A seasonal comparison of ozone photochemistry in the remote marine boundary layer

During the Austral winter of 1995, a series of measurements were made as part of the Southern Ocean Atmospheric Photochemistry Experiment-1 (SOAPEX-1) campaign at the Cape Grim Baseline Air Pollution station (41 degrees S) on the northwestern tip of Tasmania, Australia. The measurements were complimentary to similar ones made in the Austral summer during January-February 1995 and both sets of data are used to make a comparison of photochemistry in the remote marine boundary layer in summer and winter. Seasonal measurements of peroxy radicals are compared to both a steady-state analysis and a full model in order to elucidate their relationships to measured O-3, j-((OD)-D-1) and H2O. Calculations of the seasonal concentrations and relationships of OH are consistent with both model and measurements. From a detailed analysis of the ozone budgets accounting for the contributions of photochemistry, deposition and entrainment processes, it is clear that on a seasonal basis there are two major controlling factors working in opposition. The net effect of photochemistry in the remote marine boundary layer is to destroy ozone; this is balanced by entrainment of ozone into the marine boundary layer from the lower free troposphere, Using the measured data, photochemistry represents the dominant loss process for ozone on a seasonal basis varying from 1.2 ppbv d(-1) in summer (87% of the ozone loss) to 0.6 ppbv d(-1) in the winter (64% of the ozone loss). It is clear that the level of photochemistry taking place in the marine boundary layer determines the lower bound for ozone Levels. (C) 2000 Elsevier Science Ltd. All rights reserved.