Gas concentrations
Sampling of gases is done on the tall tower at heights of 4.2, 8.4, 16.8, 33.6, 50.4, 67.2, 101 and 125 m from the mast base (actual heights from the surrounding terrain are 1-1.5 m higher).
The studied gases are
- CO2
- CO
- H2O
- NOx
- O3
- SO2.
One of the mandatory continuous measurements for an ICOS atmospheric site is high precision measurements of CH4, CO2 and CO at a tall tower. At SMEAR II these gases are measured with Picarro G2401. The instrument measures also H2O and it is sampling from three levels: 16.8 m, 67.2 m and 125 m. Slightly longer description of the measurements is given and the data is described here.
Several species of biogenic and other volatile organic compounds (VOC) are measured for approximately 1.5 h in three-hour cycles with proton transfer reaction mass spectrometer connected to the sample lines.
Exmples of the measured species (protonated mass & most common compounds):
- 33 methanol
- 45 acetaldehyde
- 47 ethanol, formic acid
- 59 acetone
- 69 isoprene, methylbutenol fragment
- 71 methacrolein, methyl vinyl ketone
- 73 methyl ethyl ketone
- 79 benzene
- 87 methylbutenol
- 93 toluene
- 99 hexenal
- 101 cis-3-hexenol, hexanal
- 103 hexanol
- 137 monoterpenes
- 205 sesquiterpenes
Gas concentrations at the sample levels on the tower are measured in a sequence. Each level is measured for 1 min 15 s which means that the total cycle length is 10 min. Sample lines are 157 m long, 14 mm internal diameter PTFE tubes. A continuous by-pass flow system is used to transport the air samples through the sample lines into the measurement cottage. The by-pass flow rate is 43 l/min and the residence time in the sample lines is 35 s. The intakes, tubes, and water separators are heated to keep their temperature above the ambient. Inside the measurement cottage the sample lines are connected to 4 mm internal diameter PTFE tubes at manifolds made of PTFE, and directed to six magnetic valves. The sample flow rate in these lines is 6.0 l/min and discontinuous.