Tropospheric ozone production related to West African city emissions during the 2006 wet season AMMA campaign 1UPMC Univ. Paris 06, UMR8190, CNRS/INSU, LATMOS-IPSL – Université Versailles, St-Quentin, France
05 Jul 2011
2ISAC-Institute for Atmospheric Sciences and Climate, National Research Council, Bologna, Italy
3DLR Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
4Danish Meteorological Institute, Lyngbyvej, Denmark
5Université de Toulouse, CNRS, LA (Laboratoire d'Aérologie), Toulouse, France
Received: 01 September 2010 – Published in Atmos. Chem. Phys. Discuss.: 10 November 2010 Abstract. During African Monsoon Multidisciplinary Analyses (AMMA)
airborne measurements of ozone, CO and nitrogen oxides (NOx) were collected by
French and German Falcon aircraft near three cities
in West Africa (Cotonou, Niamey and Ouagadougou). They have been analysed to
identify the good conditions to observe ozone plumes related to city emissions
during the monsoon season.
Results show that an O3 increase of 40–50 ppbv above the summer average
concentration took place during two specific events: one near Cotonou on the
coast of the Gulf of Guinea, and the other near Niamey in the Sahel region.
In both cases a high level of NOx (3–5 ppbv) is related to the
ozone production. Air mass transport simulations with FLEXPART and a tracer
simulation with the BOLAM mesoscale model shows that Southern Hemisphere
biomass burning emissions are always at higher altitude (>3 km) compared to
the city emissions. In Niamey and Ouagadougou, the daily variability of ozone
and CO correlates with the FLEXPART analysis showing the role of air mass
stagnation near the city for 1–2 days and advection of emissions from the
vegetated areas. Absence of ozone enhancements for high CO values can be
explained by the occurrence of deep convection near the city.
In the Sahel region, convection must be accounted for to understand the small number
of observed ozone plumes but also to explain the high level of NOx in
the 3–5 ppbv range, due to increasing soil emissions after rainfall.
To verify that daily ozone production can reach 20 ppbv day−1 for the NOx and CO
conditions encountered near West African cities, a simulation of the CiTTyCAT
Lagrangian model was conducted using the observed average chemical
composition reported by other aircraft during AMMA. Such ozone production
is possible for NOx levels up to 5 ppb showing that
West African cities are potentially significant sources of tropospheric ozone.
Revised: 22 June 2011 – Accepted: 27 June 2011 – Published: 05 July 2011
Citation: Ancellet, G., Orlandi, E., Real, E., Law, K. S., Schlager, H., Fierli, F., Nielsen, J. K., Thouret, V., and Mari, C.: Tropospheric ozone production related to West African city emissions during the 2006 wet season AMMA campaign, Atmos. Chem. Phys., 11, 6349-6366, doi:10.5194/acp-11-6349-2011, 2011.