Examination of CO2 transport processes in a mountainous cool temperate deciduous forest in central Japan using atmospheric 222Rn measurement
Shohei Murayama1, Hiroaki Kondo1, Nobuko Saigusa2, Akira Wada3, Kentaro Ishijima4, Hidekazu Matsueda5, Yousuke Sawa5
1 Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST)
2 National Institute for Environmental Studies
3 Meteorological College
4 Frontier Research Center for Global Change, JAMSTEC
5 Meteorological Research Institute
2 National Institute for Environmental Studies
3 Meteorological College
4 Frontier Research Center for Global Change, JAMSTEC
5 Meteorological Research Institute
P17 in Poster presentations
In order to examine CO2 transport processes over a complex terrain, measurements of atmospheric 222Rn, a natural radioactive noble gas emitted from soil with a half life of 3.82 days, were made at Takayama, a deciduous forest site (TKY; 36.15°N, 137.42°E, 1420 m a.s.l.) in central Japan; this is one of the longest flux monitoring sites in the world. Continuous measurements of vertical CO2 profiles, as well as of CO2 fluxes and meteorological parameters, were made at two towers, one on the ridge and another one on the slope; the distance between the two towers was about 100 m. 222Rn measurements were made at the towers using an electrostatic collection method.
In the growing season, prominent CO2 diurnal cycles with a maximum during the nighttime and a minimum in the early afternoon were observed, consistent with the biological activities, while 222Rn showed a complicated diurnal variation. However, the following relationships between 222Rn and topographical winds were found:
(1) With upslope wind, 222Rn at the ridge tower tended to increase due likely to an accumulation of 222Rn emitted from the soil in the airmass flowing upward along the slope.
(2) With downslope wind, 222Rn at the slope tower tended to increase due likely to an accumulation of 222Rn emitted from the soil in the airmass flowing downward along the slope. This increase in 222Rn occurred only near the ground surface. During the periods when higher 222Rn values were observed at the slope tower, the CO2 concentrations were also observed to be higher at the slope tower than at the ridge tower, indicating a downslope transport of respiratory CO2 emitted near the surface.
Using the observed 222Rn and wind data and the 222Rn flux from soil estimated from an empirical equation at TKY, each component of the 222Rn budget (i.e., the eddy vertical transport, the advective transport and the storage) in the canopy layer was estimated for the nights when the downslope winds were observed, followed by a calculation of the effective eddy diffusivity (K). Using the CO2 concentration, wind data and the obtained K, each component of the CO2 budget in the layer was also estimated. For the 3 nights during the intensive campaign in October 2008, 222Rn transported vertically upward and downward parallel to the slope were estimated to be, on average, about two thirds and one third of the 222Rn emitted from the soil, respectively. With the assumption that the CO2 emitted from the soil is transported vertically and in parallel to the slope in the same ratio as the observed 222Rn emitted from the soil, and combined with the soil respiration estimated using an empirical equation for TKY, we calculated the vertical and ground-parallel transport components of the respiratory CO2 emitted above ground. As a result, we found that about one third and two thirds of CO2 emitted above ground were transported vertically and in parallel to the slope, respectively.
Full presentation file:
fp_P17 (07.12.2009 13:54)