Factors controlling the distribution of ozone in the West African lower troposphere  during the AMMA (African Monsoon Multidisciplinary Analysis) wet season campaign

Saunois, Marielle; Reeves, Claire E.; Mari, C.; Murphy, J. G.; Stewart, David J.; Mills, G.; Oram, D. E.; Purvis, R. M.

doi:10.5194/acp-9-6135-2009

Cited by 43 publications

(45 citation statements)

References 76 publications

(81 reference statements)

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“…Mean ozone deposition velocites have been measured to be very high in northern Congo Andreae et al, 1992) and in the Republic of Central Africa (Cros et al, 2000). Saunois et al (2009) have shown from measurements of the FAAM BAe-146 aircraft that there is a strong ozone gradient between forest and bare soil in the boundary layer in West Africa. They observed an increase of ozone mixing ratios from 25 ppbv over the forested area up to 40 ppbv over the Sahelian area in the West African lower troposphere.…”

Section: Ozone (O 3 )mentioning

confidence: 99%

Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers

et al. 2010

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Abstract. In this paper we present the long term monitoring of ambient gaseous concentrations within the framework of the IDAF (IGAC-DEBITS-AFRICA) program. This study proposes for the first time an analysis of long-term inorganic gas concentrations (1998 to 2007) of SO 2 , NO 2 , HNO 3 , NH 3 and O 3 , determined using passive samplers at seven remote sites in West and Central Africa. Sites are representative of several African ecosystems and are located along a transect from dry savannas-wet savannas-forests with sites at Banizoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d'Ivoire), Zoetele (Cameroon) and Bomassa (Congo). The strict control of measurement techniques as well as the validation and inter-comparison studies conducted with the IDAF passive samplers assure the quality and accuracy of the measurements. For each type of African ecosystem, the long term data series have been studied to document the levels of surface gaseous concentrations. The seasonal and interannual variability have also been analyzed as a function of emission source variations. We compared the measured West and Central African gas Correspondence to: M. Adon (adonatma@yahoo.fr) concentrations to results obtained in other parts of the world. Results show that the annual mean concentrations of NO 2 , NH 3 , HNO 3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite similar concentrations. Annual mean NO 2 concentrations vary from 0.9±0.2 in forests to 2.4±0.4 ppb in the dry savannas, NH 3 from 3.9±1.4 to 7.4±0.8 ppb and HNO 3 from 0.2±0.1 to 0.5±0.2 ppb. Annual mean O 3 and SO 2 concentrations are lower for all ecosystems and range from 4.0±0.4 to 14.0±2.8 and from 0.3±0.1 to 1.0±0.2 ppb, respectively. A focus on the processes involved in gas emissions from dry savannas is presented in this work, providing explanations for the high concentrations of all gases measured at the three dry savannas sites. At these sites, seasonal concentrations of all gases are higher in the wet season. Conversely, concentrations are higher in the dry season in the wet savannas. In forested regions, we measure no significant difference between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at:

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Section: Ozone (O 3 )mentioning

confidence: 99%

Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers

et al. 2010

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“…The minimum over southern West Africa is related to convective uplift of O 3 -poor air masses from forested regions (high O 3 dry deposition, Saunois et al, 2009) or oceanic regions. Reeves et al (2010) also noted latitudinal differences in ozonesonde data collected in July and August 2006 at Cotonou, Benin (6 • N, 2 • E) and Niamey, Niger (13 • N, 2 • E), further north.…”

Section: Relation To Observed Trace Gas Profilesmentioning

confidence: 99%

“…Comparison with data collected in the lower troposphere can be used as an indicator of convective uplift. Over West Africa during late July/August 2006, lower tropospheric CO varied from 145 ppbv over southern tropical forests to 80-90 ppbv north of 14 • N (Saunois et al, 2009;Reeves et al, 2010). An analysis of all aircraft vertical profile data collected during July-August 2006 showed a maximum in CO concentrations in the upper troposphere (200 hPa) and that flights influenced by local convection had more variable CO ranging from 80 to 200 ppbv compared to "non-convective" flights (100-150 ppbv).…”

Section: Relation To Observed Trace Gas Profilesmentioning

confidence: 99%

Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

et al. 2010

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Abstract. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12-18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the midlatitude lower stratosphere was under-sampled (in the midupper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westward flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the Correspondence to: K. S. Law (kathy.law@latmos.ipsl.fr) campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into "convective" and "non-convective" flights. Strong signatures, particularly in species with short lifetimes (relative to CO 2 ) like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350-365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g. O 3 -poor) or polluted air masses from Asia (high O 3 , CO, CO 2 ) into West Africa. Thus, recent uplift of CO 2 over Asia may contribute to the observed positive CO 2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL and needs to taken into consideration in derivations of mean age of air. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20-40% above 370 K) during the campaign period importing air masses with high O 3 and NO y . Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net Published by Copernicus Publications on behalf of the European Geosciences Union.

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“…West Africa is characterized by the presence of potentially important and poorly known sources of aerosol and ozone precursors. Large latitudinal variability of nitrogen oxides emissions in West Africa during the wet season was observed during AMMA and is reported by Stewart et al (2008); NO x levels in the Sahelian area are high leading to higher ozone mixing ratios (Saunois et al, 2009) while ozone precursors from biomass burning in the southern hemispheric African continent can be transported by deep convection in the upper troposphere up to 10 • N (Mari et al, 2008;Real et al, 2010).…”

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confidence: 99%

Impact of deep convection in the tropical tropopause layer in West Africa: in-situ observations and mesoscale modelling

et al. 2011

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Abstract. We present the analysis of the impact of convection on the composition of the tropical tropopause layer region (TTL) in West-Africa during the AMMA-SCOUT campaign. Geophysica M55 aircraft observations of water vapor, ozone, aerosol and CO 2 during August 2006 show perturbed values at altitudes ranging from 14 km to 17 km (above the main convective outflow) and satellite data indicates that air detrainment is likely to have originated from convective cloud east of the flights. Simulations of the BO-LAM mesoscale model, nudged with infrared radiance temperatures, are used to estimate the convective impact in the upper troposphere and to assess the fraction of air processed by convection. The analysis shows that BOLAM correctly reproduces the location and the vertical structure of convective outflow. Model-aided analysis indicates that convection can influence the composition of the upper troposphere above the level of main outflow for an event of deep convection close to the observation site. Model analysis also shows that deep convection occurring in the entire Sahelian transect (up to 2000 km E of the measurement area) has a non negligible role in determining TTL composition.

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Factors controlling the distribution of ozone in the West African lower troposphere during the AMMA (African Monsoon Multidisciplinary Analysis) wet season campaign

Cited by 43 publications

References 76 publications

Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers

Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers

Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

Impact of deep convection in the tropical tropopause layer in West Africa: in-situ observations and mesoscale modelling

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