An overview of two years of ozone radio soundings over Cotonou as part of AMMA

Abstract

Abstract. As part of the African Monsoon Multidisciplinary Analysis (AMMA) program, a total of 98 ozone vertical profiles over Cotonou, Benin, have been measured during a 26 month period (December 2004-January 2007. These regular measurements broadly document the seasonal and interannual variability of ozone in both the troposphere and the lower stratosphere over West Africa for the first time. This data set is complementary to the MOZAIC observations made from Lagos between 0 and 12 km during the period 1998-2004. Both data sets highlight the unique way in which West Africa is impacted by two biomass burning seasons: in December-February (dry season) due to burning in the Sahelian band and in June-August (wet season) due to burning in southern Africa. High interannual variabilities between Cotonou and Lagos data sets and within each data set are observed and are found to be a major characteristic of this region. In particular, the dry and wet seasons are discussed in order to set the data of the Special Observing Periods (SOPs) into a climatological context. Compared to other dry and wet seasons, the 2006 dry and wet season campaigns took place in rather high ozone environments. During the sampled wet seasons, southern intrusions of biomass burning were particularly frequent with concentrations up to 120 ppbv of ozone in the lower troposphere. An insight into the ozone distribution in the upper troposphere and the lower stratosphere (up to 26 km) is given. The first tropospheric columns of ozone Correspondence to: V. Thouret (valerie.thouret@aero.obs-mip.fr) based on in-situ data over West Africa are assessed. They compare well with satellite products on seasonal and interannual time-scales, provided that the layer below 850 hPa where the remote instrument is less sensitive to ozone, is removed.

Measurements of volatile organic compounds over West Africa

“…A combination of modelling, in situ and remote sensing observations indicates that enhancements of O 3 and CO in the troposphere over Africa can result from the transport of biomass burning emissions from fires situated in the opposite dry-season hemisphere add (Barret et al, 2008;Sauvage et al, 2005;Thouret et al, 2009;Real et al, 2010). High acetonitrile observations provided clear evidence of biomass burning outflow during three flights on which the PTR-MS was operated.…”

“…Crutzen and Andreae, 1990), and VOC such as acetonitrile can serve as tracers of biomass burning influence. This is particularly useful in West Africa, where observations of enhanced O 3 and CO in the mid-troposphere during the wet season have been attributed to long range transport from fires across the equator (Sauvage et al, 2005;Thouret et al, 2009) Published by Copernicus Publications on behalf of the European Geosciences Union. 5282 J. G. Murphy et al: Measurements of volatile organic compounds over West Africa but back trajectories struggle to represent these transport patterns .…”

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…For example, over Lagos, Nigeria, ozone-enhanced layers are observed in December-February, when burning occurs regionally, and also in July-August, when biomass burning occurs over southern equatorial Africa (Sauvage et al, , 2007b. Such processes have been further investigated in the frame of AMMA (Reeves et al, 2010;Thouret et al, 2009;Mari et al, 2008). Similarly, over Brazzaville, the seasonal cycle of ozone peaks in July-August but a secondary maximum appears in January-February due to transport of ozone and precursors from fires occurring in the Northern Hemisphere (Sauvage et al, , 2007b.…”

“…Reeves et al (2010) provide a clear characterisation of the ozone distribution throughout the troposphere over West Africa during the monsoon period. Owing to previous analysis based on MOZAIC data (Sauvage et al, 2007a and the regular ozone soundings over Cotonou over 2 years (Thouret et al, 2009), it has been shown that the biomass burning seasons in both hemispheres impact the tropospheric ozone profiles in both hemispheres as well. As a consequence, a picture like the so-called "ozone paradox" (Thompson and Hudson, 1999) was shown to be no longer valid (Sauvage et al, 2006) owing to the insight provided by additional in situ observations.…”

What caused extreme ozone concentrations over Cotonou in December 2005?

“…In the frame of AMMA, an ozone sounding station was set up in Cotonou, Benin (6.21 • N, 2.23 • E) thanks to the IRD (Institut de Recherche et Développement) and the SMN/ASECNA (Service Météorologique National/Agence de securité et de navigation aérienne) people located there. These regular soundings were performed between December 2004 and January 2007 and aimed to better assess the seasonal and interannual variability of ozone vertical profiles in Equatorial Africa as described by Thouret et al (2009). They found that December 2005 was characterized by very high ozone concentrations throughout the troposphere compared to the rest of the sampled period.…”