ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus GmbHGöttingen, Germany10.5194/essd-7-311-2015CO2-flux measurements above the Baltic Sea at two heights: flux gradients in the surface layer?LammertA.andrea.lammert@uni-hamburg.dehttps://orcid.org/0000-0002-1506-4299AmentF.Meteorological Institute, University of Hamburg, Bundesstr. 55, 20146 Hamburg, GermanyA. Lammert (andrea.lammert@uni-hamburg.de)16November20157231131719June201513July201523October201526October2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://essd.copernicus.org/articles/7/311/2015/essd-7-311-2015.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/7/311/2015/essd-7-311-2015.pdf
The estimation of CO2 exchange between the ocean and the atmosphere is essential to
understand the global carbon cycle. The eddy-covariance technique offers a very direct approach to observe these
fluxes. The turbulent CO2 flux is measured, as well as the sensible and latent heat flux and the
momentum flux, a few meters above the ocean in the atmosphere.
Assuming a constant-flux layer in the near-surface part of the atmospheric boundary layer, this flux equals
the exchange flux between ocean and atmosphere. The purpose of this paper is the comparison of long-term
flux measurements at two different heights above the Baltic Sea to investigate this assumption. The
results are based on a 1.5-year record of quality-controlled eddy-covariance measurements.
Concerning the flux of momentum and of sensible and latent heat, the constant-flux layer theory can
be confirmed because flux differences between the two heights are insignificantly small more than 95 % of the time. In contrast, significant differences, which are larger than the measurement
error, occur in the CO2 flux about 35 % of the time. Data used for this paper are
published at http://doi.pangaea.de/10.1594/PANGAEA.808714.
FINO2: position in the Baltic Sea (top, right), the whole mast (left), and the platform with
the boom and instrument installation at a height of 6.8 and 13.8 m above sea surface (bottom).
Instrument boom at FINO2 with the turbulence sensors at both heights and instrument installation in more detail (inset).
Introduction
The chemical composition of the atmosphere is influenced to a very great extent by the exchange of
gases between the ocean and the atmosphere. Particularly the exchange of carbon dioxide (CO2) is
of interest due to the climate-relevant effects of CO2 and the role of the ocean as a major sink
of anthropogenically produced CO2. A frequently used and very direct method to measure
turbulent fluxes of momentum, heat and trace gases (e.g., CO2) is the eddy-covariance technique.
The technique itself has been proved and enhanced for more than 30 years e.g.. Eddy-covariance systems have been installed on research vessels, buoys, and
platforms to measure the near-surface CO2 fluxes above the oceans, mostly on a short timescale of
a few weeks e.g.,. This lower layer of the atmosphere, the Prandl layer, is approximated by
a height-constant turbulent flux. With the assumption of the constant-flux layer, it is possible to
obtain the CO2 flux at the boundary between water and atmosphere from a flux measurement
at a height of several meters. Measurements at one height are common practice for the
determination of CO2 fluxes and the estimation of the carbon net ecosystem
exchange above land, too e.g.,. To test the assumption of the constant-flux layer, two
eddy-covariance systems at different heights (i.e., 6.8 and 13.8 m above the sea surface) were
installed in 2008 at the research platform FINO2 (Forschungsplattform in Nord- und Ostsee 2) in the Baltic Sea. Each system consisted of a fast
sonic anemometer and an open-path infrared gas analyzer for CO2 and H2O. This publication has
the goal of testing the constant-flux theory with respect to the CO2 flux on the basis of long-term
measurements of turbulent fluxes and CO2 over 1.5 years. Therefore, the CO2 flux will be
estimated and compared at both heights with the standard eddy-covariance technique in combination with
the standard correction terms (see Sect. ). To highlight the special characteristics of
the CO2 flux, the latent and sensible heat flux as well as the momentum flux will be analyzed
additionally to serve as a reference. The data described in this paper are published in the PANGAEA
system (Data Publisher for Earth & Environmental Science; ).
