Abstract
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 Infrared Atmospheric Sounding Interferometer (IASI) instrument aboard 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 southward and northward. 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 and drive the location of the observed UT CO maxima. Anthropogenic CO contribution is mostly from Africa during the entire year, with a low seasonal variability. 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.
Highlights
Tropospheric ozone (O3) has a significant impact on the oxidative capacity of the troposphere by being a major source of hydroxyl radicals (OH) (e.g., Lelieveld et al, 2016; Logan et al, 1981) and on climate by being a powerful greenhouse gas (e.g., Myhre et al, 2013)
We developed a methodology to localize the Intertropical Convergence Zone (ITCZ) with In-service Aircraft for a Global Observing System (IAGOS) meteorological data which allowed us to delimit empirically two periods of the year of strongest interest in this region, namely June to October (JJASO) and December to March (DJFM), with the intermediate months corresponding to transitional periods
The ITCZ was found to extend over 14.5–4.5◦ S during JJASO and 4.5–9.5◦ N during DJFM, in good agreement with NCEP/NCAR reanalysis
Summary
Tropospheric ozone (O3) has a significant impact on the oxidative capacity of the troposphere by being a major source of hydroxyl radicals (OH) (e.g., Lelieveld et al, 2016; Logan et al, 1981) and on climate by being a powerful greenhouse gas (e.g., Myhre et al, 2013). The meridional position of the ITCZ has a strong seasonal variability over Africa (e.g., Suzuki, 2011), causing the alternation of dry and wet seasons This meteorology impacts the O3 and CO distributions in the African UT (e.g., Sauvage et al, 2007d) and their propagation in the global atmosphere (e.g., Zhang et al, 2016). Using upper-tropospheric measurements of O3 and relative humidity (RH) from 1994 to 2004, Sauvage et al (2007d) showed an ozone minimum collocated with a RH maximum at the ITCZ Both vertical transport in the ITCZ followed by photochemical production in the upper branches of the Hadley cells were found to contribute to meridional O3 gradient creation. Flight trajectories span a wide range of latitudes (between 22◦ S and 51◦ N) for a limited variation of longitude (between 17 and 0◦ E), which allows investigating the meridional distribution of both trace gases
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