FIGURE 2.1 Ozone Air Quality Standards (AQS) in ppb (nanomoles of O3 per mole of dry air). Different national and international standards are noted as well as estimates for northern midlatitudes of the preindustrial background (i.e., O3 abundances with all anthropogenic emissions of NOx, CO, VOC, and CH4 cut off, and before current climate and stratospheric O3 change) and the present-day baseline abundances (i.e., the statistically defined lowest abundances of O3 in air flowing into the continents, typical of clean-air, remote marine sites). The pre-2008 U.S. AQS was 0.08 ppm, which through numerical roundoff meant that an AQS violation was 85 ppb or greater (D.J. Jacob, personal communication, 2009).

FIGURE 2.1 Ozone Air Quality Standards (AQS) in ppb (nanomoles of O3 per mole of dry air). Different national and international standards are noted as well as estimates for northern midlatitudes of the preindustrial background (i.e., O3 abundances with all anthropogenic emissions of NOx, CO, VOC, and CH4 cut off, and before current climate and stratospheric O3 change) and the present-day baseline abundances (i.e., the statistically defined lowest abundances of O3 in air flowing into the continents, typical of clean-air, remote marine sites). The pre-2008 U.S. AQS was 0.08 ppm, which through numerical roundoff meant that an AQS violation was 85 ppb or greater (D.J. Jacob, personal communication, 2009).

(including premature mortality) across the three years was estimated to be $4.9-$5.7 billion. If the analysis used the highest annual maximum 8-hr concentration, impacts would have increased by a factor of two to three. While highly uncertain (see Hubbell et al., 2005 for details), such estimates give a sense of the magnitude of impacts associated with ozone pollution.

In most metropolitan airsheds O3 has a daily cycle peaking in the late afternoon. It also varies with weather patterns and over seasons. An example of the variability of surface O3 abundance is shown is Figure 2.2 for a site in Michigan. This result is typical (i.e., violation of the U.S. NAAQS occurs episodically throughout the summer months) often for a few days in a row.


Atmospheric Chemistry and Dynamics Ozone is a highly reactive gas that is constantly being produced and destroyed by the natural cycles of atmospheric chemistry throughout the troposphere and stratosphere. About 90 percent of the column of O3, which protects living organisms from



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