SURFACE-LEVEL OZONE - CLIMATE CHANGE AND EVIDENCE FOR TRENDS

As a result of emissions of hydrocarbons, carbon monoxide, and nitrogen oxides from combustion processes, recent investigations indicate that the concentration of ozone in the Earth's atmosphere may be changing. Because ozone acts as a greenhouse gas, an increase in ozone concentration in the free troposphere may have climatic consequences. In the planetary boundary layer, increases in surface ozone may affect human health, the ecosystem, and the atmospheric chemical system. Using surface ozone measurements, this paper reviews the literature concerning (1) increases in baseline surface ozone concentrations from the mid-1800s to the present and (2) trends in ozone concentrations measured at the surface. The monthly average ozone concentrations measured at surface level in the last half of the nineteenth century appear to be lower than those currently measured at many rural locations in the world. The evidence is not conclusive that the surface ozone concentrations currently monitored at "clean" rural locations are approximately double those measured in the last half of the nineteenth century in Europe or North America. Although results for the past 10 to 25 years suggest that surface ozone levels in Europe may be rising, the evidence for increasing trends in surface ozone is not consistent among monitoring sites. The identification of trends is often a function of the period selected for analysis. A review of the limited number of available longer records from either Europe or North America suggests that it is difficult to detect any trends on a region-wide basis. For assessing trends in surface ozone concentrations, it is important that world-wide monitoring at remote locations be continued and expanded so that an adequate database becomes available.