THE ANNUAL CYCLE OF PEROXIDES AND OZONE IN MARINE AIR AT CAPE-GRIM, TASMANIA

The concentration of gas-phase peroxides has been measured almost cont inuously at the Cape Grim baseline station (41 degrees S) over a perio d of 393 days (7702 h of on-line measurements) between February 1991 a nd March 1992. In unpolluted marine air a distinct seasonal cycle in c oncentration was evident, from a monthly mean value of > 1.4 ppbv in s ummer (December) to < 0.2 ppbv in winter (July). In the summer months a distinct diurnal cycle in peroxides was also observed in clean marin e air, with a daytime build-up in concentration and decay overnight. B oth the seasonal and diurnal cycles of peroxides concentration were an ticorrelated with ozone concentration, and were largely explicable usi ng a simple photochemical box model of the marine boundary layer in wh ich the central processes were daytime photolytic destruction of ozone , transfer of reactive oxygen into the peroxides under the low-NOx amb ient conditions that favour self-reaction between peroxy radicals, and continuous heterogeneous removal of peroxides at the ocean surface. A dditional factors affecting peroxides concentrations at intermediate t imescales (days to a week) were a dependence on air mass origin, with air masses arriving at Cape Grim from higher latitudes having lower pe roxides concentrations, a dependence on local wind speed, with higher peroxides concentrations at lower wind speeds, and a systematic decrea se in peroxides concentration during periods of rainfall. Possible phy sical mechanisms for these synoptic scale dependencies are discussed.