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Uncertainty of eddy covariance flux measurements over an urban area based on two towers

Abstract. The eddy covariance (EC) technique is the most direct method for measuring the
exchange between the surface and the atmosphere in different ecosystems.
Thus, it is commonly used to get information on air pollutant and greenhouse
gas emissions, and on turbulent heat transfer. Typically an ecosystem is
monitored by only one single EC measurement station at a time, making the
ecosystem-level flux values subject to random and systematic uncertainties.
Furthermore, in urban ecosystems we often have no choice but to conduct the
single-point measurements in non-ideal locations such as close to buildings
and/or in the roughness sublayer, bringing further complications to data
analysis and flux estimations. In order to tackle the question of how
representative a single EC measurement point in an urban area can be, two
identical EC systems – measuring momentum, sensible and latent heat, and
carbon dioxide fluxes – were installed on each side of the same building
structure in central Helsinki, Finland, during July 2013–September 2015. The
main interests were to understand the sensitivity of the vertical
fluxes on the single measurement point and to estimate the systematic
uncertainty in annual cumulative values due to missing data if certain,
relatively wide, flow-distorted wind sectors are disregarded. The momentum and measured scalar fluxes respond very differently to the
distortion caused by the building structure. The momentum flux is the most
sensitive to the measurement location, whereas scalar fluxes are less
impacted. The flow distortion areas of the two EC systems (40–150 and
230–340∘) are best detected from the mean-wind-normalised turbulent
kinetic energy, and outside these areas the median relative random uncertainties of
the studied fluxes measured by one system are between 12 % and 28 %. Different
gap-filling methods with which to yield annual cumulative fluxes show how using data
from a single EC measurement point can cause up to a 12 %
(480 g C m−2) underestimation in the cumulative carbon fluxes as
compared to combined data from the two systems. Combining the data from two
EC systems also increases the fraction of usable half-hourly carbon fluxes
from 45 % to 69 % at the annual level. For sensible and latent heat,
the respective underestimations are up to 5 % and 8 % (0.094 and
0.069 TJ m−2). The obtained random and systematic uncertainties are in
the same range as observed in vegetated ecosystems. We also show how the
commonly used data flagging criteria in natural ecosystems, kurtosis and
skewness, are not necessarily suitable for filtering out data in a densely built
urban environment. The results show how the single measurement system can be
used to derive representative flux values for central Helsinki, but the
addition of second system to other side of the building structure decreases
the systematic uncertainties. Comparable results can be expected in similarly
dense city locations where no large directional deviations in the source area
are seen. In general, the obtained results will aid the scientific community
by providing information about the sensitivity of EC measurements and their
quality flagging in urban areas.
