Evidence for photochemical control of ozone concentrations in unpolluted marine air

OZONE in the troposphere is an important greenhouse gas, and a key participant in the oxidation of other trace species, but the mechanisms for its formation and destruction are not fully understood. In polluted regions of the Northern Hemisphere, seasonal increases in ozone concentration have been observed 1,2 ; such changes could arise from photochemical reactions 3–6 , but they could also involve transport from the ozone-rich stratosphere 7 . In remote, unpolluted (low-No x ) regions, photochemical theory predicts net destruction of ozone 8 . Here we present observations of a large summer minimum in ozone concentration in the unpolluted marine boundary layer of the Southern Hemisphere. Our results show a clear link between ozone loss and hydrogen peroxide production in the region, demonstrating that in situ photo-chemistry, rather than transport, is the major cause of the seasonal ozone cycle in the boundary layer. These findings emphasize the role of photochemical processes in the lower atmosphere, and may suggest only a limited role for transport in other, more polluted regions.