Investigation of Low Ozone in the Tropical Tropopause Layer (TTL) in the Western Tropical Pacific

The ATTREX (Airborne Tropical TRopopause EXperiment) mission from January to March 2014 (ATTREX 3) by NASA's Global Hawk to the western tropical Pacific Ocean observed exceptionally low values of ozone (near 20 ppb on average) across multiple flights in the tropical tropopause layer (TTL). Ozone levels in the lower tropical troposphere are generally lower than in the extratropics, and frequent intense convection can lift air with low ozone up to the tropopause. The Brewer-Dobson circulation further facilitates circulation of this air away from the tropics, toward the mid-latitudes. What is less understood is why the levels of ozone were so low within the TTL (~12 to 17 km): if slow upward transport were the primary mechanism we might expect to see ozone values that are indeed low, but with a slight rise in this layer above the line where air motion is characterized by slow mean ascent. This was not always the case during ATTREX 3—at times, the minimum values of ozone occurred near the top of the TTL. In addition, two prior ATTREX deployments (ATTREX 1 and 2) with flights from California to the central and eastern tropical Pacific Ocean observed minimum ozone values typically around 40 to 50 ppb, in contrast to the very low minimum of ~20 ppb in ATTREX 3. One possible reason for this anomaly is destruction of ozone by ozone-reactive species such as halogen compounds, or by hydrocarbon oxidation under conditions with low concentrations of nitrogen oxides (NOx). Alternatively, there may simply not be much ozone to start with in the TTL, because of deep convection bringing very low ozone air up from the tropical marine boundary layer. To investigate these possibilities, atmospheric tracers are used in this analysis to help determine what the likely composition of the air in question was, as well as from where it came—these tracers include CO, CO2, CH4, N2O and SF6, all of which were simultaneously measured with ozone in-flight. These data are coupled with data from NCAR's Gulfstream V aircraft taken during the CONTRAST (CONvective TRansport of Active Species in the Tropics) mission, which flew in the same area but at lower altitudes, as well as ozone profiles from the AURA satellite to provide a more complete set of observations. The findings of this investigation have significant implications in understanding the complex atmospheric transport and chemistry occurring in the western Pacific TTL.