7.3.10 Quality control of chemical analysis for precipitation and dry deposition

   As part of the GAW programme, JMA observes pH of precipitation and ion concentrations to monitor global-scale atmospheric pollutions. The procedures of sampling, analysis of precipitation samples, and quality control of analytical values are in accordance with the Manual for the GAW Precipitation Chemistry Programme (WMO-GAW Report No.160).

  Precipitation samples are collected at Ryori and Minamitorishima on a daily basis, and sent to the JMA headquarters on a monthly basis, where the samples are analyzed. Analytical results are made publicly available after passing through quality control.

   Quality control is performed for ion balance between the chemical equivalents of cations (pH, Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) and anions (Cl^-, NO₃^2- and SO₄^2-), and for conductivity balance between the measured electrical conductivity and that theoretically calculated from pH and ion concentrations. Analytical results that fall within the prescribed limits for both ion and conductivity balances are made publicly available.

Comparison of cations and anions:

Ion difference [%] = 100×(CE-AE)/(CE+AE)

  AE : Total - The anion equivalent (µeq/l) is calculated using the following equation:
    AE=1000× [ΣC_Ai/(Eq.Wt.)_Ai]+5.1/10^(6-pH)
    C_Ai : concentration of i-th anions (mg/l)
    (Eq.Wt.)_Ai : Equivalent of i-th anions
    5.1/10^(6-pH) : concentration of bicarbonate ions (calculated concentration at 25°C, pH>5.0, µeq/l)

  CE : Total - The cation equivalent (µeq/l) is calculated using the following equation:
    CE=1000×[ΣC_Ci/(Eq.Wt.)_Ci]+[10^(6-pH)]
    C_Ci : concentration of i-th cations (mg/l)
    (Eq.Wt.)_Ci : Equivalent of i-th cations
    10^(6-pH) : H⁺ ion concentration (µeq/l)

Comparison of electrical conductivity:

    Δk(%)=100×[(k-k_meas)/k_meas]
    k_meas : Measured conductivity (µS/cm)
      k : Calculated conductivity (µS/cm)
        k = Σ_ci× Λi⁰
        ci: Molar concentration of i-th ion (mmol/l)
        Λi⁰:Ionic conductivity of i-th ion per 1 mol or equivalent amount (S cm²/mol)

        Calculated by the following equation:

        k=10^(3-pH)×349.7+c(SO₄²⁻)×160.0+c(NO₃⁻)×71.4
         + c(Cl⁻)×76.3+c(H₄⁺)×73.5+c(Na⁺)×50.1+c(K⁺)×73.5
         + c(Ca²⁺)×119.0+c(Mg²⁺)×106.0+c(HCO₃⁻)×44.5
        c(HCO₃⁻)=5.1/H⁺(25°C, 5.1/10^(3-pH) when it is pH>5.0)

  Observation of precipitation and dry deposition requires high analytical techniques. To maintain and improve analytical techniques and quality control of analytical data at participating laboratories in the GAW programme, analytical techniques are compared internationally. The GAW Quality Assurance/Science Activity Centre (QA/SAC) - Americas (http://qasac-americas.org/) is in charge of organizing and implementing international comparison.

  Annual international comparison started in 1978, but heavy metals were excluded from GAW by the decision of a meeting of GAW in December of 1999. At the same meeting, it was decided that pH and ions be analyzed as before by QA/SAC, and that the international comparison be carried out biannually starting in 2001. As a result, it became necessary to participate in the international comparison in order to participate in GAW and observation data from non-participating laboratories were no longer recognized as GAW data. JMA has been participating in the international comparison for pH and ions since the 6th intercomparison in 1983.

  As for heavy metals,in January 2000, it has been decided that non-European countries can also join the international comparison in the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air pollutants in Europe (EMEP), which is an initiative of the United Nations Economic Commission for Europe, focusing on the monitoring and evaluation of long-range transmission of air pollutants including heavy metals and acid rain. The international comparison is coordinated and implemented by the EMEP Chemical Coordinating Centre established in the Norwegian Institute for Air Research (NILU) (http://www.nilu.no/projects/ccc/index.html). JMA has been participating in the EMEP international comparison for heavy metals since the second comparison in 2002.

  The international comparison for precipitation is carried out as follows: QA/SAC distributes 3 samples of aqueous solution in different comparison, which are adjusted close to the levels of precipitation, to the participating laboratories with the comparison kept hidden. Each laboratory analyzes the samples and reports the results to QA/SAC. The international comparison for heavy metals is carried out in a similar manner. NILU distributes 4 samples of aqueous solution in which heavy metals are diluted differently close to the levels of precipitation, to the participating laboratories with the composition kept hidden. Each laboratory analyzes the samples and reports the results to NILU. For both intercomparison studies, the analytical results reported are compiled and evaluated by the organizers, and are made public with the laboratory names uncovered. Analytical results out of the limits are flagged, and the results serve to evaluate the ability of each laboratory. JMA has so far provided results within the limits.