BACHELOR THESIS 15 CREDITS DEPARTMENT OF GEOPHYSICS New concept for monitoring SO2 emissions from Tavurvur volcano in Papua New Guinea Author: Supervisor: Julia WALLIUS Prof. Olafur GUDMUNDSSON A thesis submitted in fulfillment of the requirements for the degree of Bachelor in Physics December 22, 2017 Sammanfattning Nytt koncept för att övervaka SO2 utsläpp från vulkanen Tavurvur i Papua Nya Guinea Julia Wallius Under en period på 29 dagar under oktober månad 2016 kördes ett dubbelstrålat DOAS instrument med brett synfält på ön Matupit i Papua Nya Guinea. Detta för att mäta svaveldioxidutsläppen från vulkanen Tavurvur, som ligger på Gazelle halvön i provinsen East New Britain. På grund av otillförlitliga inställningar under de första 17 dagarna så utgick de slutgiltliga resultaten från mätningar tagna mellan den 19:e och 30:e oktober med ett två dagars uppehåll under den 25:e och 26:e oktober då instru- mentet användes för att istället erhålla vindhastighetdata. Det genomsnittliga flödet av kg SO2 bestämmdes för denna period till att ligga på 0.27 s med en vindhastighet på m 3.9 s . Användandet av ett dubbelstrålat DOAS instrument med brett synfält rekom- menderas vid Tavurvur när gasutsläppen är låga och vindriktningen är övervägande sydvästlig. En vinkel på 13 grader över horisonten användes för att erhålla refer- ensspektra av himlen för varje skanningsuppsättning men på grund av SO2 spår i dessa spektra så anses denna vinkel vara för liten och att höja denna mellan 5-10 grader för referensspektra rekommenderas för framtida mätningar. Nyckelord: Optisk fjärranalys, DOAS, Vulkanövervakning, SO2-utsläpp Examensarbete C i geofysik, 1GE037, 15 hp, 2017 Handledare: Olafur Gudmundsson Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se) Hela publikationen finns tillgänglig på www.diva-portal.org Abstract New concept for monitoring SO2 emissions from Tavurvur volcano in Papua New Guinea Julia Wallius During a period of 29 consecutive days during the month of October 2016 a dual- beam wide-field-of-view DOAS instrument was deployed at Matupit Island in Papua New Guinea. This was to measure the sulphur dioxide emissions from Tavurvur vol- cano located on the Gazelle Peninsula of the island East New Britain. Due to unreliable settings for the first 17 days the final results used were derived from measurements taken between October 19th and 30th with a two day gap during the 25th and 26th of October when the instrument was used for obtaining wind speed data. The average i kg flux of SO2 was determined for the period to be 0.27 s with a wind speed estimation of m 3.9 s . The use of a dual-beam wide-field-of-view DOAS instrument is recommended at Tavurvur when the degassing levels are low and the wind is predominantly southwest- erly. An angle of 13 degrees above the horizon was used to obtain the sky reference spectrum for each set of scans, but on account of SO2 traces in these spectra this an- gle is considered too low and lifting the sky reference 5-10 degrees is recommended for future measurements. Keywords: Optic remote sensing, DOAS, Volcano monitoring, SO2 emissions Independent Degree Project C in Geophysics, 1GE037, 15 credits, 2017 Supervisor: Olafur Gudmundsson Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se) The whole document is available at www.diva-portal.org ii Acknowledgements This project would not have been possible without the invaluable help from Rabaul Vol- canological Observatory, in particular Kila Mulina and Ima Itikarai. Special thanks also go out to John Bosco and Ezequiel from RVO for their work assembling and installing the instrument on site. And finally, a huge thank you to Bo Galle and Santiago Arel- lano from Chalmers University for allowing me to be a part of their Papua New Guinea field campaign and all the help and support they provided both on site and before de- parture. Funding for this project was provided by a SIDA Minor Field Study grant and DCO-DECADE. iii A note on the text As part of the larger Rabaul caldera Tavurvur in itself is not a volcano but merely a sub-vent. However, for simplification, Tavurvur is referred to as a volcano throughout this thesis. iv Contents Acknowledgements iii List of Abbreviations vii 1 Introduction1 1.1 Rabaul caldera................................1 1.2 Method and purpose of study........................2 2 Background5 2.1 Differential Optical Absorption Spectroscopy................5 2.1.1 Lambert-Beer’s Law.........................6 2.2 DOAS Measurements and Corrections...................7 2.2.1 Fraunhofer spectrum.........................8 2.2.2 Ring effect..............................8 2.2.3 High-pass filtering..........................9 2.2.4 Dark current.............................9 2.2.5 Sky reference.............................9 2.3 Dual-beam wide-field-of-view DOAS....................9 2.4 Elevation angle................................ 