New Insights into Tropical Tropopause Layer Structure and Composition in the Deep Tropics During Northern Hemisphere Winter

Ozone-depleting substances emitted at the surface as a result of anthropogenic activity can reach the stratosphere and alter the chemical composition and radiative balance of this layer of the atmosphere. These changes in the stratosphere can have a profound impact on climate and life on Earth. Previous studies have shown that the preferred entry point into the stratosphere occurs in the deep tropics, in particular over the Western Tropical Pacific (WTP) region where strong and frequent convection can readily transport surface air to high altitudes. In this study, we examine high-spatial and temporal aircraft measurements of trace gases in the deep tropics, which we define as 12 ^oS - 15 ^oN, along with diabatic backward trajectories with convective influence and NOAA ground-based measurements of greenhouse gases (GHG). The aircraft measurements were obtained during the NASA ATTREX campaign (Eastern/Central Tropical Pacific in February 2013 and WTP in January-March 2014). These data sets allow us to elucidate physical and chemical properties of the under-sampled transition region between the main tropical convective outflow ( 10-14 km) and the cold point tropopause (CPT) ( 17-18 km), a region known as the Tropical Tropopause Layer (TTL). The TTL imprints the chemical boundary condition of air entering the global stratosphere; therefore it plays a critical role in stratospheric composition. GHG data reveal significant spatial heterogeneity within the TTL across the Pacific Ocean, with sharp transitions at the CPT and between hemispheres particularly over the WTP. Backward trajectories corroborate the dominant role of the WTP as a source of TTL air across the Pacific and reveal a significant contribution of Southern Hemisphere surface air to the TTL in both hemispheres, consistent with GHG observations. During the 2014 campaign, we find both local (to the WTP) and far-reaching (from equatorial Africa) polluted air deep into the TTL, even above the CPT, as evidenced by elevated anthropogenic compounds of varying lifetimes (e.g., non-methane hydrocarbons, halogenated hydrocarbons), a rare observation to date. The implications of the observed spatial heterogeneity, origin of surface air, and presence of local and non-local pollution within the TTL will be discussed.