Fundamental ozone photochemistry in the remote marine boundary layer: the soapex experiment, measurement and theory

During the Austral summer of 1995, a series of measurements were made as part of the SOAPEX (Southern Ocean Atmospheric Photochemistry EXperiment) campaign at the Cape Grim Baseline Air Pollution station (41°S) on the north western tip of Tasmania, Australia. These measurements included peroxy radicals, photolysis parameters, speciated peroxides, NOx and other related chemical quantities. The climatology of Cape Grim is such that it frequently receives “clean” remote marine boundary layer air off the Southern Ocean and in this air in summer, there is clear evidence for the net photochemical destruction of ozone. The factors influencing the photochemical control of ozone are investigated by means of the relationship between peroxy radicals and jO(1D). From the levels of peroxy radicals and jO(1D), mid-day hydroxyl radical concentrations of (6±0.9)×106 molecules cm-3 are calculated, in agreement with other estimates in this region. An apparent discrepancy between measured and calculated peroxy radical levels in the “clean” atmosphere is investigated both from theoretical treatments and from the speciated peroxide record. The possible role of aerosols in the suppression of peroxy radicals are also discussed. Using simple photochemical theory the relative roles of photolysis and reaction with radicals are investigated in the observed −2.5 ppbv per day photochemical destruction of ozone. Measured values of jO(1D), calculated [OH] and the corrected peroxy radical concentration are used successfully to produce a simple fit to the ozone diurnal, illustrating the magnitude of the various contributing components, i.e. photolysis and reaction.