Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters
Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters
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DOI：
10.5194/bg-12-67-2015
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发表时间：
2015-01-01
期刊：
影响因子：
4.9
通讯作者：
Borges, A. V.
中科院分区：
文献类型：
--
作者：
Abril, G.;Bouillon, S.;Borges, A. V.
在全球范围内，内陆沃茨被认为是大气中二氧化碳（CO2）的重要来源。根据气体传输速度和CO2分压的水气梯度（pCO（2））计算了水体与大气之间的CO2通量。目前，直接测量淡水中的pCO（2）仍然很少，大多数已发表的pCO（2）数据是根据温度，pH值和总碱度（TA）计算的。在这里，我们比较计算（pH值和TA）和测量（平衡和顶空）水pCO（2）在一个大的阵列的温带和热带淡水。761个数据点涵盖了TA（0至14 200 μ mol L-1）、pH（3.94至9.17）、pCO（2）（36至23 000 ppmv）和溶解有机碳（DOC）（29至3970 μ mol L-1）的广泛值。在60%的样品中，计算的pCO（2）比测量的pCO（2）高> 10%（与测量的pCO（2）相比，计算的pCO（2）的中值高估为2560 ppmv），在25%的大多数富含有机物和酸性样品中，计算的pCO（2）比测量的pCO（2）高> 100%（中值高估为9080 ppmv）。我们认为，pCO（2）计算值相对于实测值的过高估计是由于两种累积效应的综合作用：（1）在碳酸盐碱度低、DOC浓度高的沃茨中，有机酸阴离子对TA的贡献更大;（2）碳酸盐体系在低pH下的缓冲能力较低，这增加了计算的pCO（2）在酸性和富含有机物的沃茨中对TA的敏感性。无法从我们的数据集中推导出经验关系，以校正该偏倚的pCO（2）计算值。由于广泛分布的酸性，富含有机物的淡水，我们的结论是，区域和全球的估计CO2出气的淡水pH值和TA数据的基础上，最有可能被高估，虽然高估的幅度需要进一步的定量分析。一般而言，建议在内陆沃茨，特别是酸性、缓冲性差的淡水沃茨中直接测量pCO（2）。
Inland waters have been recognized as a significant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO2 (pCO(2)). Currently, direct measurements of water pCO(2) remain scarce in freshwaters, and most published pCO(2) data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO(2) in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 mu mol L-1), pH (3.94 to 9.17), measured pCO(2) (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 mu mol L-1). Calculated pCO(2) were > 10% higher than measured pCO(2) in 60% of the samples (with a median overestimation of calculated pCO(2) compared to measured pCO(2) of 2560 ppmv) and were > 100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO(2) with respect to measured pCO(2) are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO(2) to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO(2) for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO(2) are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.