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Lidar Techniques for Environmental Monitoring

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Lidar Techniques for Environmental Monitoring Lidar Techniques for Environmental Monitoring Rasmus Gr¨onlund Doctoral Thesis 2007 Lidar Techniques for Environmental Monitoring © 2007 Rasmus Gr¨onlund All rights reserved Printed in Sweden by Media-Tryck, Lund, 2007 Atomic Physics Division Department of Physics Faculty of Engineering, LTH Lund University P.O. Box 118 SE–221 00 Lund Sweden http://www.atom.fysik.lth.se ISSN 0281-2762 Lund Reports on Atomic Physics, LRAP-384 ISBN: 978-91-628-7251-9 To Friends Abstract Atoms and molecules absorb and emit light at certain, well- defined wavelengths, and this forms the basis of spectroscopy. The wavelengths are given by the discrete energy levels of the atoms/molecules. When illuminated by light of a suitable wave- length, the light will be absorbed, making it possible to deduce information on the presence and concentration of various elements and compounds. When atoms or molecules have been excited, they may sponta- neously relax to their ground state, emitting species-specific light, defined by the energy levels. This concept can be used to induce fluorescence in the material, which again can be used to analyse the properties of the sample. Light detection and ranging techniques can be used to perform remote spectroscopic measurements. The sample is probed with a laser beam and the resulting light can be collected with a telescope of sufficient size. Useful results can be deduced at ranges as large as several kilometres, although, in this work the range was never more than a kilometre. In the work presented in this thesis, differential absorption lidar measurements have been performed to determine mercury fluxes from pollution sources. These measurements have been performed as field campaigns in different parts of Europe. Some measure- ments have been made at chlor-alkali factories, and one campaign at an abandoned mercury mine. Moreover, fluorescence lidar techniques have been used for re- mote investigation of fa¸cades of historical buildings and electrical insulators. These measurements have been performed both in a laboratory setting (albeit still remotely through an ordinary at- mosphere) and as field campaigns. Biological contamination and surface treatment chemicals can be monitored, and types of stone in fa¸cades can be characterized. Finally, the remote laser-induced breakdown spectroscopy tech- nique has been extended to make imaging possible. These mea- surements can be used to characterize materials in surface layers, which may be useful in cultural heritage monitoring. v Popularvetenskaplig¨ Sammanfattning Ljus kan beskrivas som en elektromagnetisk v˚agoch har d˚aen viss v˚agl¨angd. F¨argen p˚aljuset f¨or¨andras beroende p˚av˚agl¨angden. Av det synliga ljuset har violett ljus den kortaste v˚agl¨angden och r¨ott ljus den l¨angsta.Det finns ¨aven elektromagnetisk str˚alning utanf¨or det synliga omr˚adet,fr˚anR¨ontgenstr˚alning till radiov˚agor. Det vi kallar det optiska omr˚adetstr¨acker sig fr˚anultraviolett str˚alning, med kortare v˚agl¨angd ¨anviolett ljus, till infrar¨odstr˚alning, med l¨angre v˚agl¨angd¨anr¨ottljus (¨aven kallat v¨armestr˚alning). Om man h˚aller upp en r¨odglasbit mot en lampa, ser man ett r¨ottljus. Detta beror p˚aatt glasbiten har absorberat det ljus som inte ¨arr¨ottoch sl¨apper bara igenom det r¨odaljuset. P˚a samma s¨attabsorberar atomer och molekyler i v˚aromv¨arld ljus med vissa specifika v˚agl¨angder. Olika ¨amnen absorberar ljus vid olika v˚agl¨angder, de har ett specifikt absorptionsspektrum. N¨aren atom har absorberat ljus blir den exciterad, den har mer energi ¨ann¨odv¨andigt. Alla atomer str¨avar efter att ha s˚al˚ag energi som m¨ojligt, s˚ade kommer spontant att falla tillbaka till sitt grundtillst˚and.Detta kan de g¨ora,t.ex. genom att f¨orlorasin energi till en annan atom i en kollision eller genom att s¨anda ut sin extra energi i form av ljus. Detta ljus kan, p˚asamma s¨attsom vid absorptionen, bara ha vissa v˚agl¨angder. Atomer har allts˚aocks˚a ett emissionsspektrum, som best˚arav karakteristiska v˚agl¨angder. Studien av dessa spektra kallas spektroskopi. Denna avhandling handlar om den spektroskopiska metoden li- dar (som ¨aren f¨orkortning av det engelska uttrycket “light detec- tion and ranging”), ¨aven kallat laser-radar. N¨aren kort laserpuls skickas ut i atmosf¨arenkommer ljuset att spridas n¨ardet tr¨affar molekyler och partiklar i luften. En liten del av ljuset sprids i bak˚atriktningen och kan samlas upp av ett teleskop och detek- teras. Eftersom det uts¨anda ljuset ¨aren kort laserpuls kan man veta hur l˚angt bort ljuset varit genom att m¨atahur l˚ang tid det tar innan det ˚aterv¨ander. vii Popul¨arvetenskaplig Sammanfattning Anta att man skickar ut en laserpuls vid en specifik v˚agl¨angd d¨ar ett visst ¨amne absorberar. Ljuset kommer