Series: Physics, Chemistry and Technology Vol. 4, No 1, 2006, pp. 71 - 81
USING LASERS TO MEASURE POLLUTANTS
Mira Terzić1, Janez Možina2, Darja Horvat2
1Faculty of Science, University of Novi Sad, Trg Dositeja Obradovića 4,
21000 Novi Sad, Serbia & Montenegro
2Faculty of Mechanical Engineering, University of Ljubljana,
Aškerčeva 6, 1000 Ljubljana, Slovenia
Abstract. In recent years, a large number of linear and nonlinear laser-based diagnostic
techniques for detection of pollutions in different environments have been developed.
Applications of laser spectroscopy constitute a vast field, which is difficult to cover
comprehensively in a review. Due to that, here are presented only a few spectroscopic methods,
chosen to illustrate the power of applied laser spectroscopy in environmental pollution
investigation. The paper also gives a brief presentation of main laser spectroscopy methods.
Key words: Laser spectroscopy, trace gas detection, optoaocoustic spectroscopy
Lasers have revolutionized spectroscopy, greatly expanding the field of laser spectroscopy
in many areas. Immediately after the discovery of the laser (1960), the laser systems were
developed for atmospheric studies . The most common principle employed for the detec-
tion of environmental pollutants involves the interaction of the trace species with laser light
(absorption, scattering). Up to now, various diagnostic methods based on physical process
caused by laser light–environmental species [2–6] are developed.
Ideally, detection and monitoring laser based spectroscopic technique should fulfill the
• High selectivity, for a particular gas species, with no observable cross-response from other
species; to measure accurately trace gas concentrations of less than a part per billion;
• High sensitivity, to detect a very low concentration , below ppt (parts-per-trillion, 10-12 );
• Possibility to detect numerous compounds with one instrument;
• Wide dynamic range to monitor high and low concentrations with a single instrument, in
a real–time response that can be linear over more than four decades of concentration;
• Fast Response, with measurement speeds of fractions of a second, or signal averaging to
achieve still higher sensitivity;
• Good temporal resolution for on–line monitoring;
• Noninvasiveness and nondestructiveness, which do not disturb the sample under analysis.
Received October 14, 2005
M. TERZIĆ, J. MOŽINA, D. HORVAT
Laser–based analysis and diagnostics may be classified by a variety criteria such as physical
processes caused by interaction of laser radiation with matter, sampling procedure, wavelength
region, analytical methods, according to their power–law dependence on external electric field.
In respect to sampling procedure measurements can be performed:
In situ technique, the pollutants are measured directly and in many cases non–destructively.
The data is obtained from local space, with less sensitivity, but with real–time monitoring.
Extractive techniques: samples are collected, and then analyzed and processed in labora-
tory. These techniques has high sensitivity and selectivity. But, sometimes may the contami-
nation problems arise, the data is not obtained in real time and does not allow continuous
Remote sensing technique, use of lasers to measure samples at some distance from the la-
ser system. The advantage is the contactless measurement, measurements results give three–
dimensional concentration or integrated profile of pollutions.
An important technical criterion of spectroscopic method is the wavelength region used:
visible, ultraviolet (UV), infrared (IR) and microwave spectroscopy. For species identifica-
tion IR region is particularly attractive, because the most of main atmospheric molecules
(H20, CO2, O3, CH4, N2O, CFCs), have well defined and highly characteristic spectral features
in the mid-IR spectral region (the so called fingerprint region) where molecular line intensi-
ties are reasonably large.
ABSORPTION–BASED LASER SPECTROSCOPY
The primary process is absorption of photons by atoms or molecules. Hence, Absorption–
based laser spectroscopy is one of the most widely used analytical tools for detection of a
specific molecule and one of the most important techniques for gas sensors in environmental.
Direct laser absorption spectroscopy based on the Beer –Lambert absorption law is often used
for quantitative measurements. A drawback this technique arise for very weak absorption
signals. Traditionally, this problem is surpassed with multi–pass cells . But it is limited by
instrument size. High sensitivity can be obtained with other spectroscopic tecniques based on
the detection of phenomena induced by absorption of light, such as fluorescence in laser in-
duced fluorescence (LIF) , ions in resonant enhanced multiphoton ionization (REMPI)
[9,10], or pressure changes in optoacoustic spectroscopy (OAS) [11,12]. Another form of
laser absorption spectroscopy is Cavity Ring Down Spectroscopy (CRDS) developed by
O'Keefe 1988 . CRDS is based
on measurement of changes in
relaxation time of a high-finesse
optical cavity upon introducing an
absorbing species. These tech-
niques permit researchers to obtain
an effective optical path length of
several kilometers in a very small
volume. In CRDS, intensity meas-
urements are replaced with time
measurements. CRDS promise very
high sensitivity for trace gas de-
tection (Table 1) [14–17].
Table 1. CRDS detection limits
ppt – ppm
ppt – ppm
ppt – ppm
ppt – ppb
ppt – ppb
ppt – ppb
ppt – ppb
Using Lasers to Measure Pollutants
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UPOTREBA LASERA ZA ISPITIVANJE POLUTANATA
Mira Terzić, Janez Možina, Darja Horvat
Poslednjih godina razvijen je velik broj raznovrsnih instrumenata zasnovanih na laserskim
sistemimima. Oni omogućuju da se može pouzdano i u realnom vremenu uraditi monitoring okoline,
industrijskih procesa, monitoring bioloških funkcija čoveka, selektivna detekcija molekula u
tragovima... Uporedo sa razvojem novih laserskih i detekcionih sistema razvijen je niz linearnih i
nelinearnih dijagnostičkih metoda za nedestruktivna merenja koncentracija polutanata u različitim
sredinama. Kako je teško pokriti široku primenu lasera u rešavanju problema vezanih za zaštitu
čovekove okoline, ovde je dat kratak prikaz, nekih savremenih metoda.