055 1999 3.7 0.04 0.018 0.107 0.0053 2000 3.4 0.04 0.017 0.100 0.0051