Trace gases, Amersfoort, South Africa

Monthly gases concentrations (in ppb) of SO2, HNO3, NH3, NO2 and O3 are measured using passive sampling techniques. Passive sampling techniques use 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 techniques are that they are generally easy to carry out and low cost compared to active sampling techniques to achieve mean or integrated-time concentrations. The WMO-GAW report 122 reports on passive samplers for atmospheric chemistry measurements and their role in GAW: https://library.wmo.int/index.php?lvl=notice_display&id=11054.
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 each measurement site. Samplers were sent back to Laero for chemical analysis using ionic chromatography (IC). The Laero analytical service participates bi-annually to the WMO-GAW quality assurance program for soluble trace compounds measured by Ion Chromatography (IC). Results 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.