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Diel cycle of lake-air CO2 flux from a shallow lake and the impact of waterside convection on the transfer velocity
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
2015 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 1, 29-38 p.Article in journal (Refereed) Published
Abstract [en]

Two years of eddy covariance measurements of lake carbon dioxide (CO2) fluxes reveal a dielcycle with higher fluxes during night. Measurements of partial pressure in the air (pCO2a) and in the water(pCO2w), during 4 months, show that the high nighttime fluxes are not explained by changes in the differencebetween pCO2a and pCO2w. Analyzing the transfer velocity (k600,meas), which is a measure of the efficiencyof the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain thediel cycle. During nighttime, when the fluxes are high, the wind is normally low. Thus, a solely wind-basedparameterization of the transfer velocity (ku,CC) results in large errors compared to k600,meas, especially forwind speeds lower than 6ms1. The mean absolute percentage error between ku,CC and k600,meas is 79%. By subtracting ku,CC from k600,meas, we investigate how waterside convection influence k600,meas. Our resultsshow that the difference (k600,measku,CC) increases with increasing waterside convection. Separating thetransfer velocity parameterization in two parts, one depending on the wind speed and one depending onwaterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO2 fluxes can, to a large extent, be explained bywaterside convection and that a transfer velocity parameterization based on both wind speed and watersideconvection better fits the measurements compared to a parameterization based solely on wind speed.

Place, publisher, year, edition, pages
2015. Vol. 120, no 1, 29-38 p.
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-241576DOI: 10.1002/2014JG002781ISI: 000349899200003OAI: oai:DiVA.org:uu-241576DiVA: diva2:779888
Available from: 2015-01-13 Created: 2015-01-13 Last updated: 2015-07-31Bibliographically approved
In thesis
1. Lake Fluxes of Methane and Carbon Dioxide
Open this publication in new window or tab >>Lake Fluxes of Methane and Carbon Dioxide
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Recent studies have shown that lakes, although they cover a small area of the globe, can be very important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange.

By using the eddy covariance method for lake flux studies, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes.

Eddy covariance measurements at a Swedish lake revealed a diel cycle in the fluxes of both CH4 and CO2, with higher fluxes during nighttime than daytime. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to waterside convection. For the diel cycle of CH4 flux it was suggested that waterside convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition.

Simultaneous flux measurements of CH4 and CO2 have been presented using both the eddy covariance method and the floating chambers method of which the latter is the traditional measuring method for lake fluxes. For CO2 the two methods agreed well during some periods but differed considerably during others. Disagreement between the methods might be due to horizontal heterogeneity in partial pressure of CO2 in the lake. The methods agreed better for the CH4 flux measurements. However, it is clear that due to the discontinuous nature of the floating chambers, this method will likely miss important high flux events.

The main conclusions of this thesis are:

1) the two gas flux methods are not directly comparable and should be seen as supplementary to each other

2) waterside convection enhances the fluxes of both CH4 and CO2 over the water-air surface. If gas flux measurements are not conducted during nighttime, potential high flux periods might be missed and estimates of the total amount of gas released from lakes to the atmosphere may be biased.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 41 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1221
air-lake gas exchange, carbon dioxide, eddy covariance, floating chambers, methane, waterside convection
National Category
Natural Sciences Meteorology and Atmospheric Sciences
Research subject
urn:nbn:se:uu:diva-241984 (URN)978-91-554-9152-9 (ISBN)
Public defence
2015-03-20, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Available from: 2015-02-26 Created: 2015-01-19 Last updated: 2015-03-18Bibliographically approved

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Podrajsek, EvaSahlée, ErikRutgersson, Anna
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