Daily means of measured quantities at a height of 13.8 m above sea surface: vertical wind
speed (w), horizontal wind speed (ff), air temperature (T), absolute humidity (AH), and CO2
density from June 2008 to December 2009.
FINO2 – site and instrumentation
In 2007 the FINO2 platform was installed in the southwest of the Baltic Sea, in the tri-border
region between Germany, Denmark, and Sweden (see Fig. ). The platform collects
meteorological (at several heights of between 30 and 101 m), oceanographic and biological data. In the framework of the
research project SOPRAN (Surface Ocean Processes in the Anthropocene; see http://sopran.pangaea.de),
the platform was equipped with additional sensors in June 2008. A combination of three-component sonic
anemometers (USA1) and open-path infrared gas analyzers for CO2 and H2O (LI-COR 7500) were
installed on a 9 m long boom on the southern side of the platform at two heights, at 6.8 and 13.8 m above sea
surface. Figure shows the boom with the instrumentation and the alignment of the
sonic anemometer and the LI-COR instrument, which is identical at both heights. The sonic anemometer is installed overarm and the LI-COR
instrument below the sonic anemometer. This setting was chosen to minimize the distance of the measuring volumes
of both instruments (the distance is 20 cm) and to create a sector as large as possible without
flow distortion. For the same reason the instruments at the different heights are installed on different sides of the boom, so the horizontal distance of the installations is nearly 1 m.
Additionally, slow temperature and humidity sensors were installed at each height. The gas
analyzer systems were calibrated before the installation and worked continuously without any
calibration during the whole measurement period of 1.5 years.
In this paper continuous measurements over 1.5 years (June 2008 to December 2009) are
analyzed and the fluxes at both heights are compared to each other.
Data processing
Both instrument types, the sonic anemometer and the LI-COR instrument, yield measurements with a temporal
resolution of 10 Hz. The high-frequency data were filtered due to spikes and rain. The FINO platform
itself has an influence on the measurement in the case of northerly winds. Therefore, the
data are filtered in the case of wind directions between 285 and 35∘ to exclude not only possible flow
distortion but also influence of the platform generator on the CO2 measurements.
On the basis of 10 min mean values over time periods with steady conditions (e.g., between different
maintenance periods), we used a so-called sector-wise tilt correction as alignment correction. This
procedure is similar to a planar fit correction but applied to 10∘ sectors instead of the
whole plane.
For both instruments, the comparison of the high-frequency measurements with the measurements of the slow sensors showed
no significant long-term drift in temperature and H2O.
Drifts on smaller timescales (on the order of days) due to contamination with sea salt, were
cleaned naturally by rain. The drift of both quantities had no influence on the fluctuations on the
eddy timescale, which, in contrast to the mean values, are important for the flux estimation.
Daily means of momentum flux (Fm), sensible and latent heat flux (H and LE), and CO2 flux
at a height of 13.8 m from June 2008 to December 2009.
Comparison of turbulent fluxes at two different heights: upper (13.8 m) vs. lower height (6.8 m).
The temporal resolution is 30 min. Top: momentum flux Fm (left) and sensible heat flux
H (right); bottom: latent heat flux LE (left) and CO2 fluxes (right). C gives the correlation
coefficient.
Distribution of flux differences (top to bottom) for momentum (Fm), sensible (H) and latent
heat (LE), and CO2 flux (CO2), based on 30 min values for 1.5 years. M gives the mean
difference for each; class stands for the width of class for each flux difference. The dotted lines give
the measurements uncertainties, each derived from the RMSE.
Measurement quantities
The time series at a height of 13.8 m of vertical wind speed (w), horizontal wind speed
(ff), air temperature (T), absolute humidity (AH), and the CO2 density (CO2) are plotted as
daily means in Fig. . Over the time interval of 1.5 years an annual
cycle, typical for the Baltic Sea, is recognizable for temperature and humidity (for comparison, see
). The maximum temperature, around 20∘C, is observed in August, the minimum,
around 0∘C, in winter. The absolute humidity ranges from 3 to 13 gm-3.
