Opening of a Lidar Observatory at Otlica above Ajdovščina

In June 2005 the Centre for Atmospheric Research at Otlica above Ajdovščina opened a lidar observatory, which contains a lidar for detecting aerosols, developed by the Laboratory for Astroparticle Physics. An upgrade of the existing aerosol lidar to the Raman version is planned; this will, in addition to the measurements of aerosols, also enable the assessment of the vertical profiles of water vapour and nitrogen content in the atmosphere. The development of a mobile lidar for detecting aerosols is underway. With the simultaneous use of meteorological models, the mobile lidar will enable the study of the spreading of pollution in the troposphere.

The lidar at Otlica enables the detection of aerosols in the atmosphere and the assessment of their impact on the optical properties of the atmosphere. Its transmitter, a Nd:YAG laser with a basic spectral line multiplied three times, transmits pulses of ultraviolet light into the atmosphere at a wavelength of 355 nm, which last 5 ns and have the energy of 120 mJ. The more aerosols the atmosphere contains, the higher is the portion of transmitted light scattered on them. Because aerosols scatter light in all directions, a part of that light returns back to the lidar receiver. The lidar receiver, which is represented by a parabolic mirror with a focal distance of 41 cm and an 80 cm diameter with 1.5 m2 of active surface, collects the received light and directs it to the sensitive device, the photomultiplier. The latter detects the light, enhances it, and converts it to a measurable electric signal. The electric signal is then converted into data on the strength of the scattered light. From the time delay between the transmission of light and the reception of its scattered part, the distance of the layer of aerosols from our location is calculated; from the strength of the received light, the thickness of that layer is calculated. A 40 MHz analogue/digital converter provides the digitalisation. The digitiser is connected to a computer, which stores and processes the data, via a data bus. The laser, parabolic mirrors, and the photomultipliers are attached onto a metal construction, which enables the movement of the entire system in any direction within the field of vision. The lidar at Otlica enables two-dimensional measurements of the optical properties of the atmosphere and the presence of aerosols in the atmosphere.

Since aerosols often represent indicators of pollution in the atmosphere, the lidar measurements at Otlica show the presence of pollutants in the air over a broad area of the Trnovski gozd plateau, the Vipava Valley, the Kras and the coastal region, up to an altitude of several 10 km above the surface. With the use of meteorological models we can assess from where the pollution, detected by the lidar, came to our territory. The simultaneous use of lidar measurements and meteorological models provides the study of the typical paths of aerosols, or the answer to the question how many air currents and from where do they most often carry the pollution to the area of Slovenia. Such an example is the pollution above the highly industrialised Padan Plain, which is carried over to the territory of Slovenia by the air currents of the SW winds; the first larger natural obstacle it encounters is the Trnovski gozd plateau. Everyone is also familiar with the phenomenon of Sahara sand, which is lifted into the atmosphere by desert storms and travels several thousand kilometers, before reaching our area.