CARBON DIOXIDE CAPTURE BY SUBARCTIC SCOTS PINE TREES

Carbon dioxide Capture and Storage (CCS) refers to techniques with which we can get rid of the excess carbon dioxide that in the atmosphere can cause climatic warming. The ultimate storage of the excess carbon dioxide may be at the bottom of oceans, under depleted natural gas fields, or in deep saline aquifers.

Growing forests can be part of the CCS-techniques; they are the intermediate storage of carbon. While growing, the trees capture carbon dioxide molecules from the forest atmosphere, assimilate them first into sugars and finally transform them into carbon-rich wood. When the wood is burned, for example as pellets by cofiring with coal in a CCS power plant, all the resulting carbon dioxide can be captured from flue gases. And furthermore: carbon dioxide can be stored, irrespective of the carbon dioxide origin.

Carbon dioxide capture of young Scots pine (Pinus sylvestris) trees has been monitored in the Smear – studies of Varrio Subarctic Research Station, on Kotovaara hill, since 1991. The capturing starts by installing a cuvette to the top of the Scots pine tree.

24Cuvette012.jpg CO2-capturing cuvette installed in a young Scots pine. Kotovaara Smear – station in Varrio Strict Nature Reserve, Finnish Lapland.

Cuvette is a transparent, cylinder-shaped chamber which is made of plexiglas. A living Scots pine twig is installed into the cuvette through a small hole in one end. The other end is a lid with hinches, so that the cuvette can be either open (reference state) or closed (like in the photo).

In the reference state the cuvette air has the same carbon dioxide content as is the case in the outside air just above the forest stand. In spring 2006, for instance, the reference carbon dioxide content in Finnish Lapland was about 380 ppm.

In the capturing state the cuvette lid closes. The living Scots pine needles continue assimilating carbon dioxide from the cuvette air. See operation of the cuvette in a videoclip here.

In the closed state the carbon dioxide should diminish from the cuvette air. If this is the case, there is carbon dioxide sink in the needles. They have captured carbon dioxide from the atmosphere. This phenomenon is visible in the measurements of Varrio Smear station.

24CarbonDrop.jpg Drop of carbon dioxide (ppm) in the cuvette air. Example from Varrio Smear – station on 22 May 2006, in the morning at 10:35 hours.

The analyzer pumps the air from the cuvette into IRGA (Infra Red Gas Analyzer) apparatus for 60 seconds. Every tenth second the carbon dioxide content is snapshot and recorded from the cuvette air stream. In a suitable sunny and warm day the carbon dioxide content in the cuvette air drops by about 50 ppm during one minute, like in the example time point: 22.5.2006 at 10:35. The drop depicts the carbon dioxide sink which the Scots pine twig has as inbuilt.

With the cuvette method the carbon dioxide capture of Scots pines can be recorded through the whole growing season. There are considerable differences in the carbon dioxide capture, between day and night on one hand, and between spring, midsummer and autumn days on the other hand.

23SpringCapture.jpg 23SummerCapture.jpg 23AutumnCapture.jpg

Course of carbon dioxide capture over one week period during early spring (left), mid summer and late autumn. Recorded in young Scots pine forest in 2005 at Varrio Smear – station, Varrio Strict Nature Reserve, Finnish Lapland.

The carbon dioxide capture follows always the course of the daily light (sun). Highest capture is seen in the middle of the day when the sun is at its highest. In the night slight negative values are recorded. They tell that the nightly respiration surpasses the carbon dioxide capture. In the night or during the dim mornings and evenings the Scots pine emit carbon dioxide back to the atmospere. The overall daily balance, however, is almost every day positive. This means that the Scots pine forest in Kotovaara of Varrio Strict Nature Reserve, is a genuine carbon dioxide sink.

In early spring the carbon dioxide capture is slight, but clearly observable in the middle of the day. The maximum carbon dioxide capture is about 5 ppm per minute. It should be noted that at the end of April the ground in Finnish Lapland is still frozen and covered by snow of over half a meter. The stand, however, is free from snow and can receive the abundant sun radiation of the spring. There is some respiration in the night time, as small negative values are recoded.

In the midsummer week, the Scots pine trees are in full growth. There is continuous light and we can see high results for carbon dioxide capture, on those days at its highest about 30 ppm per minute. There is clear midnight drop in the capture, however, even if we see with human eye a continuous midsummer light, day and night. In some days, in the middle of the day, like on 22 June there are strong irregularities in the carbon dioxide capture. Such irregularities are caused by the occasional midday clouds which are normal at this time of the summer.

During late autumn the carbon dioxide capture is still relatively high, at its maximum about 10 ppm per minute. It is concentrated in the midday moments only. The dark nights are long, and we can see it as almost zero capture for the most of the day. Negative values are regularly observed in the night, which tells that the soil and air are still sufficiently warm for the trees to respire.

The snow cover and winter frost come to Varrio Strict Nature Reserve usually at the end of October. Thus the Scots pine trees, while capturing carbon dioxide still in the beginning of October, are suprisingly well adapted to utilize the last warmth of the growing season.

Carbon dioxide way after capturing into Scots pine trees, continues through forest operations. The trees are harvested for instance in first thinnings for energy, processed into chips, or into more storable and movable wood pellets. The carbon remains chemically locked and as stored especially in the pellets. The wood pellets are transported into large power plants and can be burned via co-firing technologies together with coal.

Carbon dioxide is captured for the second time from the flue gases of the power plant. Recent advances in these technologies are described in the IPCC report from 2005. Since then a practical application for CCS-power plant has been launched on 29 May 2006 in Germany by Vattenfall.

Combination of biological and technological means introduces thus the principle of Double Capture of Carbon Dioxide. The carbon dioxide is first captured by living trees from the atmosphere. For the second time the carbon dioxide is captured from the flue gases of a technologically advanced wood-powered co-generation plant.

In the combat against climatic warming the double capture has a fundamental advantage. If the single capture of capture of carbon dioxide from coal-powered plants stops the emissions, it can stagnate the current excess carbon dioxide flow into the atmosphere. Double capture can do more. It is a method with which we start to clean the atmosphere from the excess carbon dioxide.

References

IPCC 2005. Carbon Dioxide Capture and Storage: Special Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. ISBN 052186643X. Full download in http://www.ipcc.ch/activity/srccs/index.htm

Vattenfall 2006. Carbon dioxide free power plant.

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text by Veli Pohjonen 24.7.2006, update 11.1.2007

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