Spatial Distribution of Methane Over Lake Baikal Surface

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Spatial Distribution of Methane Over Lake Baikal Surface

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    Spectrochimica Acta Part A 66 (2007) 788795

    Review

    Spatial distribution of methane over Lake Baikal surface

    V.A. Kapitanov a,, I.S. Tyryshkin a, N.P. Krivolutskii a, Yu.N. Ponomarev a,M. De Batist b, R.Yu. Gnatovsky c

    a Institute of Atmospheric Optics, SB RAS, 1 Akademicheskii ave, 634055, Tomsk, Russiab Renard Center of Marine Geology, Ghent University, Belgium

    c Limnology Institute, SB RAS, Irkutsk, Russia

    Received 15 October 2006; accepted 17 October 2006

    Abstract

    The results of application of a high sensitivity methane laser detector to investigations of the methane concentration in the atmosphere overBaikal lake are presented as well as methane flows from the water into the atmosphere. The measurements were conducted at a stationary stationand aboard the research vessel Vereschagin during two summer expeditions in 2003 and 2004. Mean background concentration was equal to(2.00 0.16) ppm in August 2003 and (1.90 0.07) ppm in June 2004. The areas of methane emission through the waters surface are found to bedistinctly localized and to have a characteristic size of about 150300 m in diameter. The methane concentration in the centers of these areas canreach approximately 27 ppm. Methane flows into the atmosphere in some Baikal regions were measured as well. 2006 Elsevier B.V. All rights reserved.

    Keywords: Diode laser detector; Lake Baikal; Atmospheric methane; Concentration measurement

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7882. Laser gas-analyzer of methane and measurement technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7893. Detector specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7904. Measurement procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7905. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7926. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795

    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795

    1. Introduction

    Methane is the most important representative of organic sub-stances in the atmosphere [1] and its concentration significantlyexceeds the concentration of other organic compounds. In recentyears, the amount of methane in the atmosphere increased ata rate about 1% a year, and this increase favors intensificationof the greenhouse effect, because methane efficiently absorbsthe Earths thermal radiation in the IR spectral region. The

    Corresponding author. Tel.: +7 3822 492645; fax: +7 3822 492086.E-mail address: kvan@asd.iao.ru (V.A. Kapitanov).

    contribution of methane to the greenhouse effect is about 30%of that due to carbon dioxide.

    By estimates, the methane emission from natural andanthropogenic sources into atmosphere is approximately500600 Mt/year; and approximately one-third of this amount isdue to the natural sources: wetlands, termites, forest fires, GlobalOcean, and fresh water bodies. Although the general balance ofmethane in the atmosphere is well known, the contribution fromindividual sources remains poorly studied.

    The discovery in 1969 of methane gas-hydrates in the Earthsinterior made by Vasilev et al. [2] has aroused an increasedinterest in gas-hydrates and stimulated a series of investigations,which found a huge fuel reserve in the form of gas-hydrates in the

    1386-1425/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2006.10.036

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    V.A. Kapitanov et al. / Spectrochimica Acta Part A 66 (2007) 788795 789

    bottom of the Global Ocean. According to the estimates avail-able, the reserves of the hydrocarbon fuel (mostly, methane)in the gas-hydrate form markedly exceed the combined fuelreserves in all other forms on the Earth. Gas-hydrates attractincreased attention not only because they can be used as fuel andchemical raw materials, but they may also be connected to pos-sible methane emissions into the atmosphere both in future fielddevelopments and due to relatively small changes in thermody-namic (climatic) conditions leading to a violation of the stabilityof the gas-hydrates, which may cause additional methane emis-sions into the atmosphere leading to additional ecological andclimatic problems.

    Lake Baikal is the largest fresh water body on the Earth. Thearea of its water surface is about 31,722 km2. Quite recentlygas hydrates were found in its bottom [3,4]. Lake Baikal is aconvenient object for investigation of such formations, as wellas processes of methane emission from gas-hydrates. As wasshown in ref. [5], despite the fact that the first direct or indirectdata on gas emissions into water and atmosphere were obtainedas long as 200 years ago, the methane concentrations in the waterand atmosphere have not yet been measured.

    The goal of our work was to study methane concentrationfields and flows, as well as to find anomalies in the methanedistribution in the atmosphere over the Lake Baikal surface usinga high-sensitive laser methane detector.

    2. Laser gas-analyzer of methane and measurementtechnique

    To measure methane concentrations and flows, we developedan improved version of the detector originally designed at the

    Institute of General Physics RAS based on a multifrequencynear-IR diode laser and a multi-pass cell [6]. The detector isshown schematically in Fig. 1.

    It consists of:

    (1) Optical block including a diode laser (DL) with aFabriPerrot cavity, two optical cells: a multi-pass cell witha tunable optical way length (analytical channel) and a ref-erence one; two photoreceivers (PR);

    (2) personal computer (Pentium);(3) many-functional Input/Output board AT-MIO-16E1 (or

    AT-MIO-16E4), produced by National Instruments Inc.company, NI-DAQ board installed into the PCI bus;

    (4) interface module located in the computer front panel;(5) programs controlling the detector.

    The detector employs a GaInPAs diode laser as a sourceof radiation at a working temperature of 0 to +50 C. Chang-ing the magnitude of current and temperature, it is possible totune the laser frequency in the range from 6000 to 6080 cm1(1.6451.666 m), where rather strong absorption lines ofmethane are observed. The diode laser emits several (510)longitudinal modes. The radiation power in one of them isabout 70% of the total laser power (3 mW); the width of anindividual mode is 103 cm1. The laser is mounted on aPeltier element. Radiation frequency tuning of the highest-power mode to the frequency of methane absorption line centeris performed by changing the DL temperature. The temper-ature is measured by a thermal sensor (thermistor), situatednear the laser. The DL long-term temperature stability is 102degree.

    Fig. 1. Methane detector block diagram.

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    The diode laser operates in a repetitively pulsed modewith a period of 4.5 ms and a pulse duration of 4 ms. Currentpulses supplying the laser have a trapezoid shape. This allowsscanning the laser frequency in the range within 1 cm1 duringa single pulse and recording the transmission spectrum of themethane individual line. The diode laser emits in two oppositedirections. The principal laser beam comes into the multipassanalytical optical cell with a photodetector at the exit. Atthis time atmospheric air is continuously blown through thecell.

    The laser radiation emitted in the opposite direction passesthrough the reference cell filled with a mixture of methaneand nitrogen at a preset methane c