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Long Term Measurements of Sulfur Dioxide, Nitrogen Dioxide, Ammonia, Nitric Acid and Ozone in Africa Using Passive Samplers

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Long Term Measurements of Sulfur Dioxide, Nitrogen Dioxide, Ammonia, Nitric Acid and Ozone in Africa Using Passive Samplers Atmos. Chem. Phys., 10, 7467–7487, 2010 www.atmos-chem-phys.net/10/7467/2010/ Atmospheric doi:10.5194/acp-10-7467-2010 Chemistry © Author(s) 2010. CC Attribution 3.0 License. and Physics Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers M. Adon1,2, C. Galy-Lacaux1, V. Yoboue´2, C. Delon1, J. P. Lacaux1, P. Castera1, E. Gardrat1, J. Pienaar3, H. Al Ourabi4, D. Laouali5, B. Diop6, L. Sigha-Nkamdjou7, A. Akpo8, J. P. Tathy9, F. Lavenu10, and E. Mougin11 1Laboratoire d’Aerologie,´ UMR 5560, Universite´ Paul-Sabatier (UPS) and CNRS, Toulouse, France 2Laboratoire de Physique de l’Atmosphere,` Universite´ de Cocody, Abidjan, Cote d’Ivoire 3School of Chemistry, North-West University, Potchefstroom 2520, South Africa 4Departement´ de Physique, Universite´ d’Alep, Alep, Syria 5Departement´ de Physique, Universite´ Abdou Moumouni, Faculte´ des Sciences, Niamey, Niger 6Departement´ de Physique, Universite´ de Bamako, Bamako, Mali 7Centre de Recherches Hydrologiques,Yaounde,´ Cameroon 8Departement´ de physique, Universite´ Abomey Calavi, Cotonou, Benin 9Direction Gen´ erale´ de la Recherche Scientifique et Technologique, Brazzaville, Congo 10Centre d’etudes´ spatiales de la biosphere,` UMR 5126, Toulouse, France 11Laboratoire des Mecanismes´ et Transferts en Geologie,´ UMR 5563 UR154 UPS/CNRS/IRD, Toulouse, France Received: 18 December 2009 – Published in Atmos. Chem. Phys. Discuss.: 12 February 2010 Revised: 9 July 2010 – Accepted: 26 July 2010 – Published: 10 August 2010 Abstract. In this paper we present the long term monitoring concentrations to results obtained in other parts of the world. of ambient gaseous concentrations within the framework of Results show that the annual mean concentrations of NO2, the IDAF (IGAC-DEBITS-AFRICA) program. This study NH3, HNO3 measured in dry savannas are higher than those proposes for the first time an analysis of long-term inorganic measured in wet savannas and forests that have quite simi- gas concentrations (1998 to 2007) of SO2, NO2, HNO3, NH3 lar concentrations. Annual mean NO2 concentrations vary and O3, determined using passive samplers at seven remote from 0.9±0.2 in forests to 2.4±0.4 ppb in the dry savan- sites in West and Central Africa. Sites are representative of nas, NH3 from 3.9±1.4 to 7.4±0.8 ppb and HNO3 from several African ecosystems and are located along a transect 0.2±0.1 to 0.5±0.2 ppb. Annual mean O3 and SO2 concen- from dry savannas-wet savannas-forests with sites at Bani- trations are lower for all ecosystems and range from 4.0±0.4 zoumbou (Niger), Katibougou and Agoufou (Mali), Djougou to 14.0±2.8 and from 0.3±0.1 to 1.0±0.2 ppb, respectively. (Benin), Lamto (Cote d’Ivoire), Zoetele (Cameroon) and Bo- A focus on the processes involved in gas emissions from dry massa (Congo). The strict control of measurement tech- savannas is presented in this work, providing explanations niques as well as the validation and inter-comparison stud- for the high concentrations of all gases measured at the three ies conducted with the IDAF passive samplers assure the dry savannas sites. At these sites, seasonal concentrations quality and accuracy of the measurements. For each type of all gases are higher in the wet season. Conversely, con- of African ecosystem, the long term data series have been centrations are higher in the dry season in the wet savan- studied to document the levels of surface gaseous concen- nas. In forested regions, we measure no significant differ- trations. The seasonal and interannual variability have also ence between wet and dry seasons. This unique database been analyzed as a function of emission source variations. of long term gases concentrations monitoring is available at: We compared the measured West and Central African gas http://medias.obs-mip.fr/idaf/. Correspondence to: M. Adon ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 7468 M. Adon et al.: Sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa 1 Introduction the IDAF African network at seven sites in West and Central Africa. Martins et al. (2007) have already published the long Measurement programs play a critical role in air pollution term monitoring of gases at the three IDAF sites located in and atmospheric chemistry studies. Moreover, long term South Africa. All the monitored gases play a major role in the measurement programs are crucial to documenting changes chemistry of the atmosphere and they were included in the in atmospheric composition and long term monitoring allows measurement program of the atmospheric chemistry interna- the characterization of seasonal and interannual variability. tional network DEBITS/IGAC. The monitored compounds This is particularly important in tropical regions where sea- are sulfur dioxide (SO2), nitrogen dioxide (NO2), nitric acid sonal cycles related to natural and anthropogenic