d˚aatt ab- sorberas om ¨amnetp˚atr¨affas. Om man simultant m¨aterp˚aen n¨arliggande v˚agl¨angd, som dock inte absorberas, ser man en skill- nad i det bak˚atspridda ljuset, som enbart beror p˚aatt den ena v˚agl¨angden absorberats. Med denna metod, som kallas differen- tiell absorptions-lidar kan man f˚aen profil av koncentrationen av det efters¨okta¨amnetl¨angsmed laserns riktning. Genom att svepa laserriktningen ¨over ett omr˚adekan man skapa en koncentra- tionskarta. Detta kan kombineras med m¨atningar av vindhastig- heten i omr˚adet f¨oratt m¨atafl¨odetav en f¨ororening fr˚anen ut- spridd k¨alla. I detta arbete har differentiell absorptions-lidar anv¨ants f¨or att m¨atakvicksilverutsl¨appfr˚anolika typer av k¨allor. B˚ade klor- alkali-fabriker p˚atre olika platser i Europa och en nedlagd kvick- silvergruva har unders¨okts. M¨atningarna p˚aklor-alkali-fabriker har gjorts inom ett st¨orreEU-projekt d¨areffekterna av kvicksil- verutsl¨app p˚am¨anniskor boende n¨aradessa fabriker unders¨oktes. De fl¨odesm¨atningar som gjordes var viktiga indata till sprid- ningsmodeller och beh¨ovdesf¨oratt f˚aett m˚attp˚ahur mycket kvicksilver som sl¨apptes ut. M¨atningar av kvicksilverinneh˚allet i gr¨onsaker och fisk i omr˚adena,samt epidemiologiska m¨atningar av arbetare och m¨anniskor som bodde i n¨arhetenvar ocks˚adelar av projektet. Om man vill unders¨oka fasta material kan en annan metod till¨ampas. N¨arett objekt belyses med ett ultraviolett ljus exci- teras molekyler i objektet. N¨arde ˚aterfaller till sitt grundtillst˚and s¨anderde ut fluorescensljus. Det ¨ardenna effekt som g¨oratt en vit skjorta p˚aett diskotek lyser bl˚aaktigt.Samma effekt anv¨ands f¨or att f¨orsv˚araf¨orfalskning av t.ex. sedlar och kreditkort. N¨ardessa belyses med ultraviolett ljus ses en fluorescensbild som ¨arosynlig i vanlig belysning. Genom att belysa objekt med en kraftig ultraviolett laserpuls kan man inducera fluorescens i objektet. Fluorescens-str˚alningen kan vara olika stark i olika f¨arger,beroende p˚avilka molekyler objektet inneh˚aller. Genom att analysera fluorescens-spektret kan man d˚aidentifiera materialet. Denna teknik kallas laser-inducerad fluorescens, eller, n¨arden anv¨ands p˚aavst˚and,fluorescens-lidar. Metoden har i detta arbete anv¨ants f¨oratt studera historiska bygg- nader samt elektriska isolatorer. Det finns ett behov av att unders¨oka historiska byggnader, b˚ade f¨oratt monitorera om ˚atg¨arder beh¨over vidtagas samt f¨oratt iden- tifiera material och dokumentera byggnaden. Med fluorescens- lidar kan man, genom att svepa laserstr˚alen ¨over byggnaden och m¨atafluorescens-spektret i varje punkt, identifiera biologisk p˚av¨axt,omr˚aden d¨arytbehandlingskemikalier anv¨ants samt olika typer av material. I vissa fall kan man ¨aven se skillnader mellan samma typ av material, vilket t.ex. kan bero p˚a˚alder,skador eller viii Popul¨arvetenskaplig Sammanfattning f¨orslitningar. Elektriska isolatorer p˚akraftledningar ¨arute i v¨ader och vind och om de blir p˚av¨axtamed alger kan dessa binda vatten vilket kan orsaka elektriskt genomslag. Det finns d¨arf¨orett behov att kon- trollera s˚aatt detta inte intr¨affar, vilket ¨arm¨ojligt med fluorescens- lidar. Om laserpulsen som s¨ands mot ett objekt ¨artillr¨ackligt inten- siv och fokuseras ner till en liten punkt kan man skapa en liten explosion. Detta beror p˚aatt man tillf¨ors˚amycket energi att man bryter s¨onder materialet och det skapas en liten gnista, in- neh˚allande fria atomer och joner fr˚anytan. Eftersom man har tillf¨ort mycket energi kommer dessa atomer och joner att bli excite- rade. N¨aratomerna faller tillbaka till sitt grundtillst˚ands¨ander de ut specifika v˚agl¨angder och d¨armedkan man analysera vilket ¨amne som fanns i provet. Metoden kallas “laser-induced breakdown- spectroscopy”, eller gnist-spektroskopi. Genom att sedan svepa laserstr˚alen¨over ytan kan man skapa en karta ¨over de ¨amnensom finns i omr˚adet. I detta arbete har allts˚atre olika typer av spektroskopi p˚a avst˚andutf¨orts; dels m¨atningar av luftf¨ororeningar, med differen- tiell absorptions-lidar, dels m¨atningar p˚abyggnader och isolatorer med fluorescens-lidar och dels laser-inducerad gnist-spektroskopi. ix List of Publications This thesis is based on the following papers, which will be referred to by their Roman numerals in the text. I Elemental Mercury Emissions from Chlor-Alkali Plants Measured by Lidar Techniques R. Gr¨onlund, M. Sj¨oholm,P. Weibring, H. Edner, and S. Svanberg. Atmospheric Environment 39, 7474–7480 (2005). II Mercury Emissions from the Idrija Mercury Mine Measured by Differential Absorption Lidar Techniques and a Point Monitoring Absorption Spectrometer R. Gr¨onlund, H. Edner, S. Svanberg, J. Kotnik, and M. Horvat. Atmospheric Environment 39, 4067–4074 (2005). III Joint Application of Doppler Lidar and Differential Absorption Lidar to Estimate the Atomic Mercury Flux from a Chlor-Alkali Plant M.
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