Greater Resistance to Reducing the Mid Range of Concentrations
The analysis indicated that there is a greater resistance to reducing the hourly average concentrations in the mid range than the hourly average concentrations above 0.09 ppm. The following two slides indicate that the higher hourly average concentrations (i.e., above 90 ppb) decreased at a faster rate (greater negative rate per year) than the hourly average concentrations in the mid-level range. The number of hourly average concentrations in the low end of the distribution also decreased. Apparently, both the high and low ends of the distribution are moving toward the center of the distribution.