Technical Note: Large overestimation of <i>p</i>CO<sub>2</sub> calculated from pH and alkalinity in acidic, organic-rich freshwaters

Abstract

Abstract. Inland waters have been recognized as a significant source of carbon dioxide (CO 2 ) to the atmosphere at the global scale. Fluxes of CO 2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO 2 (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 µmol L −1 ), pH (3.94 to 9.17), measured pCO 2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 µmol 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 organicrich 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 CO 2 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.

Regional-scale lateral carbon transport and CO<sub>2</sub> evasion in temperate stream catchments

“…In our study, only 7 % of the pH values were < 7. For pH > 7 the median and mean relative errors are 1 and 15%, respectively (Abril et al, 2015). streams.…”

“…According to Abril et al (2015), high uncertainties of pCO 2 estimates from pH and alkalinity measurements occur at pH values < 7. In our study, only 7 % of the pH values were < 7.…”

Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed

“…For samples with a very low pH (<4.5), we bubbled the water with atmospheric air in order to degas the CO 2 . Consequently, the initial pH increased above the value of 5, and the TA titration could then be performed (Abril et al, 2015).…”

“…However, the latter studies used the Glorich database (Hartmann et al 2014) and thus calculated pCO 2 from pH, alkalinity and temperature. As a consequence, the CO 2 degassing estimation is probably overestimated, notably in low, buffered and high DOC waters such as boreal and tropical rivers, which strongly contribute to the global CO 2 degassing (Abril et al, 2015). Furthermore, CO 2 degassing is mostly estimated from the water-air CO 2 gradient, gas transfer velocity and stream surface areas.…”

Biogeochemical contrast between different latitudes and the effect of human activity on spatio-temporal carbon cycle change in Asian river systems

“…The model further clarified that these mechanisms are closely related to the slight increase in DIC and the decrease in POC in nighttime in three rivers. (Nakayama, 2017a(Nakayama, , 2017b in order to estimate the carbon cycle both in vertical flux such as CO2 evasion to the atmosphere and sediment storage, and in horizontal transport such as TOC, DOC, POC, DIC-flux to the ocean in previous data of these rivers (Schlunz and Schneider, 2000;Coynel et al, 2005;Rasera et al, 2008;Dubois et al, 2010;Alin et al, 2011;Butman and Raymond, 2011;Dai et al, 2012;10 Striegl et al, 2012;Abril et al, 2015;Lauerwald et al, 2015;Long et al, 2015). There are no data about CO2 evasion but only pCO2 and average efflux data in Mekong River Long et al, 2015), the author estimated CO2 evasion by multiplying the average efflux (gC/m 2 /yr) and water area (km 2 ).…”