酸雨观测中pH测量负偏差来源

通过现场调查、实验室测试,对酸雨观测中发现的pH测量负偏差现象进行了验证和分析。现场个例调查发现,在正常保存、使用期限内的pH电极也有可能出现老化现象,老化的pH测量电极给出的pH测量结果为负偏差,其数量级与文献指出的在观测资料质量评估中发现的pH测量负偏差相吻合。实验室测试结果显示,老化的pH测量电极的测量负偏差与模拟雨水样品的电导率和pH值有关:电导率越小,pH测量偏差则越大;在电导率差别不大时,该负偏差大小与水样pH值呈现一定的正相关线性关系,且电导率越小时两者的线性斜率越大。经过综合分析,初步确认pH电极的老化是造成历史观测资料中pH测量负偏差的重要原因。根据分析结果,提出了在台站检测pH测量电极老化的替代性技术方法,以更好地保证酸雨观测数据质量。

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.