Tahkuse Observatory is located in a sparsely populated rural region, 27 km northeast of the city of Pärnu, Estonia (58o31'N 24o56'E), not far from the Soomaa National Park.

Historical survey

The first measurements of air ions were carried out in this place in the framework of AEL research projects in 1984. A special air ion spectrometer with corona ionizer was used that time. A PC controlled apparatus performed the measurements of negative and positive air ions every 2 min. Hourly average values with standard deviations were saved on magnetic tape. The measurements lasted 2 months in summer.

The mobility spectrum of small natural air ions was regularly measured by an original spectrometer from June 10, 1985 to June 2, 1986. The air intake was at a height of 5 m above ground. Information about negative and positive ion spectra was obtained every 5 min. The hourly average spectra with standard deviation were saved on magnetic tape. The accident in Chernobyl nuclear power plant fell into the same period and, in consequence of that, enhanced concentrations of air ions were recorded in May 1986.

An extended instrumentation for measurements of the air ion spectra in a wide mobility range was set up at the same place in July 1988. Besides the spectra, main meteorological parameters are recorded: wind speed, wind direction, air temperature, pressure and relative humidity.

Since 1991, an analyzer of atmospheric NO2, donated by the University of Turku, has been operating continuously. A pyranometer for the measurement of solar radiation was added in 1994, and a device M-83 for the measurement of total column ozone in 1995. Analysis of the ion composition of precipitations has been carried out since 1996. Aerosol measurements by means of the electrical aerosol spectrometer of the University of Tartu were performed episodically.

Contemporary activities

The following apparatus is operating at Tahkuse Observatory in 1998:

  1. Automated air ion spectrometer with a mobility range of 0.00032-3.2 cm2V-1s-1, designed at the University of Tartu
  2. NO2 concentration meter designed at the University of Turku
  3. Ozone meter M-83 for measurements of total column ozone
  4. Pyranometer M-80M
  5. Sensors for recording of wind speed, wind direction, air temperature, pressure, and relative humidity
  6. Devices for collection of precipitations.
Most of the apparatus is operating continuously; the data are saved as average values every 5 min.

Thus the apparatus at Tahkuse Observatory enables to monitor the mobility spectrum of air ions (charged nanometer particles) and its variations in detail and during a long period. On the ground of the mobility spectrum, it is possible to roughly derive the size spectrum of aerosol particles in a diameter range of 1-90 nm, and the ionization rate (the exposure rate). The NO2 concentration and the main meteorological parameters are measured simultaneously. There exists a need to add an SO2 meter to the complex, since SO2 is believed to be an essential factor of gas-to-particle conversion.

Hilja Iher MSc is the direct maintenance servicer of the apparatus; her work is financed by the Ministry of Environment of Estonia. She acts also as supervisor of international GLOBE program at the local school.

Selected published papers

1. Hõrrak, U., H. Iher, A. Luts, J. Salm, and H. Tammet, Mobility spectrum of air ions at Tahkuse Observatory, J. Geophys. Res., 99, 10679–10700, 1994.

2. Hõrrak, U., J. Salm, and H. Tammet, Bursts of intermediate ions in atmospheric air, J. Geophys. Res. Atmospheres, 103, 13909-13915, 1998.

3. Hõrrak, U., A. Mirme, J. Salm, E. Tamm, and H. Tammet, Air ion measurements as a source of information about atmospheric aerosols, Atmos. Res., 46, 233-242, 1998.