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电导率加和性质及其在酸雨观测数据质量评估中的应用
The Conductivity Additivity of Ionic Components in Precipitation and Its Application to the Data Evaluation of Acid Rain Monitoring
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摘要: 大气降水中离子成分的电导率具有可加和性, 国际上已经普遍应用该原理对大气降水离子成分观测的数据质量进行分析和评估, 即所谓的相对电导率差 (conductance percent difference, CPD) 方法。该文应用CPD方法对我国4个大气本底观测站的电导率和降水离子成分数据进行了数据质量分析, 针对我国的降水电导率范围偏高等特点, 对国外推荐的CP D统计检验指标的适用性进行了讨论, 指出在我国应用CPD方法时宜采取pH值分组的方式, 并根据这种分组方式的统计分析, 估计4个大气本底观测站的部分pH值观测数据可能存在-0.05左右的测量误差。该文还依据降水中离子成分电导率的可加和性原理, 提出K-pH不等式方法, 可以用于中国气象局酸雨观测站网的pH值和电导率数据的现场校验和数据质量的分析评估。应用K-pH不等式方法对观测数据进行现场质量检验时, 对pH值小于5.0范围的酸性降水效果尤其显著。应用K-pH不等式方法对1992—2005年间全国酸雨观测站观测数据的统计分析显示, 部分酸雨观测站的pH值测量数据可能存在-0.1~-0.3左右的系统性负偏差。Abstract: 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.
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图 1 实测电导率与计算电导率K H++K OH-的比较
Fig. 1 The comparison of the measured conductivity with calculated K H++K OH-
图 2 不同pH值范围条件下CPD值的频率分布
(方点为统计数据, 实线为拟合的高斯分布曲线) (a) pH≤4.0, (b) 4.0
Fig. 2 The distribution of CPD values in different pH ranges
(solid squares denote the statistical values, and curves denote the regressed normal distributions )(a) pH≤4.0, (b)4.0
图 3 实测电导率和K H++K OH-的差值分布随pH值的变化
(竖线表示中心90%的数据分布范围, 方框表示中心50%的数据分布范围, 中间横线表示中值)
Fig. 3 The differences of measured conductivities with K H++K OH- in different pH values
(vertical lines denote the ranges of central 90% data, the squares denote the ranges of central 50% data, the bars denote medians)
表 1 大气降水中主要离子的当量电导率*(单位: S·cm2·eq-1)
Table 1 The mole conductance of major ions in precipitation (unit:S·cm2·eq-1)
表 3 不同pH值时的K H+, KOH-和K H++KOH-(单位: μS·cm-1)
Table 3 The calculated K H+, K OH- and K H++K OH-in solutions with different pH (unit:μ S·cm-1)
表 4 pH值测量误差ΔpH导致K H++K OH- (单位: μ S·cm-1) 计算偏差的变化
Table 4 The error of calculated K H++K OH-(unit:μ S·cm-1) due to pH measurement error ΔpH
表 5 4个大气本底站CPD值的统计分析 (单位:%)
Table 5 The statistics of the CPD values for 4 stations (unit:%)
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