The Cause of pH Measurement Negative Bias in Acid Rain Monitoring

Precipitation pH and conductivity are measured routinely in a nation wide network with more than 300 stations known as Acid Rain Monitoring Network (ARMN), under the infrastructure of China Meteorological Administration (CMA) since early 1990s. Recent reports indicate that a small part of precipitation pH data in the historic ARMN CMA dataset may suffered negative bias, upmost to-0.3 pH unit, when meticulous statistical method being applied in the data quality evaluation. To seek a better understanding on the causes of this negative bias, a field investigation is designed and conducted to track the performance of pH electrode at site by using the method of routine measurement of target samples. Two target samples prepared by the central lab in Beijing, with pH value of 4.8 and 6.5, are measured at two dozen of selected CMA ARMN stations once every ten days. At one of the selected stations, obvious negative bias in pH measurement appears after a new pH electrode being put into use for only 3 months. The magnitude of the negative bias produced by this short lived electrode is relatively steady in following 6 months, which ranges from 0.2 to 0.4 pH unit and is similar with that found in the historic pH dataset of ARMN CMA. As the regulated usage period for a pH electrode in ARMN CMA is 12 months, the lack of on site pH electrode examining/testing method at ARMN CMA stations may result in using a short lived electrode unwittingly in the routine precipitation pH measurement. Hence, the aging of pH electrode is considered as the most likely cause for the negative bias found in the historic pH dataset of ARMN CMA. Further test for the found aged pH electrode, by using a group of prepared solutions with different pH values and conductivities, shows that the negative bias of aged pH electrode is not only affected by ion strength of the solution, but also by the acidity of the solution. The aged pH electrode tends to give larger negative bias in the solution of lower ion strength. Also, the negative bias given by the aged pH electrode is correlated to the pH value of the solution. For the solution with conductivity in range of 30—70 μS·cm-1, the slope of the negative bias to the pH value of the solution is about 0.08, which tends to be smaller for the solutions of higher ion strength. Based on the results of the field investigation and the laboratory test, target sample measurement is suggested as an on site quality control method for pH measurement at stations.