14. starptautiskÄ konference 2012 - Latvijas JÅ«ras akadÄmija

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Proceedings of 14th International conference „Maritime Transport and Infrastructure - <strong>2012</strong>”DIFFERENTIAL OPTICAL ABSORPTION SPECTROSCOPYAS A TOOL TO MEASURE EMISSIONS FROM SHIPS IN HARBOURJanis Kleperis*,**, Līga Grīnberga** and Anatolijs Šarakovskis***Department of Housing and Environment of Riga City Council, 49/53 Brivibas str., LV-1010, Riga, Latvia;E-mail: Janis.kleperis@riga.lv**Institute of Solid State Physics, University of Latvia, 8 Kengaraga str., LV-1063, Riga, Latvia;E-mail: Anatolijs.sarakovskis@cfi.lu.lvAbstractDifferential optical absorption spectroscopy (DOAS) is used to monitor concentrations of airpollutants in Riga Harbour region close to an enterprise involved in the oil transit business from1999. Light beam (630 m) analyzed in DOAS is separating oil transit company and apartmentbuildings with hundreds of people. Always when wind is blowing from direction of oil storage tanksand tanker docks which are involved in handling operations, higher SO 2 and hydrocarbonconcentrations are registered. Results from monitoring are analysed for a period of 12 years bycomparing selected measured pollution values and meteo data.KEY WORDS: Differential optical absorption spectroscopy, air pollution monitoring, SO 2 fromships.1. IntroductionThree main methods are available to measure air pollution in an open area: passive sampling,active sampling and automatic sampling [1]. Passive sampling refers to diffusion tubes or badges thatprovide a simple and inexpensive indication of average pollution levels over a period of weeks ormonths. Plastic tubes or discs open at one end to the atmosphere and with a chemical absorbent at theother is collecting specific gases (depending from the type of absorbent) from environment for definedtime period, then analyzed in laboratory with gas chromatograph. The low cost per tube allowssampling at a number of points and is useful in highlighting "hotspots" where more detailed study maybe needed. Active sampling involves the collection of samples, by physical or chemical means, forsubsequent laboratory analysis. Typically, a known volume of air is pumped through a filter orchemical collector for a known period of time - the collector then subjected to laboratory analysis.Automatic sampling is the most sophisticated method producing high-resolution measurements of arange of pollutants at a single point. The sample is measured on-line and in real-time, typically with15-minute averages or better, with data being collected from individual monitoring sites by telemetry.Instruments using physical scientific measurement techniques, such as chemiluminescence, UVfluorescence, IR absorption and Differential Optical Absorption Spectroscopy (DOAS), are used.DOAS instrument from OPSIS AB company (Sweden) [2] is used in Riga. Measurements are made insitu and there is no disturbance in the airflow due to pump sampling as with point analyzers. Openpathmethods are used for the remote sensing of the atmosphere and provide better estimates ofaverage pollutant concentrations over a given area, besides a single instrument can measure severalcomponents, both organic and inorganic [3].Riga City has DOAS type air quality monitoring station based on differential opticalabsorption spectroscopy in Riga Harbour region close to an enterprise involved in the oil transitbusiness since 1999. Instead of measuring pollution at a specific location, open-path methods recordthe average pollutant concentration along the all path length of a light beam (630 m in Riga Harbour)[4]. Monitoring data are analysed for a period of 12 years by comparing selected measuredconcentrations with meteorological data as well as specific cases with high pollution values areanalysed in this article.68
Proceedings of 14th International conference „Maritime Transport and Infrastructure - <strong>2012</strong>”2. Principles of Differential Optical Absorption SpectroscopyDOAS is now one of the most commonly used spectroscopic methods to measure trace gasesin the open atmosphere [5]. The basis of spectroscopic measurements of quantitative trace gasconcentrations in the atmosphere is Bouguer-Lambert–Beer’s law describing the absorption ofelectromagnetic radiation by matter:I( ) I0()exp(( )c L)(1)Here, I 0 (λ) denotes the initial intensity of a light beam with wavelength emitted by source ofradiation (high pressure xenon lamp), while I(λ) is the radiation intensity of the beam after passingthrough a gas layer with thickness L, where the absorber is present at a uniform concentration of c.The parameter σ(λ) is the absorption cross-section at wavelength λ characteristic of any specificmolecule (gas). The average trace gas concentration c can be calculated from the measured ratioI 0 (λ)/I(λ) if the light path length L is known:I ( )ln( 0 )I()c ( ) L(2)By comparing the intensity of light source with and without the polluting substance in thelight path, the integrated amount of the substance can be determined from Lambert-Beer`s law (2) andspecific absorption coefficient of this substance. Depending on the absorption spectrum of the speciesof interest, light at ultraviolet, visible or near infrared wavelengths are used. DOAS is the most widelyapplied open-path method used for air quality monitoring. The DOAS method is based on thedifference in absorption between different wavelengths [5]. It uses advanced computer technology,with software including the spectrographic fingerprint of the pollutants to be determined, to evaluatepollutant concentrations. The advantage of the DOAS method is that it does not use a sample cell andthe path length can be increased considerably, to hundreds of meters and even to several kilometers,allowing the determination of much lower concentrations than with conventional instruments. In theDOAS method, the computer rapidly sums the reference spectra while varying the proportions of thedifferent components until a match is obtained. In the DOAS system, a narrow beam of light from ahigh-pressure Xe lamp is focused onto a receiver up to 1 km away from the source. The light detectedby the receiver is transmitted through an optical fiber to a computerized spectrum analyzer. Thespectrometer in the analyzer splits the light into narrow wavelength bands using an optical grating. Anarrow slit sweeps past the detector at high speed, allowing a large number of instantaneousmeasurements to be made, building up the entire spectrum. The scan is repeated 100 times eachsecond and the registered spectrum is stored in the computer's memory. The measured spectrum iscompared with various spectra calculated by the computer based on different proportions of individualpollutants that may be present in ambient air (reference spectra of individual pollutants are stored inthe computer's memory). When the best possible match is obtained, the computer calculates theconcentrations of the different gases present in air.What gases can be measured with DOAS? Usually detectable in sub-ppb levels are so calledcriteria pollutants under the current legislation on air pollution [6]: SO 2 , NO 2 , NO, O 3 ; selectedaromatic hydrocarbons: benzene, toluene, styrene, ethylbenzene, xylenes, cresols, trimethylbenzenes,monochlorbenzene, dichlorbenzene; and typical smelling compounds: NH 3 , CS 2 , trimethylamine,formaldehyde, acetaldehyde, and phenol. In some cases toxic and exotic gases must be measured, forexample mercury (Hg), phosgene (COCl 2 ), chlorine dioxide (ClO 2 ), chlorine (Cl 2 ), sulphur trioxide(SO 3 ), hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen cyanide (HCN ), methylisocyanate(MIC) etc.69
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