Abstract
Abstract. Rates for large-scale vertical transport of air in the Tropical Tropopause Layer (TTL) were determined using high-resolution, in situ observations of CO2 concentrations in the tropical upper troposphere and lower stratosphere during the NASA Tropical Composition, Cloud and Climate Coupling (TC4) campaign in August 2007. Upward movement of trace gases in the deep tropics was notably slower in TC4 than during the Costa Rica AURA Validation Experiment (CR-AVE), in January 2006. Transport rates in the TTL were combined with in situ measurements of chlorinated and brominated organic compounds from whole air samples to determine chemical loss rates for reactive chemical species, providing empirical vertical profiles for 24-h mean concentrations of hydroxyl radicals (OH) and chlorine atoms in the TTL. The analysis shows that important short-lived species such as CHCl3, CH2Cl2, and CH2Br2 have longer chemical lifetimes than the time for transit of the TTL, implying that these species, which are not included in most models, could readily reach the stratosphere and make significant contributions of chlorine and/or bromine to stratospheric loading.
Highlights
Inputs of trace gases into the stratosphere are controlled by transport processes and chemistry in the Tropical Tropopause Layer (TTL)
The effective equivalence between the 1-D advectiondiffusion approach and more detailed models that incorporate convective inputs and/or isentropic exchange reflect the fundamental characteristic of the CO2 profiles observed in the TTL in both TC4 and Costa Rica AURA Validation Experiment (CR-AVE): in both seasons, the observed profiles are only moderately modified from the time history obtained using the CO2 index in the tropics
To examine whether our transport rates and OH radical and Cl atom concentrations which were empirically derived from the 5 August observations, are unambiguous for other flight data, we generated the vertical profiles for the chemical species stated above by using their mixing ratios observed at the base of TTL on 6 August as the initial boundary values
Summary
Inputs of trace gases into the stratosphere are controlled by transport processes and chemistry in the Tropical Tropopause Layer (TTL). We use detailed measurements of trace gases in the TTL with a wide range of chemical lifetimes, from a few days to infinite (nonreactive), to empirically constrain both the chemical lifetimes and the transport time scales for key species, emphasizing the precursors of stratospheric inorganic halogens. We use the transport properties of the TTL obtained from CO2 to evaluate observations of reactive species, in order to infer rates of photochemical removal and to understand the efficiency with which these tracers propagate into the stratosphere via large-scale ascent of air mass in the TTL versus other possible pathways. We analyze data for selected gases representing a wide range of photochemical lifetimes in the TTL Most of these species are removed primarily by photolysis and reaction with hydroxyl radical (OH), allowing us to deduce empirically the 24-h mean OH vertical distribution in the TTL from an optimal fit of the photochemical transport simulation to the observations. At a 1-s data collection rate, measurement precision is ±0.6 ppbv (STP) and average uncertainty is ∼±5%
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