emissions (HNO3), ammonia (NH3) and ozone (O3). sources of gases and particles are marked, resulting from sea- It is known that the release of acidic pollutants, which in- sonality of biomass burning, soil and vegetation emissions. clude sulfur and nitrogen compounds, into the atmosphere The international DEBITS program (Deposition of Bio- can cause adverse health effects and have the potential to geochemically Important Trace Species) was initiated in cause other environmental damage (e.g. acid rain) (Brim- 1990 as part of the IGAC/IGBP (International Global Atmo- blecombe et al., 2007). The deposition of atmospheric spheric Chemistry/ International Geosphere-Biosphere Pro- compounds containing these species are thus of major im- gramme) “core project” in order to study wet and dry atmo- portance in atmospheric studies. The oxides of nitrogen, spheric deposition in tropical regions (Lacaux et al., 2003). (NOx (NO+NO2) mainly emitted into the atmosphere as NO, In 2004, DEBITS activities were positively assessed, and which is subsequently transformed into NO2 and other ni- continue in the new IGAC II framework (task DEBITS II, trogenous species, collectively known as NOy) play impor- Pienaar et al., 2005; Bates et al., 2006). In 2009. The DEB- tant roles in controlling the oxidative chemistry of the lower ITS network includes about twenty five long term measuring atmosphere, including regulation of the photochemical pro- stations evenly distributed within the tropical belt in Africa, duction of ozone, nitric acid and organic nitrates. Nitric Asia and South America. For tropical Africa, the IDAF acid (HNO3) is the most important transformation product (IGAC/DEBITS/AFRICA) project started in 1994. From the of NOx. The measurement of HNO3 in ambient air is of beginning the IDAF program has been recognized by INSU great importance because of its significance in the acidifica- (Institut National des Sciences de l’Univers, in France) and tion of the atmosphere and its control of the levels of photo- the CNRS (Centre National de la Recherche Scientifique, in oxidants. Ammonia is also an important atmospheric pollu- France) as a member of the Environmental Research Ob- tant, with a wide variety of impacts. In the atmosphere, NH3 servatory (ORE, in France) network. The ORE IDAF has neutralizes a great portion of the acids produced by oxides of been given the mission of establishing a long-term measur- sulfur and nitrogen. Essentially, all emitted NH3 is returned ing network to study the atmospheric composition in Africa. to the surface by deposition, which, since the early 1980s, has Since 2005, the IDAF program is also associated with the been known to be one of the primary causes of soil acidifica- AMMA LOP (African Monsoon Mutidisciplinary Analy- tion (Sutton et al., 2009). Sulfur dioxide (SO2) is the main ses/Long Observation Program, Lebel et al., 2007). Within cause of acid precipitation, which adversely affects natural the IDAF framework, several studies of the chemical compo- systems as well as agriculture and building materials (Smith sition of precipitation representative of African ecosystems et al., 2001). The sulfate aerosol particles formed as a conse- have been published. These studies have helped to quantify quence of these emissions impair visibility and affect human the wet deposition of biogeochemically important elements health (Chestnut, 1995). Tropospheric ozone (O3) is also a and to estimate the contributions of different atmospheric major environmental concern because of its adverse impacts sources (Galy-Lacaux and Modi, 1998; Galy-Lacaux et al., on human health (Mc Granaham and Murray, 2003), its im- 2001; Sigha et al., 2003; Yoboue et al., 2005; Mphepya et al., pact on crops and forest ecosystems (NRC, 1991; Mauzerall 2004, 2006; Galy-Lacaux et al., 2009). To complement these and Wang, 2001) and its greenhouse gas effect responsible studies, it is appropriate to study and quantify dry deposition for climate change (Hansen et al., 2002). fluxes. Dry deposition can be estimated, on the one hand, us- In the IDAF program, SO2, NO2, HNO3, NH3 and O3 ing continuous concentration measurements of gaseous and have been measured using passive sampling. Passive sam- bulk air sampling for particulate species (ammonium and ni- plers present a good means of addressing many measurement trate particulate content). On the other hand, realistic dry issues in air pollution and atmospheric chemistry. They pro- deposition velocities need to be determined for each specific vide a cost-effective way to monitor specific species at rural, site and species in order to estimate dry deposition fluxes. regional and global scales. Passive samplers have been ex- In this paper, we present the long term monitoring of am- tensively tested in various international studies (Ferm and bient gaseous concentrations within the IDAF program. In a Rodhe, 1997; Ferm et al., 2005; Carmichael et al., 1996; second step, dry gaseous deposition fluxes will be estimated Martins et al., 2007). Within the framework of the IDAF and these results are the subject of a separate paper.
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