Observations of ozone-poor air in the Tropical Tropopause Layer

Abstract

Abstract. Ozonesondes reaching the tropical tropopause layer (TTL) over the West Pacific have occasionally measured layers of very low ozone concentrations-less than 15 ppbv-raising the question of how prevalent such layers are and how they are formed. In this paper we examine aircraft measurements from the ATTREX, CAST and CONTRAST campaigns based in By contrast, the mean ozone concentrations in profiles in the Northern Hemisphere were always above 15 ppbv and normally above 20 ppbv at these altitudes. The CAST and CONTRAST aircraft sampled the atmosphere between the surface and 120 hPa, 10 finding very low ozone concentrations only between the surface and 700 hPa; mixing ratios as low as 7 ppbv were regularly measured in the boundary layer, whereas in the free troposphere above 200 hPa concentrations were generally well in excess of 15 ppbv. These results are consistent with uplift of almost-unmixed boundary layer air to the TTL in deep convection. An interhemispheric difference was found in the TTL ozone concentrations, with values <15 ppbv measured extensively in the Southern Hemisphere but seldom in the Northern Hemisphere. This is consistent with a similar contrast in the low-level ozone 15 between the two hemispheres found by previous measurement campaigns. Further evidence of a boundary layer origin for the uplifted air is provided by the anti-correlation between ozone and halogenated hydrocarbons of marine origin observed by the three aircraft.