Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River
Aquatic CO<sub>2</sub> 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 <i>K<sub>T</sub&...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2023-07-01
|
| Series: | Water |
| Subjects: | |
| Online Access: | https://www.mdpi.com/2073-4441/15/14/2621 |
| _version_ | 1797587176560525312 |
|---|---|
| author | Anamika Dristi Y. Jun Xu |
| author_facet | Anamika Dristi Y. Jun Xu |
| author_sort | Anamika Dristi |
| collection | DOAJ |
| description | Aquatic CO<sub>2</sub> 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 <i>K<sub>T</sub></i> 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 <i>K<sub>T</sub></i> and the consequent CO<sub>2</sub> emission rate. As a case study, we utilized 3-year (2019–2021) measurements on the partial pressure of dissolved carbon dioxide <i>(p</i>CO<sub>2</sub>) in one of the world’s largest alluvial rivers, the lower Mississippi River, to determine the difference in CO<sub>2</sub> emission rate estimated through three approaches: velocity-based <i>K<sub>T</sub></i>, wind-based <i>K<sub>T</sub></i>, and a constant <i>K<sub>T</sub></i> (i.e., <i>K<sub>T</sub></i> = 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<sup>−1</sup> to 1.8 ms<sup>−1</sup>, and wind speed above the water surface fluctuated from 0 ms<sup>−1</sup> to nearly 5 ms<sup>−1</sup>. Correspondingly, we obtained a velocity-based <i>K<sub>T</sub></i> value of 7.80–22.11 m/day and a wind-speed-based <i>K<sub>T</sub></i> of 0.77–8.40 m/day. Because of the wide variation in <i>K<sub>T</sub></i> values, the estimation of CO<sub>2</sub> emission using different approaches resulted in a substantially large difference. The velocity-based <i>K<sub>T</sub></i> method yielded an average CO<sub>2</sub> emission rate (FCO<sub>2</sub>) of 44.36 mmol m<sup>−2</sup> h<sup>−1</sup> for the lower Mississippi River over the 3-year study period, varying from 6.8 to 280 mmol m<sup>−2</sup> h<sup>−1</sup>. In contrast, the wind-based <i>K<sub>T</sub></i> method rendered an average FCO<sub>2</sub> of 10.05 mmol m<sup>−2</sup> h<sup>−1</sup> with a small range of fluctuation (1.32–53.40 mmol m<sup>−2</sup> h<sup>−1</sup>,), and the commonly used constant <i>K<sub>T</sub></i> method produced an average FCO<sub>2</sub> of 11.64 mmol m<sup>−2</sup> h<sup>−1</sup>, also in a small range of fluctuation (2.42–56.87 mmol m<sup>−2</sup> h<sup>−1</sup>). Based on the findings, we conclude that the effect of river channel geometry and flow velocity on CO<sub>2</sub> outgassing is still largely underestimated, and the current estimation of global river CO<sub>2</sub> emission may bear large uncertainty due to limited spatial coverage of flow conditions and the associated gas exchange variation. |
| first_indexed | 2024-03-11T00:33:26Z |
| format | Article |
| id | doaj.art-9968762ec9e34da2a480ef235e236add |
| institution | Directory Open Access Journal |
| issn | 2073-4441 |
| language | English |
| last_indexed | 2024-03-11T00:33:26Z |
| publishDate | 2023-07-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Water |
| spelling | doaj.art-9968762ec9e34da2a480ef235e236add2023-11-18T21:47:55ZengMDPI AGWater2073-44412023-07-011514262110.3390/w15142621Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland RiverAnamika Dristi0Y. Jun Xu1School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70803, USASchool of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70803, USAAquatic CO<sub>2</sub> 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 <i>K<sub>T</sub></i> 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 <i>K<sub>T</sub></i> and the consequent CO<sub>2</sub> emission rate. As a case study, we utilized 3-year (2019–2021) measurements on the partial pressure of dissolved carbon dioxide <i>(p</i>CO<sub>2</sub>) in one of the world’s largest alluvial rivers, the lower Mississippi River, to determine the difference in CO<sub>2</sub> emission rate estimated through three approaches: velocity-based <i>K<sub>T</sub></i>, wind-based <i>K<sub>T</sub></i>, and a constant <i>K<sub>T</sub></i> (i.e., <i>K<sub>T</sub></i> = 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<sup>−1</sup> to 1.8 ms<sup>−1</sup>, and wind speed above the water surface fluctuated from 0 ms<sup>−1</sup> to nearly 5 ms<sup>−1</sup>. Correspondingly, we obtained a velocity-based <i>K<sub>T</sub></i> value of 7.80–22.11 m/day and a wind-speed-based <i>K<sub>T</sub></i> of 0.77–8.40 m/day. Because of the wide variation in <i>K<sub>T</sub></i> values, the estimation of CO<sub>2</sub> emission using different approaches resulted in a substantially large difference. The velocity-based <i>K<sub>T</sub></i> method yielded an average CO<sub>2</sub> emission rate (FCO<sub>2</sub>) of 44.36 mmol m<sup>−2</sup> h<sup>−1</sup> for the lower Mississippi River over the 3-year study period, varying from 6.8 to 280 mmol m<sup>−2</sup> h<sup>−1</sup>. In contrast, the wind-based <i>K<sub>T</sub></i> method rendered an average FCO<sub>2</sub> of 10.05 mmol m<sup>−2</sup> h<sup>−1</sup> with a small range of fluctuation (1.32–53.40 mmol m<sup>−2</sup> h<sup>−1</sup>,), and the commonly used constant <i>K<sub>T</sub></i> method produced an average FCO<sub>2</sub> of 11.64 mmol m<sup>−2</sup> h<sup>−1</sup>, also in a small range of fluctuation (2.42–56.87 mmol m<sup>−2</sup> h<sup>−1</sup>). Based on the findings, we conclude that the effect of river channel geometry and flow velocity on CO<sub>2</sub> outgassing is still largely underestimated, and the current estimation of global river CO<sub>2</sub> emission may bear large uncertainty due to limited spatial coverage of flow conditions and the associated gas exchange variation.https://www.mdpi.com/2073-4441/15/14/2621CO<sub>2</sub> emissiongas exchange coefficient<i>p</i>CO<sub>2</sub>riverine carbonMississippi River |
| spellingShingle | Anamika Dristi Y. Jun Xu Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River Water CO<sub>2</sub> emission gas exchange coefficient <i>p</i>CO<sub>2</sub> riverine carbon Mississippi River |
| title | Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River |
| title_full | Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River |
| title_fullStr | Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River |
| title_full_unstemmed | Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River |
| title_short | Large Uncertainties in CO<sub>2</sub> Water–Air Outgassing Estimation with Gas Exchange Coefficient <i>K<sub>T</sub></i> for a Large Lowland River |
| title_sort | large uncertainties in co sub 2 sub water air outgassing estimation with gas exchange coefficient i k sub t sub i for a large lowland river |
| topic | CO<sub>2</sub> emission gas exchange coefficient <i>p</i>CO<sub>2</sub> riverine carbon Mississippi River |
| url | https://www.mdpi.com/2073-4441/15/14/2621 |
| work_keys_str_mv | AT anamikadristi largeuncertaintiesincosub2subwaterairoutgassingestimationwithgasexchangecoefficientiksubtsubiforalargelowlandriver AT yjunxu largeuncertaintiesincosub2subwaterairoutgassingestimationwithgasexchangecoefficientiksubtsubiforalargelowlandriver |