In contrast the CO2 density shows the maximum, near 0.8 gm-3, in the winter months and the
minimum, 0.6 gm-3, in summer. Neither the vertical nor the horizontal wind speed show an
annual cycle. The daily mean values of the vertical wind velocity fluctuate around 0, ranging from -0.1 to 0.1 ms-1. These values are a result of processes such as horizontal convective rolls,
large-scale advection or temperature contrasts between water and air. The average over the whole time
period is -0.004ms-1, which is below the measurement uncertainty.
The time period from June to December is comparable for all variables in both years, 2008 and 2009.
Turbulent fluxes and flux differences
The estimation of fluxes, such as momentum or CO2, are based on the correlation of highly resolved
fluctuations of the vertical wind speed with quantities such as horizontal wind fluctuations or CO2
fluctuations. The raw eddy-covariance fluxes of the momentum Fm, sensible and latent heat H
and LE, and CO2 were calculated over 30 min intervals from the fast sensors and were given by
Fm=-ρau′w′‾H=ρacpT′w′‾LE=Leρv′w′‾FCO2=w′ρc′‾,
where ρa is the density of dry air, ρc that of CO2 and ρv that of water vapor.
Le is the latent heat of vaporization, cp the specific heat, and T the air temperature.
Over-bars denote temporal means and dashes the fluctuations with respect to these means. It is
necessary to correct the raw fluxes due to correlated density effects, e.g., for the CO2
flux; therefore the latent and sensible heat flux have to be taken into account. A commonly used correction
was given by :
FCO2=w′ρc′‾+μρcρaw′ρv′‾+(1+μσ)ρ‾cw′T′‾T‾,
with the ratio of molecular masses μ=ma/mv and of densities of air constituents
σ=ρv‾/ρa‾. The subscript “v” stands for water vapor. The
latent heat fluxes are corrected according to Webb, the sensible heat flux according to Schotanus.
For a detailed description of the eddy-covariance method and its correction terms, please
see, amongst others, and .
The determination of the measurement error for turbulent fluxes with an error propagation is in
general very difficult, e.g., due to the correction terms. Assuming temporally uncorrelated
measurement errors, the root mean square deviation of preceding 30 min flux estimates provides an
upper limit for the root mean square error (RMSE) of the measurements. Similar approaches to
determine observation errors, e.g., by extrapolating the autocorrelation function to a zero
time lag, are frequently used in data assimilation e.g., and are known as the nugget effect.
The turbulent fluxes of the whole time period of 1.5 years are shown in Fig.
as daily averages. The momentum fluxes are in the range of -0.7 to nearly
0.0 kg/(m s-2). The sensible heat flux shows a clear annual signal, with maximum values in
autumn and winter. The amplitude and variability of daily latent heat fluxes is higher, compared to
the sensible heat. The minimum is in March or April, whereas high values of more then 100 Wm-2 are
observed from July till November in both years. The CO2 fluxes show very small variability with
values between -0.5 and 0.4 mg (m2 s)-1. This magnitude is in the same range as observed by other
authors, e.g., -0.2 to 0.05 mg (m2 s)-1 above the Baltic Sea , or -0.1 to
0.3 mg (m2 s)-1 near the coast above the Sea of Japan . Compared to measurements above
land surface, the fluxes of momentum, sensible heat, and CO2 show no significant diurnal (not
shown) and a much weaker annual cycle.
Figure shows the comparison of the turbulent fluxes with a 30 min resolution
at a height of 13.8 m vs. a height of 6.8 m. The scatterplots of the momentum and sensible heat flux show the
expected strong association of both heights, with a very high correlation coefficient of about 0.98
each. Both fluxes are determined by the analyses of just the sonic anemometers. For the latent heat
flux, the correlation is a bit lower, with C=0.96. In contrast, the comparison of the CO2
fluxes shows a wide spread around 0, with a very low correlation coefficient of 0.46. For both
the latent heat and the CO2 flux we have to take into account that an instrument combination of
sonic anemometers and the LI-COR instrument is used. Nevertheless, the relatively low correlation of the CO2
fluxes, compared to the other turbulent fluxes, is surprising.
