Toward a biologically significant and usable standard for ozone that will also protect plants.

Ozone remains an important phytotoxic air pollutant and is also recognized as a significant greenhouse gas. In North America, Europe, and Asia, incidence of high concentrations is decreasing, but background levels are steadily rising. There is a need to develop a biologically significant and usable standard for ozone. We compare the strengths and weaknesses of concentration-based, exposure-based and threshold-based indices, such as SUM60 and AOT40, and examine the O(3) flux concept. We also present major challenges to the development of an air quality standard for ozone that has both biological significance and practicality in usage.

The health benefits of reduced tropospheric ozone in California.

Californians are exposed daily to concentrations of ozone (O3) that are among the highest in the United States. Recently, the state adopted a new 8-hr ambient standard of 0.070 ppm, more stringent than the current federal standard. The new standard is based on controlled human studies and on dozens of epidemiologic studies reporting associations between O3 at current ambient levels and a wide range of adverse health outcomes. Clearly, the new O3 standards will require further reductions in the precursor pollutants and additional expenditures for pollution control. Therefore, it is important to quantify the incremental health benefits of moving from current conditions to the new California standard. In this paper, a standard methodology is applied to quantify the health benefits associated with O3 concentration reductions in California. O3 concentration reductions are estimated using ambient monitoring data and a proportional rollback approach in which changes are specific to each air basin, and control strategies may impact concentrations both below and above the standard. Health impacts are based on published epidemiologic studies, including O3-related mortality and morbidity, and economic values are assigned to these outcomes based on willingness-to-pay and cost-of-illness studies. Central estimates of this research indicate that attaining the California 8-hr standard, relative to current concentrations, would result in annual reductions of 630 cases of premature mortality, 4200 respiratory hospital admissions, 660 pediatric emergency room visits for asthma, 4.7 million days of school loss, and 3.1 million minor restricted activity days, with a median estimated economic value of dollar 4.5 billion. Sensitivity analyses indicate that these findings are robust with respect to exposure assessment methods but are influenced by assumptions about the slope of the concentration-response function in threshold models and the magnitude of the O3-mortality relationship. Although uncertainties exist for several components of the methodology, these results indicate that the benefits of reducing O3 to the California standard may be substantial and that further research on the shape of the O3-mortality concentration-response function and economic value of O3-related mortality would best reduce these uncertainties.

Measurements of ozone concentrations in Zagreb.

Ozone concentrations were measured in Zagreb at four sites from May 1999 to April 2001 in order to categorize the air quality with respect to ozone. In the summer of 2000, the ozone measurements were also extended to four sites in the suburbs of Zagreb. Methods of active and passive sampling with nitrite ion as a reagent were used. In the northern part of the town ozone was analyzed by an automatic device. Automatic device measurements in the years 1999 and 2000 showed that hourly averages of ozone concentrations did not exceed the Croatian recommended value of the 98th percentile (180 micro/g m(-3)). Over the two-year period, 24-h averages occasionally exceed 110 microg m(-3) in city center and in the northern part of the town. Regardless of these isolated examples, ozone was well within acceptable concentrations. Ozone concentrations measured in summer 2000 were higher in the suburbs of Zagreb than in the city. The 98th percentile values higher than 110 microg m(-3) were recorded at three sites.

European abatement of surface ozone in a global perspective.

EU's programme Clean Air for Europe (CAFE) is presently revising the policy on air quality which will lead to the adoption of a thematic strategy on air pollution under the Sixth Environmental Action Programme by mid-2005. For the abatement of surface ozone it is becoming evident that processes outside European control will be crucial for meeting long-term aims and air quality guidelines in Europe in the future. Measurements and modelling results indicate that there is a strong link between climate change and surface ozone. A warmer and dryer European climate is very likely to lead to increased ozone concentrations. Furthermore, increased anthropogenic emissions in developing economies in Asia are likely to raise the hemispheric background level of ozone. A significant increase in the background concentration of ozone has been observed at several sites in Northern Europe although the underlying causes are not settled. The photochemical formation of tropospheric ozone from increased concentrations of methane and CO may also lead to a higher ozone level on a global scale. Gradually, these effects may outweigh the effect of the reduced European ozone precursor emissions. This calls for a global or hemispheric perspective in the revision of the European air quality policy for ozone.

Understanding the effectiveness of precursor reductions in lowering 8-hr ozone concentrations--Part II. The eastern United States.

Analyses of ozone (O3) measurements in conjunction with photochemical modeling were used to assess the feasibility of attaining the federal 8-hr O3 standard in the eastern United States. Various combinations of volatile organic compound (VOC) and oxides of nitrogen (NOx) emission reductions were effective in lowering modeled peak 1-hr O3 concentrations. VOC emissions reductions alone had only a modest impact on modeled peak 8-hr O3 concentrations. Anthropogenic NOx emissions reductions of 46-86% of 1996 base case values were needed to reach the level of the 8-hr standard in some areas. As NOx emissions are reduced, O3 production efficiency increases, which accounts for the less than proportional response of calculated 8-hr O3 levels. Such increases in O3 production efficiency also were noted in previous modeling work for central California. O3 production in some urban core areas, such as New York City and Chicago, IL, was found to be VOC-limited. In these areas, moderate NOx emissions reductions may be accompanied by increases in peak 8-hr O3 levels. The findings help to explain differences in historical trends in 1- and 8-hr O3 levels and have serious implications for the feasibility of attaining the 8-hr O3 standard in several areas of the eastern United States.

A new flux-orientated concept to derive critical levels for ozone to protect vegetation.

The current European critical levels for ozone (O3) to protect crops, natural and semi-natural vegetation and forest trees are based on a relative small number of open-top chamber experiments with a very limited number of plant species. Therefore, the working group "Effects of Ozone on Plants" of the Commission on Air Pollution Prevention of the Association of German Engineers and the German Institute of Standardization reanalysed the literature on O3 effects on European plant species published between 1989 and 1999. An exposure-response relationship for wild plant species and agricultural crops could be derived from 30 experiments with more than 30 species and 90 data points; the relationship for conifer and deciduous trees is based on 20 experiments with nine species and 50 data points. From these relationships maximum O3 concentrations for different risk stages are deduced, below which the vegetation type is protected on the basis of the respective criteria. Because it is assumed that the fumigation concentrations reflect the O3 concentrations at the top of the canopy, i.e. the upper surface boundary of the quasi-laminar layer if the micrometeorological big-leaf approach is applied, the application of these maximum O3 concentrations requires the transformation of O3 concentrations measured at a reference height above the canopy to the effective phytotoxic concentrations at the top of the canopy. Thus, the approach described in this paper is a synthesis of the classical concept of toxicology of air pollutants (critical concentrations) and the more toxicological relevant dose concept.