11 2.5 Wind speed calculation........................... 12 2.6 Previous DOAS measurements on Tavurvur................ 12 3 Method 13 3.1 Materials and instrument assembling.................... 13 3.2 Configuration and testing of instrument................... 15 3.3 Installation.................................. 15 3.4 Scanner settings and properties...................... 16 3.4.1 Measurement modes........................ 16 Sensitive detection.......................... 16 Average flux............................. 16 3.5 Data evaluation................................ 19 4 Results 21 4.1 Offsets..................................... 21 4.2 SO2 fluxes.................................. 23 v 5 Discussion 25 5.1 Wind direction and speed.......................... 25 5.2 Recommendation............................... 25 6 Conclusions 26 Bibliography 27 vi List of Abbreviations COSPEC Correlation Spectroscopy D Day DB-WFOV Dual-beam wide-field-of-view DCO-DECADE Deep Carbon Observatory, Deep Earth Carbon Degassing DECADE Deep Earth Carbon Degassing DOAS Differential Optical Absorption Spectroscopy GPS Global Positioning System KG Kilogram M Meter NOVAC Network for Observation of Volcanic and Atmospheric Change O3 Ozone PPM Parts Per Million RVO Rabaul Volcanological Observatory S Second SO2 Sulphur dioxide T Ton UV Ultraviolet vii Chapter 1 Introduction For many years, volcanological monitoring has been an important tool used in order to further our understanding of the workings of volcanoes and the degree to which we can accurately predict a coming volcanic eruption (United States Geological Survey 2016). To this end, scientists have long recognized that the surveillance of volcanic gases play an important part, as gases dissolved in magma provide the driving force of volcanic eruptions. The observation of any changes in the release of certain gases from a vol- cano can therefore help forecast volcanic activity and provide insight into the processes behind eruptions (United States Geological Survey 2015). More specifically, the be- havior of magma bodies as they ascent and descent in relation to eruptions can be better understood by conducting high temporal SO2 emission measurements over long time periods (Galle et al., 2010). Together with other geophysical data it is also possi- ble to interpret the relationship between magma ascent and conduit and hydrothermal processes (Olmos et al., 2007) (Sparks, 2003). In fact, the relationship between gas signals and other geophysical measurement methods is conceivably strong, and it is possible that the process of volcanic degassing is responsible for, or at least closely related to, observed ground deformation and seismicity (Oppenheimer and McGonigle, 2004). Volcanoes also contribute to the release of many toxic gases into the atmosphere, which makes them a relevant study in context with the Earth’s climate. During recent years this specific topic has been given more and more attention, as the emission of a wide variety of volatile species, some of which are converted into aerosols during transport in the atmosphere, makes volcanism one of the main climate-forcing agents and a natural volatile source with important impact upon ecosystems at both local and regional scales (Allard et al., 2016). By measuring the gas emissions, evaluating the data and observing any underlying trends, the potential impact of volcanic emissions on the atmosphere and climate can begin to be understood. 1.1 Rabaul caldera Papua New Guinea occupies the eastern half of the island of New Guinea and its offshore islands in Melanesia, placing it in the pacific “Ring of fire” - a name given to the ring of volcanoes around the Pacific Ocean, a result of plate tectonics - which makes it highly prone to disaster. 1 Chapter 1. Introduction At the northeastern end of the province East New Britain lies Rabaul, a small town inside a large caldera known as Rabaul Volcano. For many years Rabaul was consid- ered the most important settlement of the province and served as a popular boating destination for both commercial and recreational purposes (Wikipedia 2017), before the eruptions of the sub-vents Tavurvur and Vulcan in 1994 devastated the town. The present-day shape of the caldera, with a wide opening where the sea forms Blanche Bay to the east, is reckoned to have been formed by a major eruption around 1400 years ago (Volcano Discovery 2017). Besides Tavurvur and Vulcan, other vents along its 14 km length include Turanguna, Rabalanakia, Sulphur Creek, Kombiu and the Beehives. The Rabual Volcanological Observatory (RVO) is a branch