3A04 Diode Laser Photoacoustic Detection of Automobile ExhaustEmissions◯S. Civiš , V. Horká, and J. Cihelka
J. Heyrovský Institute of Physical Chemistry, Academy of Science of Czech Republic, Dolejškova 3, 18223
Prague 8, Czech Republic

ABSTRACT
A study of the infrared spectra of the CH4, NH3, N2O and ethane gas has been carried out. These molecules
are part of atmospheric pollution caused by emissions from automobile traffic. We tested the method ofphotoacoustic (PA) detection together with the laser semiconductor emission as a new tool for monitoring oflimited concentrations of these atmospheric pollutants. The variation of the intensity of the PA signal withpressure was studied. The detection limits were determined for the individual gases in a mixture with air at theoptimum pressure. The spectra of automobile exhaust gases at various motor temperatures were measured
qualitatively and quantitatively. A new type of semi-conductor GaInAsSb/AlGaAsSb laser, working in the 4342
– 4362 cm-1 region at room and higher temperatures was used as the tunable radiation source.
RESULTS
The main target of the study was PA detection of gases absorbing in the area of emissions ofGaInAsSb/AlGaAsSb based lasers. We concentrated on detection of CH4, NH3, N2O and ethane, which arepredominant molecules in pollution of the lower troposphere. We attempted to carry out detection, quantitative
determination of the main components of automobile emissions and comparison with the individual spectra of
the pure components.
Figure 1 depicts the photoacoustic spectra of CH4, NH3, ethane and N2O in the 4360.5 – 4364.0 cm-1 region( laser current 100 – 40 mA), at a pressure of 100 Torr, and, for ammonia, 60 Torr.
optoac.

CH4optoac.

NH3optoac.

C2H6optoac.

N2OFTIR

FTIR

4355

4356

4357

4358

4359

-1Wavenumber (cm )Fig. 1. Laboratory measurement of the spectra at a pressure of 100 Torr for CH4, C2H6, and N2O and 60 Torrfor NH3. CH4 and NH3 are compared with the FTIR spectra.
A study was carried out to determine the optimum gas pressure for maximum response of the PA signal. The
optimum pressure for NH3 is 65 Torr for CH4 90 Torr and for N2O 100 Torr.
Methane can be detected down to values below 100 ppm using the absorption line at 4362.0651 cm-1(00011112-00000000 transition), ammonia below 1 ppm using the line at 4355.304 cm-1 (0110s-0000stransition) and N2O only down to 10 000 ppm using the band at 4361.647 cm-1 (0002-0000 transition).

40-4

10 000 ppm
4355,24355,44355,6
1 000 ppm

4355,84356,04355,24355,44355,6
100 ppm

4355,84356,04355,24355,44355,64355,84356,04355,84356,04355,60.1 ppm

4355,84356,04355,64355,84356,0PA-signal (mV)

0,40,0-0,40,10,0-0,1
0,01
0,00-0,010,015
0,000-0,015
0,008
0,004
0,000

10 ppm4355,24355,44355,61 ppm
4355,24355,44355,24355,4-1

Wavenumber (cm )Fig. 2 Detection limit of ammonia at a pressure of 65 Torr. The pure gas was diluted with air.
Figures 2 and 3 depicts the spectra of NH3 and comparison with the spectra of automobile emissions in the4355 cm-1 area measured at a pressure of 70 Torr. The exhaust gases were collected immediately after starting
the motor (cool) and after thorough warming of the motor by driving the vehicle (warm). The calculated contentof ammonia was approximately the same in both fractions and equalled 225 ppm in the “cool” sample and 210
ppm in the “warm” sample.

10p=70 torr

0CH4-10PA-signal (mV)

4355,84355,9104356,0NH304355,840
20

CH4- 125 ppm

4355,9NH3- 225 ppm

4356,0exhaust gas (cool)04355,880
400-40

CH4- 800 ppm

4355,84355,9NH3- 210 ppm

4356,0exhaust gas (warm)

4355,94356,0-1

Wavenumber (cm )Fig. 3. Spectra of methane, ammonia and exhaust gases just after starting the motor (cool) and from the warm
motor at a pressure of 70 Torr.
SUMMARY
The work was concerned with practical application of semiconductor lasers in combination with the PA
detection. The rotation-vibration spectra of the gases CH4, NH3, N2O and ethane were measured in the infrared
region using of the GaSb laser. The dependence of the PA signal on pressure was studied for the purecomponents and their mixtures with air. The detection limits were measured for NH3, CH4 and N2O. For
ammonia was about 0.1 ppm. The method was applied in practice to monitor the contents of the individual kindsof molecules in automobile exhaust gases.
Its effectiveness, simplicity and low cost mean that the PA method combined with diode lasers could haveextensive commercial applications.
ACKNOWLEDGEMENTS.This work was funded by the Grant Agency of the Czech Republic, grant No. 203/06/1278.
Heyrovský Institute of Physical Chemistry for financial support with 120 Bruker reconstruction

S.C. thanks J.