Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

Abstract

Abstract. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12-18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the midlatitude lower stratosphere was under-sampled (in the midupper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westward flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the Correspondence to: K. S. Law (kathy.law@latmos.ipsl.fr) campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into "convective" and "non-convective" flights. Strong signatures, particularly in species with short lifetimes (relative to CO 2 ) like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350-365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g. O 3 -poor) or polluted air masses from Asia (high O 3 , CO, CO 2 ) into West Africa. Thus, recent uplift of CO 2 over Asia may contribute to the observed positive CO 2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL and needs to taken into consideration in derivations of mean age of air. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20-40% above 370 K) during the campaign period importing air masses with high O 3 and NO y . Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net Published by Copernicus Publications on behalf of the European Geosciences Union.

Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility

“…Fierli (unpublished) and Khaykin et al (2009), do show evidence for overshooting over West Africa. As a result evaporation of the ice particles lead to hydration within the TTL and lower tropical stratosphere (Corti et al, 2008) during TROCCINOX and SCOUT-O3.This is in reasonable agreement with the analysis of Law et al (2010), although they found an influence from lower stratospheric air in the mid and upper TTL during the SCOUT-AMMA campaign period.…”

“…While N 2 O has a long lifetime, is well mixed within the troposphere, and has only one sink in the stratosphere, the ozone abundance is influenced by tropospheric photochemical sources and is not well suited as dynamical tracer in particular in the tropics. As Law et al (2010) demonstrate, this might lead to differences in lower TTL gradients because the ozone concentrations in the TTL vary significantly in dependency of location and season. For these reasons it is not surprising that the data from the campaigns are on different "branches" of the correlations shown in the right panel of Fig.…”

“…At the same time the ozone data indicate the presence of more aged air. Law et al (2010), analysed air mass origins in the TTL during August 2006 using domain filling trajectories over West Africa. Whilst most air masses were already residing in the TTL during the 10 days prior to the measurements, up to 39% of the air masses in the mid-TTL below 370 K were "perturbed" by injection of lower tropospheric air from Asia, India, and oceanic regions .…”

Emission location dependent ozone depletion potentials for very short-lived halogenated species

“…trajectories based on large-scale wind fields alone underestimate rapid vertical transport in the tropics. For example, Law et al (2010) have suggested that use of such trajectories underestimates the contribution of deep convection over Asia to the air masses in the tropical tropopause layer (TTL) observed over west Africa and estimates of convective transport based on trajectories calculated explicitly in a mesoscale model appear to show deeper transport of air masses into the tropical tropopause layer . The inclusion of the convective parameterization in the FLEX-PART trajectory code clearly has a significant impact on our results and may go some way to improving the representation of transport based on large-scale trajectories alone.…”

Tropical deep convective life cycle: Cb-anvil cloud microphysics from high-altitude aircraft observations

“…• 3D box surrounding the flight path, analogously to Law et al (2010). The closest starting point (in space, time, and height) is attributed to each point of the flight path.…”