The Statistical Characteristics of pH Measurement Bias in the Monitoring Data of CMA-ARMN

Precipitation pH and conductivity are measured routinely in the nation-wide network with more than 300 stations, known as the Acid Rain Monitoring Network (ARMN), under the infrastructure of China Meteorological Administration (CMA) since early 1990s. Previous studies indicate that a systematic negative bias in the pH measurement, caused by the aging of pH electrode, which has weakened the data quality obviously. Characteristics of the bias are investigated to describe the temporal and spatial features of the negative pH measurement bias (NBpH) and to estimate its magnitude with meticulous statistical approaches, based on the monitoring dataset obtained from CMA-ARMN during 1992—2011. The result indicates that 40% stations of CMA-ARMN suffers from NBpH problem in the early 1990s, while this ratio decreases to 25% during 2010—2011. The proportion of data infected with NBpH problem during 2010—2011 reduces to 2% from 10% in the early 1990s. This improvement of pH measurement mentioned above can be attributed to the renewal of pH meter for all CMA-ARMN stations in 2000, the re-approval of Standard Operation Manual of CMA-ARMN in 2005, and the recent trail measures on the pH electrode aging. Also, statistics shows that NBpH problem seldom occurs in the stations over the Northwest China and Qinghai-Tibet Plateau, but frequently occurs in the stations of other regions over China. On the whole, NBpH problem in southern part of China is more serious than those in the northern part of China. This geographical feature of NBpH is related with the behavior of aged pH electrode which shows bigger negative bias when measuring the solution of low pH and low conductivity. Quasi-quantitative estimation of NBpH based on the K-pH inequality principle shows that the magnitude of NBpH for about 80% data ranges from-0.6 to 0, with a few of extremes lower than-3.0. The lower pH is, the larger NBpH tends to be. To avoid NBpH problem in future, substitution of the now-using normal pH electrode in CMA-ARMN with the pH electrode for low ion strength is suggested, together with the corresponding QA/QC (quality assurance/quality control) procedures. In addition, the possibility and the example of data correction with NBpH are discussed.