The physiological potential for acquisition of atmospheric ammonia (NH $ ) was investigated in three European meadow grasses (Arrhenatherum elatius, Bromus erectus and Brachypodium pinnatum) competing in chalk grasslands. Experiments were carried out with plants cultivated for about three months on a soil-sand mixture at high root nitrogen supply or in nutrient solutions at low root nitrogen supply. Two different root nitrogen regimes were applied to the solution-grown plants : 130 µmol NO $ plant −" wk −" (approx. 50 kg nitrogen ha −" in three months) ; or 130 µmol NO $ − plus 130 µmol NH % + plant −" wk −" . Each regime was combined with two levels of NH $ fumigation (0 and 70 nmol mol −" air for 24 d). Uptake of gaseous NH $ in the shoots was investigated under controlled environmental conditions including NH $ concentrations ranging from 0 to 30 nmol mol −" air. Concurrently, photosynthesis, glutamine synthetase activity, nitrogen allocation, biomass allocation and apoplastic cation composition were measured. For A. elatius, the influence of photorespiration on NH $ acquisition was also assessed.Independently of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were 0.5 nmol mol −" . The total leaf conductance to NH $ absorption remained constant at increasing NH $ concentrations, showing that the capacity for assimilation was unaltered. Whereas internal factors in the leaves did not cause differences in the potential for NH $ acquisition between the species, other factors of NH $ acquisition were quite different : B. erectus had higher stomatal conductance and, thus, higher NH $ uptake rates per unit leaf area compared to A. elatius and B. pinnatum ; higher stomatal conductances of B. erectus were to a large extent offset by a lower leaf area per plant, resulting from a lower growth rate and thicker leaves than in the two other species. The rate of photorespiration in Arrhenatherum constituted at least 15% of the net photosynthetic rate. Surprisingly, suppression of photorespiration indicated that NH $ uptake was supported by photorespiration. Bromus responded to fumigation with 70 nmol NH $ mol −" air for 24 d by lowering the root : shoot ratio and increasing the nitrogen concentration in the stem dry matter. The total leaf conductance to NH $ uptake decreased in all three species upon exposure to NH $ , while the stomatal conductance was unaffected. The NH $ exposure caused lower apoplastic concentrations of H + , Mg# + and Ca# + in A. elatius and B. erectus.
During winter 1982/83 and in early 1985, abnormal ozone deficiencies in the stratosphere were recorded at northern midlatitude stations. Ozone variations measured by ozonesondes at Hohenpeissenberg are analysed with respect to dynamic transport variations related to the quasibiennial oscillation (QBO). Atter the elimination of seasonal fluctuations, long-term trends and average QBO related variations, ozone time series are compared to particle surface area densities derived from lidar measurements of the stratospheric aerosol layer at Garmisch-Partenkirchen. The two stations are only 37 km apart. The analysis reveals that the 1985 ozone minimum is a transport effect, as well as most of the 1982/83 minimum. But at 17 to 20 km in 1982/83, 30 to 40% of the ozone deficiency is most probably related to the aerosol perturbation of the lower stratosphere following the April 1982 eruption of the Mexican volcano El Chich6n.
The balance of the tropospheric ozone is investigated consid-•' ering the ozone sources with emphasis on tropospheric pollutants and stratospheric-tropospheric exch~nge processes. An observation during the course of our solar-terrestrial
With an accurate scanning backscatter lidar cross sections of contrails are repetitively measured .Horizontal and vertical growth and effective diffusion coefficients are determined. The lidar ratio and optical depth are calculated by means of shadow calibration. . INTRODUCTIONThe analysis of satellite data reveals an additional cloud cover due to contrails of 0.4% over mid-Europe' and up to 2% over the eastern part of the Atlantic Ocean2. The contrails consist of ice crystals and form from the additional water vapour emitted by high flying aircrafts. Their persistence (up to some hours) depends on the ambient temperature and humidity3'4. To evaluate the impact of contrails on the earth's radiation budget, their extension and optical parameters must be known. In this paper we briefly describe the scanning lidar system dedicated to the measurement of contrails and present the results of the contrail measurements analysed yet with respect to the temporal and spatial evolution and the optical depth. THE SCANNING LIDAR SYSTEMThe transmitter is a Q-switched Nd:YAG laser with a pulse energy of 0.55 J @ 532 urn, a pulse width < 7 ns. The pulse repetition rate is 10 Hz, and the beam divergence 0.5 mrad. The receiver is a 52 cm diameter Cassegrain telescope with 0.5 to 6 mrad FOV. The detectors are THORN EMI 9128A photomultipliers with modified voltage dividers and range-gate circuits. Depolarisation profiles are taken at 532 nm. Data acquisition is done by a 50 MS/s 4-channel transient recorder from Kontron/Germany with 1 MB record length/channel, yielding a best height resolution of 3 m. Data are transferred by IEEE-bus to the system PC. The system PC controls on-line scanning by the two-axes scanning mount under multiple tasking WINDOWS. With each scanning step a video picture of the contrail is saved on the PC hard disk by a frame-grabber to support the analysis of the measurements. The repeatability of the scanning-mount pointing is much better than the laser beam divergence ofO.5 mrad. Its max. turning velocity is 200/5, and the scanning ranges are from horizontal to vertical and 90° around the vertical axis. The whole system including supporting equipment is accommodated in a 20 feet standard container. Details of the system are described in Freudenthaler et.al.5. . MEASUREMENTS AND RESULTSContrail measurements are done regularly in Garmisch-Partenkirchen, which is located below several international flight routes crossing in various directions. With a computer-controlled scanning routine the contrails can be followed while they drift with the wind, and several vertical cross-section measurements can be performed as they develop with time. The youngest contrail measured was about 15 s old, the oldest about one hour. As an example for a repeated scan Fig. 1 shows four contour plots of the backscatter signal of the same contrail 1,13 mm, 6.2 mm, 10.9 mm, and 15.5 mm after the passage of the aircraft. The plots are reconstructed from 15 lidar profiles per scan, whose paths are shown as straight lines in the contour pl...
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