Results of 8-year continuous measurements of aerosol profiles in the stratosphere with discussion of the importance of stratospheric aerosols to an estimate of effects on the global climate

SummaryAfter a short description of the measuring technique used the results of an efficient lidar system to determine stratospheric aerosol profiles up to 30 km altitude are reported in detail. Technical data of the lidar system: 100 megawatt trasmitter energy, 20 n sec pulse length, 1/sec pulse repetition, 694 nm wavelength, single photon counting with use of a chopper for suppressing fluorescence of the laser crystal after the shot, fully-automated control of the measuring procedure and storage of data by computer.Shown as examples are first individual profiles of the stratospheric aerosol distribution after the most violent eruption of the recent past (El Chichon 1982). These profiles reveal the rapid variations of the profile structure caused by the different flow conditions as a function of season. The results make clear that the fastest possible sequence of lidar measurements is required to obtain significant and application-oriented data.The integrated backscattering is presented from the beginning of 1977 to the end of 1984. Its variations, resulting from a whole series of volcanic events during this period, are discussed in comparison with the back-ground conditions 1977/78.The residence time of stratospheric aerosols is derived for different layers of the stratosphere with an estimate of the mean residence time of particles<0.5 μm diameter at the respective level.The dramatic variation of the stratospheric aerosol distribution in 47o northern latitude shortly before and immediately after the El Chichon eruption and continuously until summer 1984 is shown by means of three-dimensional diagrams.The different temporal trends in the behavior of the stratospheric aerosol in midlatitudes as a function of the latitude of volcanic eruptions are discussed by means of examples. Our results of measurements are compared with those of other groups. Calculations of the optical depth are shown from early 1982 to late 1984 and are compared with background conditions from 1978. The stratospheric aerosol mass in the column above the unit surface is grphically plotted as a function of time. In several passages it is pointed out that not only volcano El Chichon as the last — even though extreme volcanic event — has to be considered but also minor volcanic eruptions which took place in different countries in late summer and fall 1983.Special sections give an overview of the available literature concerning the question of an impact on climate, especially on the northern hemispheric temperature by stratospheric turbidity, and conclusions are drawn as to the practical application of our measurement results.Based on reliable data from the literature, a volcanically-induced cooling of the order of about 0.5 K is to be expected from the turn of the year 1984/85 on, followed by a gradual normalization (provided: no new major volcanic eruptions).ZusammenfassungNach einer kurzen Beschreibung der angewandten Meßtechnik wird im Detail über die Ergebnisse der Anwendung eines leistungsfähigen Lidar-Systems (100 Megawatt abgegebene Energie, 20 n sec Pulsdauer, Pulsfrequenz 1/sec, Wellenlänge 694 nm, Einzel-Photonenzählung mit Anwendung eines Choppers zur Unterdrückung der Fluoreszenz des Laser-Kristalls nach dem Schuß, vollautomatische Steuerung des Meßvorganges und Speicherung der Daten mittels Computer) zur Erfassung der stratosphärischen Aerosol-Profile bis 35 km NN berichtet.Als Meßbeispiele werden zunächst individuelle Profile der stratosphärischen Aerosolverteilung nach dem stärksten Vulkanausbruch der jüngsten Zeit (El Chichon 1982) gezeigt. Diese Profile lassen die raschen Variationen der Profilstruktur als Folge der unterschiedlichen stratosphärischen Strömungsbedingungen in Abhängigkeit von der Jahreszeit erkennen. Die Ergebnisse machen deutlich, daß eine möglichst dichte zeitliche Folge von Lidar-Messungen notwendig ist, um signifikante und für die Anwendung relevante Daten zu erhalten.Anschließend wird die integrierte Rückstreuung von Beginn 1977 bis Ende 1984 dargestellt und deren Variationen als Folge einer ganzen Reihe von Vulkanereignissen in dieser Zeit — verglichen mit dem Background 1977/78 — diskutiert. Schließlich erfolgt eine Ableitung der Verweildauer der Aerosole in verschiedenen Schichten der Stratosphäre mit einer Abschätzung der mittleren Verweildauer von Partikeln<5 μm ϕ in den jeweiligen Stockwerken.Der dramatische Ablauf der Feinstruktur der stratosphärischen Aerosolverteilung in 47o nördlicher Breite kurz vor, unmittelbar nach der Eruption des El Chichon sowie durchgehend bis zum Sommer 1984, wird in dreidimensionalen Diagrammen dargestellt. Die unterschiedlichen Zeitabläufe des Verhaltens des stratosphärischen Aerosols in mittleren Breiten in Abhängigkeit von der Breite der Vulkaneruption werden anhand typischer Beispiele diskutiert. Vergleiche mit den Meßergebnissen anderer Gruppen werden gezogen. Die Ergebnisse einer Berechnung der optischen Dicke werden von Anfang 1982 bis Ende 1984 dargestellt und mit dem Background 1978 verglichen. Schließlich wird die stratosphärische Aerosolmasse in der Säule über der Einheitsfläche als Funktion der Zeit graphisch dargestellt. Es wird an verschiedenen Stellen darauf hingewiesen, daß nicht bloß El Chicon als letztes — wenn auch extremes Vulkanereignis — in Betracht zu ziehen ist, sondern auch kleinere Vulkaneruptionen, welche sich in verschiedenen Ländern im Spätsommer und Herbst 1983 ereignet haben.In gesonderten Abschnitten wird eine Übersicht über die vorliegende Literatur zur Frage der Beeinflussung des Klimas, insbesondere der nordhemisphärischen Temperatur durch stratosphärische Verstaubungen dargestellt und Schlüsse in bezug auf die praktische Anwendung unserer Meßergebnisse gezogen.Mit einer nordhemisphärischen, vulkanisch bedingten Abkühlung in der unteren Troposphäre in der Größenordnung von im Mittel 0.5 K ab Jahreswechsel 1984/85, gefolgt von einer allmählichen Normalisierung (keine neuen starken Vulkaneruptionen vorausgesetzt) ist, gestützt auf verläßliche Daten aus der Literatur, zu rechnen.

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