Trace gases, Djougou, Benin

Monthly gases concentrations (in ppb) of sulfur dioxide, nitric acid, ammonia, nitrogen dioxide and ozone are measured using INDAAF passive samplers technique. Passive sampling technique uses the property of molecular diffusion of gases into the sampling device where they are collected on an impregnated filter or adsorbent material. The advantages of passive sampling technique is that it is generally easy to carry out and low cost compared to active sampling technique to achieve mean or integrated-time concentrations. The sampling technique is conform to WMO-GAW guidelines (<a href="https://library.wmo.int/records/item/41112-report-on-passive-samplers-for-atmospheric-chemistry-measurements-and-their-role-in-gaw">GAW-WMO Report n°122, 1998</a>).
Samplers are exposed at about 1.5–3m above the ground using duplicates for each gas. The INDAAF passive samplers were developed and prepared at the Laboratoire d’Aérologie (LAERO) and then were sent each two months on measurement sites. Samplers were sent back to Laero for chemical analysis using ionic chromatography (IC). The LAERO analytical lab participates bi-annually to the WMO-GAW quality assurance program for soluble trace compounds measured by Ion Chromatography (IC). The results of these inter-comparisons are available under the reference 700106 at the following address: http://qasac-americas.org/. According to these WMO inter-comparison tests, analytical precision is estimated to be 5% or better for all ions.