Long term monitoring of the chemical composition of precipitation and wet deposition fluxes over three Sahelian savannas

Abstract

International audienceThe purpose of this study is to analyze a long term database of the chemical composition of precipitation at three African dry savanna sites in the Sahel. The precipitation samples were collected during the monsoon season at Agoufou (15°20′N, 01°29′W, Mali) from 2004 to 2006, Banizoumbou (13°31′N, 02°38′E, Niger) from 1994 to 2009 and Katibougou (12°56′N, 07°32′ W, Mali) from 1997 to 2008. pH and major inorganic and organic ions in precipitation were analyzed by ionic chromatography. A characterization of mean precipitation chemistry with the associated wet deposition fluxes for each species is presented. The first important result is that interannual variability of all volume-weighted mean (VWM) concentrations is low, ranging between ±5% and ±25%. Acidity in dry savannas is low and indicates the strong alkaline nature of the precipitation. The average annual pH at Agoufou is 6.28, 5.75 at Banizoumbou and 5.54 at Katibougou. This result is correlated with the important terrigenous contribution measured in the chemical content of precipitation, implying acidity neutralization by mineral species such as Ca2+ and NH4+. Mg2+ and K+ are found to play a minor role in neutralization. Enrichment factor calculations for Ca2+, SO42−, K+ and Mg2+ with respect to the sea reference reveal a significant influence of Saharan and Sahelian crustal sources. VWM concentrations of these species dominate the composition of measured precipitation. An estimation of the potential particulate and gas contribution to the total precipitation composition is given for each site: At Agoufou, the mean relative contribution in rainwater is 80% for particles and 20% for gases, while at the Banizoumbou and Katibougou sites, results indicate 70% for particles and 30% for gases. The high particulate phase contribution to precipitation emphasizes the importance of multiphase processes between gases and particles in the atmospheric chemistry typical of African semi-arid savanna ecosystems. The second highest contribution is nitrogenous, with high VWM concentrations of NO3− and NH4+ measured at the three sites. Monthly evolution of NO3− and NH4+ concentrations are studied in relation to gaseous emission sources in the Sahelian region, i.e. biogenic soil emission and ammonia sources from animals. The calculated wet nitrogen deposition flux presents a regular increase throughout the wet season at the three sites. Results suggest total mean nitrogen deposition fluxes of 1.80 kg N ha−1 yr−1 at Agoufou, 2.10 kg N ha−1 yr−1 at Banizoumbou, and 3.30 kg N ha−1 yr−1 at Katibougou. The marine contribution is lower, 23% at Agoufou, 17% at Banizoumbou and 13% at Katibougou. The last contribution concerns organic acidity, which ranges from 5% at Agoufou, 10% at Banizoumbou to 14% at Katibougou. Terrigenous and marine contributions present a negative gradient, whereas nitrogenous and organic contributions a positive gradient along the Sahelian transect defined by Agoufou-Banizoumbou-Katibougou

A pilot study of gaseous pollutants' measurement (NO<sub>2</sub>, SO<sub>2</sub>, NH<sub>3</sub>, HNO<sub>3</sub> and O<sub>3</sub>) in Abidjan, Côte d'Ivoire: contribution to an overview of gaseous pollution in African cities

“…The average concentration of NO 2 in Abidjan (17.8 ppb) is more than 3 times higher than that measured in St John's in India (4.8 ppb) (Kumar et al, 2004). This concentration in Abidjan is comparable to those measured in Bamako (16.2 ppb) and Shenzhen (20.7 ppb) reported by Adon et al (2016) and Xia et al (2017).…”

“…However, it remains lower than the concentrations obtained in Dakar (31.7 ppb) and Al Ain in the Middle East (31.5 ppb) and reported respectively by Adon et al (2016) and Salem et al (2009). The average concentration of NH 3 measured at the Abidjan traffic site (27.2 ppb) in this study is higher than those obtained in Dakar (21.1 ppb), Al Ain (17.1 ppb) and St John's (9.5 ppb), respectively reported by Adon et al (2016), Salem et al (2009) and Kumar et al (2004). Adon et al (2016) also reported a concentration of NH 3 in Bamako (46.7 ppb) 2 times higher than that obtained in Abidjan.…”

“…The IC system used for this study has been extensively described in previous studies (Adon et al, 2010;Galy-Lacaux and Modi, 1998;Hodgkins et al, 2011). The reliability of the IC analytical results are assessed by the chemistry laboratory of the LA in Toulouse by performing a quality control inter-comparison program organized by the World Meteorological Organization/Global Atmospheric Watch (WMO/GAW) twice a year (Laouali et al, 2012). Results of the WMO/GAW quality assurance program for the year 2016 show that analytical precision is estimated to be ±5 % for all ions (http://www.qasac-americas.…”

“…Most of these sites are located in the east of the city. This could be explained by the fact that gaseous HNO 3 is the result of several chemical transformation processes of NH 3 and NO x (NO 2 + NO) guided by the presence of oxidants such as H 2 O 2 , O 3 and OH radicals (Seinfeld and Pandis, 2016;Kumar et al, 2004;Hanke et al, 2003). The prevailing SW winds can also transport the precursor pollutants from the city centre in an easterly direction, and ageing in the air masses leads to the production of HNO 3 far from their source.…”

Dry deposition of nitrogen compounds (NO<sub>2</sub>, HNO<sub>3</sub>, NH<sub>3</sub>), sulfur dioxide and ozone in west and central African ecosystems using the inferential method

“…IDAF activity is based on high-quality measurements of atmospheric chemical data (gaseous, precipitation and aerosol chemical compositions) on the basis of multiyear monitoring (http://idaf.sedoo.fr). Within the framework of IDAF, several studies of precipitation chemical composition representative of great African ecosystems have been recently published (Galy- Lacaux and Modi, 1998;Galy-Lacaux et al, 2001Al-Ourabi and Lacaux, 2002;Lacaux et al, 1993Lacaux et al, , 2003Sigha et al, 2003;Yoboue et al, 2005;Mphepya et al, 2004;2006;Laouali et al, 2012).…”