PM2.5 Aerosol EC/OC, Mineral and Trace Metals Analysis, Djougou, Benin

Weekly integrated aerosol samples are collected using two automatic aerosol samplers specifically designed and developed for the INDAAF program (http://indaaf.obs-mip.fr). Each sampler is equipped with two independent air inlets connected with KNF pumps (N89 KNE-K 220V) operating at a flow rate of 9 L/min, Cole palmer ball flow meters with a micrometric valve (flow range adjustable from 0 to 10 L/min and NILU online filter holders. Each air inlet is surmounted with an aerosol cutting head corresponding to aerodynamic particles diameter. Two mini Partisol low air inlets PM2.5 and PM10 (Rupprecht and Patashnick) operates at a flow rate of 5 L/min and the air volume sample is measured with a GALLUS type G4 gas meter (accuracy ±0.01 m-3) (Ouafo-Luembe et al., 2017; Djoussou et al, 2018). Aerosol samples were collected on 47mm diameter filters on a weekly basis. For each filter batch preparation, field blanks are included with the samplers sent into the field. Before and after collection, samples and blanks filters are conditioned in petri slides boxes and stored at 4°C.

Quartz fiber filters were used for carbonaceous analysis and the determination of Elemental Carbon (EC), Organic Carbon (OC) and Total Carbon (TC). Quartz fiber filters were cleaned by burning at 450°C for 48h before exposition. Quartz filter samples are analyzed at the Laboratoire d’Aérologie (LAERO) of Toulouse using thermo-optical methods. From 2005 to 2018, analysis were performed using the Desert Research Institute (DRI) instrument and the IMPROVE analytical protocol. Since 2018, analysis are performed using the Sunset instrument and the European EUSAAR2 protocol. All analytical results are converted to concentrations (in microgram per cubic meter).

PTFE or teflon filters were used for to determine mineral compounds. PTFE filters are weighed before and after sampling using a SARTORIUS MCS21 microbalance (1 µg resolution for a maximum range of 21 g - Measuring accuracy of ± 5 µg). From 2005, we performed PM2.5 or PM10 weekly filter samples analysis at the Laboratoire d’Aérologie (Laero) to determine mineral composition (soluble major ions) using Ion Chromatography (IC). Before this date, weekly bulk aerosol sampling on teflon filters was performed (1998-2005).

Half Teflon filters samples (PM2.5 and PM10) were analyzed by ICP-MS (7500 ce Agilent technologies) using In and Re as internal standards. Filters are previously mineralized according the method described in Ouafo and Luembe, 2017. 13 trace metals are considered: Al, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ba, La, Th, Pb and Cd. The detection limit is less than 10 ppt.