Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons

Abstract

Abstract.A meso-scale model was used to understand and describe the dynamical processes driving high ozone concentrations observed during both dry and monsoon season in monthly climatologies profiles over Lagos (Nigeria, 6.6 • N, 3.3 • E), obtained with the MOZAIC airborne measurements (ozone and carbon monoxide). This study focuses on ozone enhancements observed in the upper-part of the lower troposphere, around 3000 m. Two individual cases have been selected in the MOZAIC dataset as being representative of the climatological ozone enhancements, to be simulated and analyzed with on-line Lagrangian backtracking of air masses.This study points out the role of baroclinic low-level circulations present in the Inter Tropical Front (ITF) area. Two low-level thermal cells around a zonal axis and below 2000 m, in mirror symmetry to each other with respect to equator, form near 20 • E and around 5 • N and 5 • S during the (northern hemisphere) dry and wet seasons respectively. They are caused by surface gradients -the warm dry surface being located poleward of the ITF and the cooler wet surface equatorward of the ITF.A convergence line exists between the poleward low-level branch of each thermal cell and the equatorward low-level branch of the Hadley cell. Our main conclusion is to point out this line as a preferred location for fire products -among them ozone precursors -to be uplifted and injected into the lower free troposphere.The free tropospheric transport that occurs then depends on the hemisphere and season. In the NH dry season, the AEJ allows transport of ozone and precursors westward to Lagos. In the NH monsoon (wet) season, fire products are transported from the southern hemisphere to Lagos by the southeasterly trade that surmounts the monsoon layer. Additionally ozone precursors uplifted by wet convection in the ITCZ can also mix to the ones uplifted by the baroclinic cell and be advected up to Lagos by the trade flow.

Global NO<sub>x</sub> emission estimates derived from an assimilation of OMI tropospheric NO<sub>2</sub> columns

“…The similar magnitude of the ozone underestimation and lightning source (0.061 TgN) in our CHASER simulation shows that although the global total lightning source is similar for our simulation (7.5 TgN yr −1 ) and estimates from chemical observations (mostly 6-8 TgN yr −1 ) (e.g. Martin et al, 2007;Sauvage et al, 2007), CHASER may underestimate lightning NO x sources and their-induced ozone production in the free troposphere over Mexico and North America. Errors in the stratospheric ozone transport into the troposphere may also contribute to the ozone underestimation.…”

Impact of West African Monsoon convective transport and lightning NO<sub>x</sub> production upon the upper tropospheric composition: a multi-model study

“…According to Sauvage et al (2007b), these vertical winds are primarily caused by surface gradients in temperature and humidity, with the warm dry surface being located poleward and the cooler wet surface equatorward of the ITF. Furthermore, based on a case study, Sauvage et al (2007b) have shown that during the WAM, the low-level circulation induced by these surface gradients is responsible for the uplift of biomass burning pollutants to the level of the African Easterly Jet (∼600 hPa), which results in their long-range transport to Western Africa. The large scale UT circulation is characterised by the presence of thermally induced anticyclones over low-latitudes continental regions (Hastenrath, 1991).…”

Analysis of CO in the tropical troposphere using Aura satellite data and the GEOS-Chem model: insights into transport characteristics of the GEOS meteorological products

“…The Harmattan winds become stronger and move equatorward near 700 hPa, and merge with the southeasterly trade winds around 5 • N. There is a region of strong upward mass fluxes in the MT (430 hPa) primarily north of the Equator that lofts CO from the fires to the MT and UT. Several studies discuss these transport pathways during the West Africa monsoon season, in the context of interpreting ozone and CO observations (Sauvage et al, , 2007a, including MLS CO data (Barret et al, 2008(Barret et al, , 2010. Barret et al (2008) attribute the CO maximum over northern Africa at 215 hPa (in July 2006) mainly to convective uplift of CO-rich air from biomass burning in southern Africa following these transport pathways.…”