The WMO established quality criteria to provide co-ordination between atmospheric monitoring networks and assure the quality of the data they provide. International intercomparison studies are carried out each year and both the Laboratoire d’Aérologie (LA) and ESKOM laboratories have been involved in these tests since 1996. These inter-calibration studies ensure the quality of measurements meets the standards of the DEBITS protocol. For more details, consult the WMO report 80: Report of the WMO Meeting of Experts on the Quality Assurance Plan for the Global Atmosphere Watch, (WMO/TD-NO 513).
All ion chromatography, pH and conductivity analyses carried out at the LA are evaluated through the WMO intercomparison studies. Each year, in November, 3 blind samples are received. The results of these analyses are then sent back to the WMO and referenced with WMO values. The analytical precision of the LA has been shown to be:
- 5% or better for mineral ions
- 0.6 % for pH
- 2% for conductivity
The quality of ion chromatography analyses carried out at the Laboratoire d’Aérologie is assured through the WMO comparison studies.
Ring diagram presenting WMO intercomparison study results for the period 2005-2009 from the Laboratoire d’Aérologie. Measured concentrations for the three sample tests are shown.
The WMO criteria for precipitation analysis includes a criterion concerning the ionic balance. This takes into account several critical species and requires that the total anion equivalents (or charges) should balance the cation equivalents. The ion balance is considered as an ion difference:
Ion difference (in %) = 100 * ( [CE-AE ] / [CE+AE] )
where: AE is the anion sum in µeq/l and CE is the cation sum in µeq/l
The WMO quality criteria are as follows:
|Anions + Cations (µeq/l)||Acceptable ion difference (%)|
|X < 50||± 60|
|50 < X < 100||± 30|
|100 < X < 500||± 15|
|500 < X||± 10|
A similar test is also applied, using WMO standards, to the conductivity balance. The conductivity is tested as follows:
Xmes is the measured conductivity and Xcal is the calculated conductivity:
Xcal = Ci * Li
where Ci is the ionic concentration in mmol/l and Li is the ionic conductance in S/cm2/mol.
The difference in conductivity is formally calculated from:
D X (%) = 100 * ( (Xcal-Xmes) / Xmes) )
WMO conductivity quality criteria are as follows:
|Measured conductivity (µS /cm)||Acceptable Conductivity Différence (%)|
|X < 5||X ± 50|
|5 < X < 30||X ± 30|
|X > 30||X ± 20|