The CO<sub>2</sub> tracer clock for the Tropical Tropopause Layer

Abstract

Abstract.Observations of CO 2 were made in the upper troposphere and lower stratosphere in the deep tropics in order to determine the patterns of large-scale vertical transport and age of air in the Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft over Central America and adjacent ocean areas took place in January and February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm Pool International Cloud Experiment, TWP-ICE). The data demonstrate that the TTL is composed of two layers with distinctive features: (1) the lower TTL, 350-360 K (potential temperature(θ ); approximately 12-14 km), is subject to inputs of convective outflows, as indicated by layers of variable CO 2 concentrations, with air parcels of zero age distributed throughout the layer; (2) the upper TTL, from θ=∼360 K to ∼390 K (14-18 km), ascends slowly and ages uniformly, as shown by a linear decline in CO 2 mixing ratio tightly correlated with altitude, associated with increasing age. This division is confirmed by ensemble trajectory analysis. The CO 2 concentration at the level of 360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air in the upper TTL monotonically ages while ascending. In combination with the winter slope of the CO 2 seasonal cycle (+10.8±0.4 ppmv/yr), the vertical gradient of -0.78 (±0.09) ppmv gives a mean age of 26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s −1 . TheCorrespondence to: S. C. Wofsy (swofsy@deas.harvard.edu) TTL near 360 K in the Southern Hemisphere over Australia is very close in CO 2 composition to the TTL in the Northern Hemisphere over Costa Rica, with strong contrasts emerging at lower altitudes (<360 K). Both Pre-AVE and CR-AVE CO 2 observed unexpected input from deep convection over Amazônia deep into the TTL. The CO 2 data confirm the operation of a highly accurate tracer clock in the TTL that provides a direct measure of the ascent rate of the TTL and of the age of air entering the stratosphere.

Aerosols that form subvisible cirrus at the tropical tropopause

“…Other particle types such as biomass burning aerosol, mineral dust, meteoric material, and stratospheric sulfuric acid were present as minor fractions of an external mixture with the sulfate-organic aerosol. Outside of regions influenced by recent continental convection, aerosols showed aging signatures consistent with slow ascent through the TTL over a period of several weeks (Fueglistaler et al, 2009;Park et al, 2007). Figure 4 and Table 1 compare compositional characteristics of SVC particle residues and unfrozen aerosols during the 2 February encounter.…”

“…Most tropopause level cirrus in this study were not directly associated with recent convective events and are presumed to have formed in situ from aerosols that slowly ascended through the tropical tropopause layer (TTL). This observation is based on (1) in situ data that did not show lower tropospheric characteristics for long-lived gas phase species (Park et al, 2007), shortlived gases , or aerosol composition; (2) minimal convective influence using a trajectory-based analysis ; and (3) flight camera and satellite imagery analysis. The sizes and chemical signatures of the cirrus-forming aerosols are compared to interstitial and outof-cloud aerosols.…”

Aerosol composition of the tropical upper troposphere

“…At the 350 K level and below, vertical transport is driven by convection. Convective outflow decreases rapidly above this level but is still sporadic at e.g., 360 K (Folkins et al, 2000;Gettelman et al, 2004;Park et al, 2007). Above this level, the overriding ascent mechanism in the TTL is large scale, radiatively driven upwelling.…”

“…The average ascent rate in the CR-AVE TTL was determined by Park et al (2007) to be 0.13 km day −1 , giving overall TTL residence times of around 1 month. To produce the average CR-AVE TTL aerosol organic mass in 1 month, the required production rate of condensable organic species is ∼100 molec cm −3 s −1 .…”

Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

“…Typically, air mass transport into the lower part of the tropical tropopause layer (TTL, ∼14 km to ∼18 km) is very fast through convective events. Further upward transport into the stratosphere occurs on timescales of weeks to months through the large scale dynamics of the Brewer-Dobson circulation (Park et al, 2007;Schoeberl et al, 2008;Fueglistaler et al, 2009). Hence, the tropical UT/LS is dynamically linked to the lower troposphere allowing for rapid uplift of halogen source gases or their breakdown products.…”