The Conductivity Additivity of Ionic Components in Precipitation and Its Application to the Data Evaluation of Acid Rain Monitoring

Conductivity of precipitation (K) is a primary parameter in acid rain/precipitation chemistry monitoring. As the conductivity of ions in precipitation is an addible quantity, the conductivity data is commonly used in the quality check/assessment on the analysis data of ionic components in precipitation, by comparing the measured K with the calculated K from the ionic concentration data obtained from the chemical analysis. This method is referred normally as conductance percent difference (CPD) method and widely used. A national monitoring network has been run by China Meteorological Administration for more than 15 years for the acid rain monitoring, which is called Acid Rain Monitoring Network (ARMN/CMA), as an important supplement to its Global Atmosphere Watch (GAW) program with 4 Global/regional stations. Only pH and conductivity of precipitation are measured by those ARMN/CMA stations, with a total number nearly 300 by the end of 2006. To meet the need for quality check/assessment on the basis of pH and K data obtained from ARMN/CMA, the K-pH inequality method is proposed based on the same principle of conductivity additivity with CPD method, viz. : the K calculated from H⁺ concentration reduced from pH must be smaller than the measured K.The usage of K-pH inequality method in different pH value range is discussed and it shows that this method is an effective and easy-to-use tool for the on field check and afterward data evaluation, especially for the data with pH values below 5.0. With K-pH inequality method, the historical pH and K data of ARMN/CMA from 1992 to 2005 are checked. The results show that part of the calculated K is larger than the measured K, which means errors either in pH measurement or in K measurement, or both. The distributions of the differences between the measured and the calculated K in different pH bins show that the differences with pH≤4.0 tend to be more and more negative as pH getting lower. It suggests that the errors be mainly associated with pH measurements. With this assumption, the errors in pH measurements for those data are estim ated to be from-0.1 to-0.3. This estimate is in consistence with results of the annual blind sample inter-comparison within ARMN/CMA.The CPD values of all precipitation samples from 4 GAW stations are also calculated and the frequency distributions of CPD values in different pH bins are studied to check/evaluate the data quality of the ionic component data. The results show that CPD values for samples with pH≤4.0 tend to be positive and those for samples with pH>4.0 tend to be negative. This tendency also suggests errors in pH measurements for 4 GAW stations, but the magnitude is smaller than data of ARMN/CMA, with an estimate of-0.05.