[1] We have carried out studies of the transport between the tropical boundary layer, the tropical tropopause layer, and the stratosphere during January 2001 using both atmospheric tracers in a transport model and air parcel trajectories. Most of the transport (approximately two thirds) from the planetary boundary layer (BL) into the tropical tropopause layer (TTL) occurs vertically above the Indian Ocean and the Indonesian and west Pacific regions, consistent with transport dominated by convection. Transport from the base of the TTL into the stratosphere above the dynamical tropopause is dominated (approximately 95%) by transport into the extratropical lowermost stratosphere (ELS) with a much smaller fraction entering the stratospheric ''overworld.'' Overall, transport from the BL to the ELS is sufficiently rapid that this represents an important route by which very short lived substances (VSLS), emitted at the surface, can influence lower stratospheric ozone. The two approaches, using high-resolution trajectories and a lowerresolution transport model, yield generally similar results, increasing confidence that chemistry transport models can capture transport sufficiently well for chemical assessment modeling of the role of VSLS.

Sea ice is a reflection of, and a feedback on, the Earth's climate. We explore here, using a global atmospheric chemistry-transport model, the use of sea salt in Antarctic ice cores to obtain continuous long-term, regionally integrated records of past sea ice extent, synchronous with ice core records of climate. The model includes the production, transport, and deposition of sea salt aerosol from the open ocean and "blowing snow" on sea ice. Under current climate conditions, we find that meteorology, not sea ice extent, is the dominant control on the atmospheric concentration of sea salt reaching coastal and continental Antarctic sites on interannual timescales. However, through a series of idealized sensitivity experiments, we demonstrate that sea salt has potential as a proxy for larger changes in sea ice extent (e.g., glacial-interglacial). Treating much of the sea ice under glacial conditions as a source of salty blowing snow, we demonstrate that the increase in sea ice extent alone (without changing the meteorology) could drive, for instance, a 68% increase in atmospheric sea salt concentration at the site of the Dome C ice core, which exhibits an approximate twofold glacial increase in sea salt flux. We also show how the sensitivity of this potential proxy decreases toward glacial sea ice extent-the basis of an explanation previously proposed for the lag observed between changes in sea salt flux and δD (an ice core proxy for air temperature) at glacial terminations. The data thereby permit simultaneous changes in sea ice extent and climate.

[1] A global chemistry-climate model is used to assess the impact on atmospheric composition of the regeneration and recycling of HO x in the photo-oxidation of isoprene. The impact is explored subject to present-day, pre-industrial and future climate/emission scenarios. Our calculations show that, in all cases, the inclusion of uni-molecular isomerisations of the isoprene hydroxy-peroxy radicals leads to enhanced production of HO x radicals and ozone. The global burden of ozone increases by 25-36 Tg (8-18%), depending on the climate/emissions scenario, whilst the changes in OH lead to decreases in the methane lifetime of between 11% in the future and 35% in the pre-industrial. Critically the size of the change in methane lifetime depends on the VOC/NO x emission ratio. The results of the presentday calculations suggest a certain amount of parameter refinement is still needed to reconcile the updated chemistry with field observations (particularly for HO 2 +RO 2 ). However, the updated chemistry could have far-reaching implications for: future-climate predictions; projections of future oxidising capacity; and our understanding of past changes in oxidising capacity. Citation: Archibald, A. T., et al.(2011), Impacts of HO x regeneration and recycling in the oxidation of isoprene: Consequences for the composition of past, present and future atmospheres, Geophys. Res. Lett., 38, L05804,