A STUDY OF PEROXY-RADICALS AND OZONE PHOTOCHEMISTRY AT COASTAL SITES IN THE NORTHERN AND SOUTHERN HEMISPHERES

Peroxy radicals and other important species relevant to ozone photoche mistry, including ozone, its photolysis rate coefficient jO(D-1), NOx( NO + NO2), and peroxides, were measured at the coastal sites of Cape G rim, Tasmania, in January/February 1995 during the Southern Ocean Atmo spheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ir eland, in May 1995 during the Atlantic Atmospheric Photochemistry Expe riment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2 + RO2) concentrations and jO(D-1) switche d from a square root dependence in clean oceanic or ''baseline'' air t o a first-order dependence in more polluted air. Simple algorithms der ived from a photochemical reaction scheme indicate that this switch-ov er 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 o ccur and was observed in the ozone diurnal cycles when the peroxy radi cal/jO(D-1) dependencies became first order. The peroxy radical/jO(D-1 ) relationships imply that ozone production exceeds destruction at NO levels of 55 +/- 30 parts per trillion by volume (pptv) at Mace Head d uring late spring and 23 +/- 20 pptv at Cape Grim during summer, sugge sting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere.