SOS' scientific findings in this area are distilled out of a first collective analysis and interpretation of the detailed findings described above. They pertain strictly to the air quality management aspects of the ozone problem.

AQM1. In most rural areas of the SOS region, the rate of accumulation of ozone is limited by air concentrations of NOx rather than VOC (Trainer et al., 1993; Williams et al., 1997; Roberts et al., 1998; Sillman, 2004).

AQM2. The efficiency with which airborne VOC and NOx are converted into ozone is greater in rural areas than in urban areas of the SOS region (Ryerson et al., 1998; Gillani et al., 1998b; St. John et al., 1998).

AQM3. In urban areas within the SOS region, a large fraction (typically more than 50 percent) of the ozone contributing to exceedances of the National Ambient Air Quality Standard for Ozone was transported into the urban area from regions outside the metropolitan area in which the exceedance was observed (McNider et al., 1998; Banta et al., 1998).

AQM4. Urban Airshed Model (UAM) simulations for Atlanta, Georgia suggest that ozone accumulation throughout the Atlanta metropolitan area is more sensitive to decreases in NOx emissions than to decreases in VOC emissions (Sillman et al., 1995; St. John and Chameides, 2000).

AQM5. The UAM also predicts, however, that other southern cities, with smaller emissions of biogenic VOC and/or higher afternoon concentrations of NOy, could be close to the 'transition condition' where ozone is nearly equally sensitive to decreases in emissions of both NOx and VOC (Sillman et al., 1998).

AQM6. Observation-based air-quality methods and Observation-Based Models provide a promising complement and/or alternative to emissions-based models in evaluating alternative strategies and tactics for management of ozone and other oxidants near the ground (Kleinman, 1994, 2000; Cardelino and Chameides, 1995; Kleinman et al., 1994, 1995, 1997, 1998; Sillman et al, 1997, 1998; Sillman, 1995a, 2004).

AQM7. Summertime ozone concentrations throughout the SOS region often are high enough to inhibit photosynthesis in vegetation (Heck and Cowling, 1997; Saylor et al., 1998).

AQM8. Adoption of an ecologically based (secondary) standard for ozone - a standard different in form from the present 1-hour and 8-hour federally mandated primary standards for ozone - with a lower allowable ozone concentration and a 3-month-long (or growing-season-long) averaging time - will provide an increased margin of safety for agricultural crops, forest and shade trees, and natural vegetation from the injurious and yield-decreasing effects of ambient ozone. This kind of a secondary standard (such the often proposed SUM06 standard) - will result in many rural parts (instead of the present mainly urban parts) of the SOS region being designated ozone non-attainment areas (Heck et al., 1998; Chameides et al., 1997; Saylor et al., 1998).

AQM9. Coal-fired power plants were confirmed in both the 1992 SOS Intensive Field Study in Atlanta and in the 1995 Nashville/Middle Tennessee Ozone Study to be major NOx sources. But well-fertilized crop and pasture lands and biomass burning in wildfires, controlled burning of crop lands and forests, municipal incinerators, and pulp and paper mills were as identified as additional important sources of NOx and carbon monoxide. The TexAQS2000 Air Quality Study indicated that some of these same sources of NOx and CO were important in the Texas and Louisiana parts of the SOS region. In this connection, it is significant to recall that the 1985 and 1990 NAPAP inventories overestimate the SO2/NOx emission ratio for coal-fired power plants and underestimate the CO/NOx emission ratio for pulp mills (Buhr et al., 1995b; M. Chang et al., 1996; Gillani et al., 1998b; Jobson et al., 1998; Ryerson et al., 1998; Senff et al., 1998; Luria et al., 2000; Nunnermacker et al., 1998, 2000; St. John and Chameides, 2000).

AQM10. Maximum ozone concentrations in the Atlanta and Nashville metropolitan areas appeared to occur when plumes from coal-fired power plants or other major point sources that also contain high SO2 and NOx concentrations are embedded in a broader urban plume which in turn is embedded within the more amorphous regional background concentration of ozone (Imhoff et al., 1995; St. John et al., 1998; St. John and Chameides, 2000).

AQM11. During SOS' 1992 Atlanta Intensive and the Nashville/Middle Tennessee Ozone Study, important interactions were found to occur among land use, urban heat islands, and biogenic VOC emissions. These interactions occur because natural, managed, and urban forest trees are the largest sources of biogenic VOC in the United States and they also tend to decrease local temperatures in rural and urban areas by increasing total evapotranspiration in landscapes wherever they occur. Since isoprene is typically the most abundant VOC emitted by trees and isoprene emissions increase sharply with increasing temperature, the result of these competing effects is coupling between the distribution of forest trees, variations in local climate, and biogenic VOC emissions. This coupling is most pronounced in urban and suburban areas, where city planners and developers replace forests with areas of concrete and asphalt that give rise to urban "heat islands" with increasing temperatures. Thus, destruction of trees in urban and suburban areas increased total VOC emissions. Conversely, urban planning and construction practices that modulate the intensity of urban heat islands (for example, through placement of "green spaces" within the urban core of cities and use of high-reflectivity building materials) aided in ozone pollution abatement by decreasing air temperatures in urban and suburban areas (Cardelino and Chameides, 1990; Meagher et al., 1998).

AQM12. In rural areas of the SOS region, summertime ozone production is usually limited by NOx rather than VOC (Trainer et al., 1993; St. John et al., 1998; Roberts et al., 1998; Ryerson et al., 1998; Weinstein-Lloyd et al., 1998).

AQM13. In urban areas of the SOS region (such as downtown Nashville, Tennessee and Atlanta, Georgia), summertime ozone production often is near the transition point between NOx and VOC sensitivity (Sillman et al., 1998, Meagher et al., 1998).