Published ozonesonde data, radioactive fallout measurements, and alpine ozone observations have been used to estimate the stratospheric contribution to observed ozone concentrations at ground level. Long term average effects from the stratosphere over the U.S. are on the order of 10 ppb, with a springtime maximum around 20 to 25 ppb. Short term stratospheric intrusion events resulting in one-hour-average concentrations of stratospheric ozone in excess of 80 ppb in the lower troposphere have a frequency of only about 0.2 percent. Still fewer (but some) of these events lead to ground-level impacts of such a magnitude. Tropospheric causes of high ozone concentrations away from cities have been investigated by statistical analysis of meteorological conditions and the precursor emissions occurring along air trajectories and by comparisons of weather maps and large-scale O3 distributions. At sites well removed from cities, the upwind emissions of oxides of nitrogen are more strongly related to ozone concentrations than are the emissions of hydrocarbons. Widespread violations of the federal oxidant standard are most likely to be found in association with a stagnant high-pressure system or in the warm southwesterly flow in the western portion of a high pressure area, often ahead of an approaching cold front. The results of this and other studies suggest that not all violations of the federal oxidant standard are controllable and this fact must be considered in the design of control strategies. Also, for areas within about 125 km of large cities, control might be achieved through the reduction of HC emissions. In more remote areas, control strategies involving NOx control throughout large regions must be considered.

Prepared under contract no. 68-02-2084, SRI project 4432. Includes bibliographical references.

v. I. Analysis and findings.--V. III. Appendices (analytical methods and supplementary data).