For this reason, we calculated the difference in both fluxes (upper height minus lower height) and
analyzed the distribution of these differences. In Fig. the distribution functions of
the differences are shown, additionally to the cumulative distributions, for all four turbulent
fluxes. While the momentum-flux differences are distributed in a nearly Gaussian way, the heat flux difference
distributions both have a slight positive skewness. The CO2-flux difference distribution shows a
clear negative skewness. All distributions show the maximum at zero difference. In order to
distinguish between insignificant flux differences due to random measurement error and real flux
differences, the estimated uncertainties from the RMSE of all fluxes are plotted in Fig.
as dotted lines. By means of these limits, it is clearly evident that for the
momentum flux, just less than 5 % of all differences are significant. The same is valid for the sensible
heat flux. A positive mean difference of 4.6 Wm-2 applies in the case of the latent heat flux, while 12 %
positive differences plus 3 % negative differences of all cases are significant. So the latent heat flux at the upper height is
significantly higher than at the lower height in 12 % of the observed time interval. The
CO2 flux, with the negative skewness in the difference distribution, is significantly higher
at 13.8 m than at 6.8 m in just 5 % of all time steps, but in nearly 30 % of all analyzed cases,
the differences are significantly negative. In summary, the measurements at the FINO2 platform
indicate significant CO2 flux differences at heights of 6.8 and 13.8 m in 35 % of the time.
Conclusions
The eddy-covariance technique is a well-established method to measure turbulent fluxes of trace
gases such as CO2 in the surface layer. With the assumption of height-constant vertical fluxes in
this part of the boundary layer, measurements at only one height could be used to characterize the
flux at the surface. In this paper we have presented long-term measurements of the vertical CO2,
momentum, and sensible and latent heat flux above the Baltic Sea at two heights. The flux
uncertainties were estimated on the basis of the root mean square deviation between subsequent flux
estimates. The validity of the constant-flux layer assumption could be confirmed for the momentum
and the sensible heat flux: nearly 95 % of the time, the differences between the two measurements heights are smaller than the measurement uncertainty. Likewise both flux measurements are
highly correlated, with a correlation coefficient of 0.98 each. The latent heat flux, with a
correlation of 0.96 between the two heights, differs significantly 15 % of the time.
In contrast, 35 % of all CO2 flux differences are significant, i.e., larger than the
measurement error. Consequently the estimated surface flux will depend considerably on the choice of
measurement height. In general, measurements are only performed at a single and arbitrarily chosen
measurement height. Some discrepancy between various observational studies, e.g., the large
scatter between observed CO2 transfer velocity reported by , may partly be attributed
to vertical CO2 flux gradients in the surface layer. The mean difference for the year 2009 between
both heights is 0.018 mg (m2 s)-1, with a mean CO2 flux of -0.019 mg (m2 s)-1 for the lower and
-0.036 mg (m2 s)-1 for the upper height level. So, the mean difference is of the same magnitude as
the flux itself. Although this paper cannot provide an explanation for vertical CO2 flux
differences, it is worthwhile to document this effect, since it should be taken into account while
interpreting eddy-covariance CO2 flux measurements above the ocean.
A suggestion to bear in mind for future research is that one possible reason for those differences could be a gradient in the originally
constant flux layer. An other explanation could be found in the measurement uncertainty of the
instruments. It has to be checked whether the resolution of 10 Hz is enough to characterize small eddies, too, and if there are differences in the high-frequency loss at both heights. In this
case, the measurement could be used to determine a height-dependent error in the measured CO2 flux. We
have planned a subsequent paper to answer these questions.
Acknowledgements
This research was funded by the German Federal Ministry of Education and Research (BMBF)
through a grant in the context of the project SOPRAN, Surface Ocean Processes in the Anthropocene. We are grateful to
Gerhard Peters, who initiated the measurement campaign at the FINO2 platform, and the
Max Planck Institute for Meteorology, Hamburg, for providing the instruments. In particular, we thank
Hans Münster for the excellent technical support.
Edited by: G. König-Langlo
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