| Eddy covariance (EC) is state-of-the-art in directly and continuously measuring
turbulent fluxes of carbon dioxide and water vapor. However, low signal-to-noise ratios,
high flow rates and missing or complex gas analyzers limit it’s application to few
scalars.
True eddy accumulation, based on conditional sampling ideas by Desjardins in 1972,
requires no fast response analyzers and is therefore potentially applicable to a wider range of
scalars. Recently we showed possibly the first successful implementation of True Eddy
Accumulation (TEA) measuring net ecosystem exchange of carbon dioxide of a
grassland. However, most accumulation systems share the complexity of having to
store discrete air samples in physical containers representing entire flux averaging
intervals.
The current study investigates merging principles of eddy accumulation and eddy
covariance, which we here refer to as “true eddy accumulation in transient mode” (TEA-TM).
This direct flux method TEA-TM combines true eddy accumulation with continuous
sampling. The TEA-TM setup is simpler than discrete accumulation methods while
avoiding the need for fast response gas analyzers and high flow rates required for
EC.
We implemented the proposed TEA-TM method and measured fluxes of carbon dioxide
(CO2), methane (CH4) and water vapor (H2O) above a mixed beech forest at the Hainich
Fluxnet and ICOS site, Germany, using a G2301 laser spectrometer (Picarro Inc., USA). We
further simulated a TEA-TM sampling system using measured high frequency CO2 time
series from an open-path gas analyzer. We operated TEA-TM side-by-side with open-,
enclosed- and closed-path EC flux systems for CO2, H2O and CH4 (LI-7500, LI-7200,
LI-6262, LI-7700, Licor, USA, and FGGA LGR, USA).
First results show that TEA-TM CO2 fluxes were similar to EC fluxes. Remaining
differences were similar to those between the three eddy covariance setups (open-,
enclosed- and closed-path gas analyzers). Measured TEA-TM CO2 fluxes from our
physical sampling system closely reproduced dynamics of simulated TEA-TM
fluxes.
In conclusion this study introduces a new approach to trace gas flux measurements using
transient-mode true eddy accumulation. First TEA-TM CO2 fluxes compared favorably with
side-by-side EC fluxes, in agreement with our previous experiments comparing discrete TEA
to EC. True eddy accumulation has thus potential for measuring turbulent fluxes of a range of
atmospheric tracers using slow response analyzers. |