12 Mar 2021

12 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Origins and characterization of CO and O3 in the African upper troposphere

Victor Lannuque1,a, Bastien Sauvage1, Brice Barret1, Hannah Clark4, Gilles Athier1, Damien Boulanger2, Jean-Pierre Cammas5, Jean-Marc Cousin1, Alain Fontaine1, Eric Le Flochmoën1, Philippe Nédélec1, Hervé Petetin6, Isabelle Pfaffenzeller2, Susanne Rohs3, Herman G. J. Smit3, Pawel Wolff1, and Valérie Thouret1 Victor Lannuque et al.
  • 1Laboratoire d’Aérologie, CNRS, UPS, Toulouse, France
  • 2Observatoire Midi-Pyrénées, Université de Toulouse, CNRS, UPS, Toulouse, France
  • 3Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, IEK-8 Troposphere, 52425 Jülich, Germany
  • 4IAGOS-AISBL, 98 Rue du Trône, Brussels, 1050, Belgium
  • 5Observatoire des Sciences de L’univers de la Réunion, UMS3365, la Réunion, France
  • 6Barcelona Supercomputing Center, Barcelona, Spain
  • anow at: CEREA, Joint Laboratory École des Ponts ParisTech/EDF R&D, Université Paris-Est, 77455 Marne-la-Vallée, France

Abstract. Between December 2005 and 2013, the In-service Aircraft for a Global Observing System (IAGOS) program produced almost daily in situ measurements of CO and O3 between Europe and southern Africa. IAGOS data combined with measurements from the IASI instrument onboard the Metop-A satellite (2008–2013) are used to characterize meridional distributions and seasonality of CO and O3 in the African upper troposphere (UT). The FLEXPART particle dispersion model and the SOFT-IO model which combines the FLEXPART model with CO emission inventories are used to explore the sources and origins of the observed transects of CO and O3.

We focus our analysis on two main seasons: December to March (DJFM) and June to October (JJASO). These seasons have been defined according to the position of Intertropical Convergence Zone (ITCZ), determined using in situ measurements from IAGOS. During both seasons, the UT CO meridional transects are characterized by maximum mixing ratios located 10° from the position of the ITCZ above the dry regions inside the hemisphere of the strongest Hadley cell (132 to 165 ppb at 0–5° N in DJFM and 128 to 149 ppb at 3–7° S in JJASO), and decreasing values south- and north-ward. The O3 meridional transects are characterized by mixing ratio minima of ~ 42–54 ppb at the ITCZ (10–16° S in DJFM and 5–8° N in JJASO) framed by local maxima (~ 53–71 ppb) coincident with the wind shear zones North and South of the ITCZ. O3 gradients are strongest in the hemisphere of the strongest Hadley cell. IASI UT O3 distributions in DJFM have revealed that the maxima are a part of a crescent-shaped O3 plume above the Atlantic Ocean around the Gulf of Guinea.

CO emitted at the surface is transported towards the ITCZ by the trade winds and then convectively uplifted. Once in the upper troposphere, CO enriched air masses are transported away from the ITCZ by the upper branches of the Hadley cells and accumulate within the zonal wind shear zones where the maximum CO mixing ratios are found. Anthropogenic and fires both contribute, by the same order of magnitude, to the CO budget of the African upper troposphere.

Local fires have the highest contribution, drive the location of the observed UT CO maxima, and are related to the following transport pathway: CO emitted at the surface is transported towards the ITCZ by the trade winds and further convectively uplifted. Then UT CO enriched air masses are transported away from the ITCZ by the upper branches of the Hadley cells and accumulate within the zonal wind shear zones where the maxima are located. Anthropogenic CO contribution is mostly from Africa during the entire year, with a low seasonal variability, and is related to similar transport circulation than fire air masses. There is also a large contribution from Asia in JJASO related to the fast convective uplift of polluted air masses in the Asian monsoon region which are further westward transported by the tropical easterly jet (TEJ) and the Asian monsoon anticyclone (AMA). O3 minima correspond to air masses that were recently uplifted from the surface where mixing ratios are low at the ITCZ. The O3 maxima correspond to old high altitude air masses uplifted from either local or long distance area of high O3 precursor emissions (Africa and South America during all the year, South Asia mainly in JJASO), and must be created during transport by photochemistry.

This analysis of meridional transects contribute to a better understanding of distributions of CO and O3 in the intertropical African upper troposphere and the processes which drive these distributions. Therefore, it provides a solid basis for comparison and improvement of models and satellite products in order to get the good O3 for the good reasons.

Victor Lannuque et al.

Status: open (until 07 May 2021)

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Victor Lannuque et al.

Victor Lannuque et al.


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Short summary
African intertropical troposphere is one of the world areas where the increase in ozone mixing ratio is most marked since 1980 and where high carbon monoxide mixing ratios are found in altitude. In this article, IAGOS aircraft measurements, IASI satellite instrument observations and SOFT-IO model products are used to explore the seasonal distribution variations and the origin of ozone and carbon monoxide over the African upper troposphere.