Summertime horizontal and vertical advective carbon dioxide fluxes measured in a dense Douglas-fir forest on a slope

The eddy-covariance (EC) method has been used for fifteen years to quantify the carbon (CO2) balance of ecosystems on an annual basis (Wofsy et al., 1993). It is normally assumed that advective CO2 fluxes, i.e. transport due to mean winds, are negligible at EC sites. The vertical turbulent flux and the canopy storage flux are therefore used exclusively to determine the net ecosystem exchange (NEE) with the atmosphere. However, EC measurement in complex terrain may invalidate this assumption (Lee, 1998; Finnigan, 1999).

An observational experiment was conducted during July and August, 2009, at the Canadian Carbon Program's British Columbia Flux Station to determine the magnitude of advective CO2 fluxes. The site is located in a 35-m tall, dense (1100 stems/ha) Douglas-fir forest on a 5-10 degree slope. Katabatic flow can occur down the slope during both the day and the night; the trunk space is occasionally decoupled from the atmosphere above during these events. Initiation of down-slope flow during the summer usually occurs at around 16:00 h and lasts for several hours.

Instruments were placed on the main flux tower and 73.5 m upslope, creating a two-dimensional transect. Wind vector measurements were made using five field-calibrated Campbell Scientific Inc. CSAT3 sonic anemometers. Four were mounted on the flux tower at heights of 1, 2.6, 8 and 20 m, and one was mounted at the upper end of the transect at the 2.6 m height. CO2 concentration measurements were made at all four heights at both ends of the transect. Eight thermostated LI-COR Inc. LI-7000 and LI-6262 infrared gas analyzers (IRGAs) were used for the CO2 measurements; they were paired using a solenoid switching system placed close to the IRGAs to create four differential analyzers (cf. Black and McNaughton, 1971). The switching system was combined with a twice-daily IRGA calibration (3:00 and 15:00 h) consisting of 6 gas concentrations (0, 360, 380, 400, 450 and 500 ppm) injected via solenoid valves attached to each sampling point. The Synflex sampling tubes were 60 metres long x 4 mm inner diameter; air was drawn through each of the 8 tubes at a rate of 2 litres per minute using a single Gast Manufacturing Inc. linear pump.

Lab and field comparisons indicated that the four CO2 differencing systems were able to measure 6-minute averaged horizontal CO2 differences (one switching cycle) with an error normally less than 0.5 ppm. Calibration of the 8 IRGAs using a second order polynomial (Burns et al., 2009) also provided an estimated 1 ppm accuracy in the vertical direction, based on lab tests. The system recorded continuous CO2 traces at all sampling points, with a 5.6% data loss due to a 10-s pressure equilibration after each solenoid valve switch.

Horizontal and vertical advective CO2 fluxes as well as canopy storage fluxes will be presented for select periods of the dataset. These fluxes will be compared to the vertical turbulent CO2 flux measured at the 42-m height on the main flux tower. Implications for determining NEE at this site using the eddy-covariance method will be discussed, including effects on both daytime and nocturnal measurements.

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Burns, S., Delany, A., Sun, J., Stephens, B., Oncley, S., Maclean, G., Semmer, S., Schroter, J. and Ruppert, J. (2009) An evaluation of calibration techniques for in situ carbon dioxide measurements using a programmable portable trace-gas measuring system. Journal of Atmospheric and Oceanic Technology. 26: 291-316.

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