Mean Ages and Age Spectra for the Tropical Tropopause Layer From Observations of CO2: Implications for Air Transport and Distributions of Short-Lived Chemical Species

We present extensive CO2 observations of the TTL as related to the characteristics of TTL structure and transport. The results from the CR-AVE and TC4 missions show that air entering the upper TTL above ~360K retains imprint of the global seasonal cycle derived from the well-measured CO2 at the surface, suggesting that data for CO2 can be used as a principal tool to infer the mean age and age spectrum for the TTL, as in the lower stratosphere (Andrews et al., 2001a, 2001b; Boering et al., 1996; Park et al., 2007). The transport rates in the upper TTL inferred from CO2 are applied to generate vertical distributions in the TTL of short-lived organic halogens, non-methane hydrocarbons and alkyl nitrates, which have potentially major impacts on stratospheric ozone and humidity. New data from the Whole Air Sampler obtained over Central America in August, 2007 demonstrate that direct inputs of these transient tracers are largely limited to altitudes ~2 km below the tropical tropopause, and reactive species are efficiently removed with local lifetimes less than or comparable to the mean age of air in the upper TTL (i.e., ranging from a few days to several months). Interestingly, the observations in the subtropical lowermost stratosphere reveal higher concentrations of these species than at the same isentropic level of the TTL, indicating direct loading by summertime midlatitude convection as one of input pathways of these short-lived species into the stratosphere.