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Remote Sensing of Aerosol Optical Depth in a Global Surface Network

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Remote Sensing of Aerosol Optical Depth in a Global Surface Network Research Collection Doctoral Thesis Remote Sensing of Aerosol Optical Depth in a global surface network Author(s): Wehrli, Christoph Johannes Publication Date: 2008 Permanent Link: https://doi.org/10.3929/ethz-a-005659798 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library DISS. ETH NO. 17591 Remote Sensing of Aerosol Optical Depth in a Global Surface Network A dissertation submitted to ETH ZURICH for the degree of Doctor of Sciences presented by Christoph Johannes Wehrli Dipl. Phys. ETHZ born 9. December 1952 citizen of Bischofszell (TG) and Berg (TG) accepted on the recommendation of Prof. Dr. Atsumu Ohmura, examiner Dr. Leonard A. Barrie, Prof. Dr. Stefan Brönnimann and Dr. Claus Fröhlich, co-examiners 2008 Contents ABSTRACT ......................................................................................................................................... III ZUSAMMENFASSUNG........................................................................................................................IV CHAPTER 1 INTRODUCTION ......................................................................................................1 CHAPTER 2 HISTORY OF TURBIDITY MEASUREMENTS...........................................................5 CHAPTER 3 SUN PHOTOMETRY .................................................................................................9 3.1 BASIC EQUATION FOR AOD...........................................................................................................................9 3.2 PATH LENGTH OR OPTICAL AIR MASS CALCULATIONS .............................................................................10 3.3 SOLAR ELEVATION AND REFRACTION ........................................................................................................14 3.4 OPTICAL DEPTHS............................................................................................................................................19 3.5 DIFFUSE STRAY LIGHT ...................................................................................................................................23 3.6 CLOUD SCREENING........................................................................................................................................24 CHAPTER 4 CALIBRATION OF SUN PHOTOMETERS...............................................................29 4.1 ATMOSPHERIC EXTRAPOLATION .................................................................................................................29 4.2 COMPARISON TO REFERENCE RADIOMETER..............................................................................................35 4.3 RADIOMETRIC CALIBRATION........................................................................................................................36 4.4 STRATOSPHERIC BALLOON FLIGHT .............................................................................................................41 4.5 SUMMARY OF CALIBRATION METHODS .......................................................................................................41 CHAPTER 5 DESIGN OF THE PFR INSTRUMENT ...................................................................43 5.1 WAVELENGTH SELECTION AND NUMBER OF CHANNELS.........................................................................45 5.2 INTERFERENCE FILTERS................................................................................................................................46 5.3 VIEW GEOMETRY AND CIRCUMSOLAR STRAY LIGHT.................................................................................49 5.4 POINTING MONITOR......................................................................................................................................49 5.5 DESIGN FOR RADIOMETRIC STABILITY .......................................................................................................51 5.6 DATA LOGGING..............................................................................................................................................53 5.7 SUMMARY.........................................................................................................................................................54 CHAPTER 6 GLOBAL PFR NETWORK .....................................................................................55 6.1 GLOBAL AND TEMPORAL EXTENT...............................................................................................................55 6.2 NETWORK OPERATION.................................................................................................................................57 6.3 NETWORK INTERCOMPARISONS ..................................................................................................................60 6.4 FIRST RESULTS OF THE GAW PFR NETWORK...........................................................................................63 6.5 MONTHLY CLIMATOLOGY.............................................................................................................................75 6.6 ANNUAL AND MULTI-YEAR CLIMATOLOGY................................................................................................76 CHAPTER 7 CONCLUSIONS AND OUTLOOK ..........................................................................81 APPENDIX TABLES OF MONTHLY MEAN AOD....................................................................83 REFERENCES ....................................................................................................................................85 ACKNOWLEDGEMENTS ..................................................................................................................94 CURRICULUM VITÆ .........................................................................................................................95 ii Abstract Atmospheric aerosols provide, through their direct and indirect effects, a predominantly negative or cooling contribution to the global radiation budget. Their integral optical activity is summarized in the aerosol optical depth (AOD) that can be derived from measurements of transmitted sunlight. In the early nineties, the World Meteorological Organization (WMO) deemed the quality of their long-time AOD observations insufficient for climate studies. A World Optical depth Research and Calibration Center (WORCC) was established at Davos in 1996 and given the task by WMO to develop stable instrumentation and improved methods of calibration and observation of AOD. These new developments should be demonstrated in a global pilot network. The present thesis gives an account of the research and development work performed during implementation of these tasks. After introducing the role of aerosols in climate change and the difficulties in their characterization, a personal review of the history of turbidity or AOD measurements is given. Algorithms for AOD, methods of calibration, and the construction of a Precision Filter Radiometer (PFR) are discussed. The world-wide AOD network of PFR instruments is presented. First results of AOD measurements at network stations are given as time series and climatological values. iii Zusammenfassung Atmosphärische Schwebstoffe oder Aerosole üben durch ihre direkten und indirekten Wechselwirkungen mit dem Sonnenlicht einen signifikanten, meist abkühlenden, Einfluss auf die Strahlungsbilanz der Erde aus. Die integrale optische Wirkung der Aerosole kann in ihrer optischen Dicke (AOD) zusammengefasst und aus Messungen der transmittierten Sonnenstrahlung abgeleitet werden. Die Meteorologische Weltorganisation (WMO) hatte Anfang der 90er Jahre die Qualität ihrer langjährigen AOD Messungen als für klimatologische Zwecke ungenügend beurteilt. Das 1996 begründete Kalibrier- und Forschungszentrum für optische Dicke (WORCC) in Davos erhielt von der WMO den Auftrag, stabilere Instrumente und verbesserte Kalibrations- und Messmethoden zu entwickeln und in einem weltweiten Pilotnetzwerk zu demonstrieren. Die vorliegende Dissertation gibt eine Darstellung der in Erfüllung dieses Auftrags geleisteten Entwicklungs- und Aufbauarbeit, sowie erster Resultate von AOD Messungen an den Stationen des Messnetzes. In einer Einführung wird der Einfluss der Aerosole auf den Strahlungshaushalt in Zusammenhang mit der Klimaänderung gestellt. Ein historischer Rückblick zeigt die Ent- wicklung der AOD oder Trübungsmessungen bis zur Errichtung des WORCC aus persönlicher Sicht. Die Algorithmen zur Bestimmung der AOD, Methoden der Kalibrierung und die Konstruktion des Präzisions-Filter-Radiometers (PFR) werden detailliert besprochen, und auf mögliche Fehler hin untersucht. Das weltweite Messnetz von PFR wird vorgestellt und erste Resultate in Form von Zeit- reihen und klimatologischen Werten angegeben. iv Introduction CHAPTER 1 Chapter 1 Introduction Aerosols are an important constituent part of the atmosphere not only with respect to climate change, but also to air quality including transboundary transport of air pollution, human health and mortality, which is affected by respirable particles, or aesthetic aspects like impaired visibility range. Human activities are modifying the concentration and composition of atmospheric aerosols through combustion of fossil fuels, changing land cover, biomass burning, as well as urban and industrial emissions. Growing recognition of the role of atmospheric aerosols
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