Peroxy radicals and other important species relevant to ozone photochemistry, including ozone, its photolysis rate coefficient jO(1D), NOx(NO+NO2), and peroxides, were measured at the coastal sites of Cape Grim, Tasmania, in January/February 1995 during the Southern Ocean Atmospheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ireland, in May 1995 during the Atlantic Atmospheric Photochemistry Experiment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2+RO2) concentrations and jO(1D) switched from a square root dependence in clean oceanic or baseline air to a first-order dependence in more polluted air. Simple algorithms derived from a photochemical reaction scheme indicate that this switch-over occurs when atmospheric NO levels are sufficient for peroxy radical reaction with NO to compete with radical recombination reactions. At this crucial point, net tropospheric ozone production is expected to occur and was observed in the ozone diurnal cycles when the peroxy radical/jO(1D) dependencies became first order. The peroxy radical/jO(1D) relationships imply that ozone production exceeds destruction at NO levels of 55¿30 parts per trillion by volume (pptv) at Mace Head during late spring and 23¿20 pptv at Cape Grim during summer, suggesting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere. ¿ 1997 American Geophysical Union