Measurements of the vertical flux of CO2 above a surface

Energy2green Wind And Solar Power System

Wind Energy DIY Guide

Get Instant Access

Atmospheric studies have the advantage of covering large areas of a heterogeneous landscape, but it is still necessary to understand the fluxes at a more detailed spatial scale to arrive at a mechanistic understanding of processes. Over the last 15 years, towers between 10 and 200 m tall have been springing up over a range of forests and other vegetation types, hosting instrumentation that directly measures the vertical exchange of carbon dioxide between the surface and the atmosphere (Fig. 4a). The most commonly used instrumentation is based on the technique of eddy covariance. This technique is based on the fact that the CO2 is transported by means of turbulent eddies, and that if both the vertical wind velocity and concentration of CO2 at a point can be measured with sufficient frequency to adequately capture these eddies, the covariance of these two measurements will correspond to the vertical flux of carbon dioxide at that point. For example, in daytime over a forest, air carried out of the forest canopy will be depleted in CO2 when compared with air sinking into the forest canopy. The greater the depletion, the larger the net carbon flux. The vertical wind velocity is usually measured with a sonic anemometer, which measures the difference in transit time between upwards and downwards pointing pulses

Fig. 4 (a) Measuring the flows of carbon in and out of a forest. A fully instrumented tower, measuring turbulent flows of carbon dioxide (photo: D. Baldocchi) (b) Watching the forest's breath: Four years of measurements from a sitka spruce plantation forest near Aberfeldy, Scotland. The horizontal axis is time of day and the vertical axis is time of year. Dark colours indicate periods of carbon uptake and light colours periods of carbon release. The maximum rates of carbon release and uptake are in early summer. The effect of varying day length on carbon uptake is clear. Overall, this plantation is taking up carbon at a rate of 7 Mg C ha- 1 year-1 (data provided by R. Clement).

Fig. 4 (a) Measuring the flows of carbon in and out of a forest. A fully instrumented tower, measuring turbulent flows of carbon dioxide (photo: D. Baldocchi) (b) Watching the forest's breath: Four years of measurements from a sitka spruce plantation forest near Aberfeldy, Scotland. The horizontal axis is time of day and the vertical axis is time of year. Dark colours indicate periods of carbon uptake and light colours periods of carbon release. The maximum rates of carbon release and uptake are in early summer. The effect of varying day length on carbon uptake is clear. Overall, this plantation is taking up carbon at a rate of 7 Mg C ha- 1 year-1 (data provided by R. Clement).

of ultrasound over a fixed distance. Both vertical wind speed and CO2 concentration measurements are usually taken at frequencies of 1 once per second or greater, and then averaged over half-hour or one-hour periods. The derived flux usually represents an average for a region between 100 m and 5 km upwind of the tower, depending on tower height and local meteorological conditions

The diurnal and seasonal variation of carbon uptake over a sitka spruce forest in Scotland is illustrated in Fig. 4(b). At night-time, respiration and microbial decomposition are the only carbon cycling processes active, and there is a steady net efflux of carbon dioxide out of the forest (light colours). In the daytime, photosynthesis dominates over respiration, and there is a net uptake of carbon by the forest (dark colours). The net carbon balance of the forest on any particular day is the difference between this nighttime loss and daytime uptake. This net balance varies with meteorological conditions, with stage of vegetation development, and with season. Such observations are powerful tools for understanding the specific mechanisms that are controlling the uptake and release of carbon at daily, seasonal and interannual timescales, particularly when combined with detailed process studies in the forest canopy or soil. It is possible to witness the daily "breath" of the forest, and understand how that breath varies with weather and season. There are now over 100 flux towers set up around the world, continuously monitoring the breath of the earth's terrestrial ecosystems. Most of these are clustered into regional networks (such as CARBO-EUROFLUX in Europe, AMERIFLUX in North America, LBA in Brazil), which cluster under the umbrella of a global flux tower network, FLUXNET (http://www-eosdis.ornl.gov/fluxnet; [Baldocchi et al. (2001)].

The net carbon fluxes estimated by the tower measurements appear too large to be consistent with other measurements of carbon uptake in biomass and soils, with model expectations, or with global expectations of the magnitude of the carbon sink.

What could be causing such a large overestimate of carbon uptake? A number of causes have been hypothesised, including a bias in study site selection towards growing forests recovering from natural disturbance, unmeasured loss of carbon in the form of dissolved organic carbon in river water, or volatile hydrocarbon emissions from leaves, but these do not seem sufficient. The most favoured explanation, from the point of view of this author at least, has to do with the nature of air movement at night. On calm nights the condition of continuous, spatially homogeneous turbulent transfer that is the basic requirement for eddy covariance measurements no longer applies. Instead, air moves in a much more complex manner, draining sluggishly along pressure and topographic gradients, occasional generating turbulence at trigger points in the landscape, or being buffeted and scooped up by occasional turbulence reaching down from the higher atmosphere. Under such spatially heterogeneous conditions, it is extremely unlikely that a measurement at a single tower is likely to capture the spatial heterogeneity of vertical carbon flow, and in fact is likely to underestimate the efflux, with the tower more likely to be located in a meteorologically benign surface region rather than a region of active flux transfer. An underestimate of night-time efflux results in an overestimate of total 24-hour carbon uptake, and therefore an overestimate of the net carbon sink.

So, whilst flux measurements can provide detailed mechanistic understanding of the processes controlling carbon uptake and release, the complex nature of night-time surface meteorology means they often fail to accurately determine the net carbon balance. Moreover, flux towers are relatively high-cost technology, and therefore are too few in number to sample the spatial variations within an ecosystem. To get this information, we need to directly measure and monitor the stocks of carbon in vegetation biomass and soils.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment