Large Uncertainties in CO2 Water–Air Outgassing Estimation with Gas Exchange Coefficient KT for a Large Lowland River

Abstract

Aquatic CO2 emission is typically estimated (i.e., not measured) through a gas exchange balance. Several factors can affect the estimation, primarily flow velocity and wind speed, which can influence a key parameter, the gas exchange coefficient KT in the balancing approach. However, our knowledge of the uncertainty of predictions using these factors is rather limited. In this study, we conducted a numeric assessment on the impact of river flow velocity and wind speed on KT and the consequent CO2 emission rate. As a case study, we utilized 3-year (2019–2021) measurements on the partial pressure of dissolved carbon dioxide (pCO2) in one of the world’s largest alluvial rivers, the lower Mississippi River, to determine the difference in CO2 emission rate estimated through three approaches: velocity-based KT, wind-based KT, and a constant KT (i.e., KT = 4.3 m/day) that has been used for large rivers. Over the 3-year study period, river flow velocity varied from 0.75 ms−1 to 1.8 ms−1, and wind speed above the water surface fluctuated from 0 ms−1 to nearly 5 ms−1. Correspondingly, we obtained a velocity-based KT value of 7.80–22.11 m/day and a wind-speed-based KT of 0.77–8.40 m/day. Because of the wide variation in KT values, the estimation of CO2 emission using different approaches resulted in a substantially large difference. The velocity-based KT method yielded an average CO2 emission rate (FCO2) of 44.36 mmol m−2 h−1 for the lower Mississippi River over the 3-year study period, varying from 6.8 to 280 mmol m−2 h−1. In contrast, the wind-based KT method rendered an average FCO2 of 10.05 mmol m−2 h−1 with a small range of fluctuation (1.32–53.40 mmol m−2 h−1,), and the commonly used constant KT method produced an average FCO2 of 11.64 mmol m−2 h−1, also in a small range of fluctuation (2.42–56.87 mmol m−2 h−1). Based on the findings, we conclude that the effect of river channel geometry and flow velocity on CO2 outgassing is still largely underestimated, and the current estimation of global river CO2 emission may bear large uncertainty due to limited spatial coverage of flow conditions and the associated gas exchange variation.