The atmospheric measurement station Lutjewad was set up in the year 2000 by the Centre for Isotope Research (CIO) of Groningen University to determine multi-year records of CO2 and other greenhouse gases and related tracers. The station is located on the northern coast of the Netherlands, at 6º 21’ E, 53º 24’ N, 1 m a.s.l. and is situated directly behind the Wadden Sea dike. On the land side (to the south) the 60 m-tower is overlooking an almost perfectly flat rural landscape. Within this agricultural area the closest small village is at two kilometers distance. On the sea side, sporadically flooded salt marshes next to the dike pass into the Waddensea with its tidal flats. It stretches about six kilometers to the north where the island Schiermonnikoog marks the transition to the North Sea. The spot on the coast was chosen in order to be able to sample maritime air with northern wind and continental air with southerly winds (figure 1). The station consists of an instrumenten tower with a laboratorium in the nearby building (figure 2)
Ambient air is taken in at 7 m, 40 m and 60 m above ground and pre-dried with a Nafion membrane dryer (Neubert et al., 2004) to prevent condensation of water vapour during transportation to the nearby laboratorium. The following meteorological measurements are made for a better characterisation of the air samples:
- Air temperature, relative humidity and wind velocity at 7, 40 and 60 m height; wind direction at 60 m; atmospheric pressure at 7 m; precipitation at the ground and radiation -incoming and outgoing shortwave and longwave- at 2 m height.
- At 50 m height, a 3d-sonic anemometer / infrared CO2 and H2O analyzer combination is running since the summer of 2006 to measure the exchange of CO2, latent heat and sensible heat. Given the large height, the measurements inform on exchange in an area of a few kilometers in upwind direction, so well extwending the common single field scale. With southern wind, the measurements inform on exchange from the agricultural area of northern Groningen and with northern wind on exchange from the salt marshes and Wadden Sea.
Lutjewad laboratory equipment
Remaining water vapour is dried from the air samples in a glass cold trap that can be cooled to -40°C, or heated to +40°C. Continuous operation is obtained using two drying systems which are used alternating: if one is cooling, the other is heated to remove the collected water.
After drying, the atmospheric samples are continuously analysed with a Picarro analyzer for mixing ratios of CO2, CO and CH4 and with an optimized Agilent HP6890 gas chromatograph for mixing ratios of N2O and SF6. The measurements are calibrated on reference cylinders that are supplied by NOAA.
Up to 20 flasks are automatic sampled, see photo below, for off-line analysis at the CIO laboratorium to extend the range of gases to O2 (from the O2/N2 ratio) and the stable isotopes: 13C and 18O in CO2. The flasks are flushed at prescribed intervals until at the CIO laboratorium a signal is given that the flask is to be removed from the flushing sequence for off-line analysis.
Combined measurements of CO2 and O2/N2 concentration are obtained with the Oxzala (photo below). As CO2 uptake and O2 release are complementary during photosynthesis and respiration but only CO2 can solve into water, the temporal change in the concentrations of CO2 and O2 informs on oceanic uptake of CO2.
The auto-sampler flasks are used to take hourly samples. All these flasks will be analyzed for all concentrations (CO2, CO, CH4, N2O and SF6) and stable isotopes and the radio-active gas 14CO2. Carbon dioxide samples are weekly or monthly absorbed in larger quantities (see photo below) using a column with sodium hydroxide (NaOH). Three columns are used: one is filled with northern wind for marine air, the second is filled with southern winds for continental air and the third is used for all wind directions to determine the long term average.
The distinction between maritime and continental air can even better be made from the mixing ratio of the radio-active gas 222Rn that is emitted at the continent from the decay of 236U while emission from the oceans and other water bodies is negigible. Another reason to measure 222Rn is that the almost constant emission from land enables the calculation of greenhouse gas emissions from the Netherlands using simultaneous changes in 222Rn and the greenhouse gas of concern (Van der Laan et al., 2009b). The method was also used for the major greenhouse gas CO2, but then the anthropogenic CO2 was first separated from natural emissions and absorptions of CO2 (mainly by vegetation) using mixing ratios of CO and 14CO2 (Van der Laan, 2010). 222Rn is detected using a two-filter dual-loop system with a flow of 80 l/min in a 1500 l delay chamber where 222Rn decay products are sampled on a filter and decay of the 222Rn daughters is detected by a photo-multiplier.
Neubert, R.E.M., L.L. Spijkervet, J.K. Schut, H.A. Been and H.A.J. Meijer, 2004: A Computer-Controlled Continuous Air Drying and Flask sampling station. J. Atmos. Ocean. Techn. 21, 651-659.
Van der Laan, S., R.E.M. Neubert, and H.A.J. Meijer, 2009a: A single gas chromatograph for atmospheric mixing ratios of CO2, CH4, N2O, SF6 and CO. Atmos. Meas. Tech. 2, 549-559.
Van der Laan, S., R.E.M. Neubert, and H.A.J. Meijer, 2009b: Methane and nitrous oxide emissions in the Netherlands: ambient measurements support the national inventories. Atmos. Chem. Phys., 9, 9369-9379.
Van der Laan, S., U. Karstens, R.E.M. Neubert. I.T., Van der Laan-Luijkx and H.A.J Meijer. 2010: Observation-based estimates of fossil fuel-derived CO2 emissions in the Netherlands using 14C, CO and 222Rn, Tellus, 62B, 389-402.
|Laatst gewijzigd:||12 april 2017 14:40|