Effects of a nonlinear response of the fourier-transform infrared open-path instrument on the measurements of some atmospheric gases.

The response of a Fourier-transform infrared (FTIR) instrument to changes in absorbance is inherently nonlinear for a number of reasons. One is that the interferogram acquired by the FTIR is truncated and then apodized before further processing of the data is accomplished. A commonly used apodization function in open-path FTIR research is triangular apodization, and all the research presented here has been done with that function. We calculated a set of absorption spectra by using the HITRAN database, covering ranges in both concentration and temperature for water, ammonia, and methane. Plots of these data reveal nonlinear results. The commonly used analysis technique, classical least squares, assumes that the response is linear. We describe some of the effects of this nonlinearity and present ways to address these effects.

Gas-phase photoacoustic determination of the total carbon content of aerosol deposits.

A prototype system was constructed to determine the total carbon content of ambient aerosols trapped on quartz fiber filters. The measurement technique is based on carbon combustion to CO(2), cryogenic precon-centration, and subsequent photoacoustic monitoring of produced CO(2). A common sample set was independently analyzed by two established combustion method instruments and the photoacoustic system. Statistical comparison of data showed good agreement with accepted carbon values indicating feasibility for photoacoustic application to routine carbon analysis.

Helmholtz resonator enhancement of photoacoustic signals.

A Helmholtz resonator attached to a nonresonant photoacoustic cell enhances the responsivity to trace gas absorption by as much as a factor of 15.3. A simple system model based on a lumped parameter approach predicts the experimentally determined resonance frequency f(0) within 6% for all five resonator volumes tried and gives a pressure amplitude response at resonance proportional to f(0)(-5/2), which is the approximate experimental dependence. Optimization of response based on the model shows a pressure amplitude dependence on a(1/2), the square root of the radius of the cylindrical tube connecting the cell and the resonator.

Long-path laser monitor of carbon monoxide: system improvements.

A system for measurement of long-path absorption of carbon monoxide has been redesigned for improved performance. The diode laser source is frequency stabilized by automatically correcting the laser current and is limited to a single-wavelength output by a tunable étalon. As a result, sensitivity variations of the system to CO are limited to +/-0.7%, and system response is linear with respect to CO concentrations.

CO(2) Laser Absorption Coefficients for Determining Ambient Levels of O(3), NH(3), and C(2)H(4).

A carbon dioxide laser source was used to determine absorption coefficients for dilute absorber-air mixtures at wavelengths corresponding to several vibration-rotation lines in each branch of the 00 degrees 1-02 degrees 0 band at 9.4 microm and the 00 degrees 1-10 degrees 0 band at 10.4 microm. For all samples the total pressure was 1 atm and the temperature was 300 K; the concentrations ranged from 10 ppm (parts per million by volume) to 357 ppm for NH(3) and C(2)H(4), and from 10 ppm to 80 ppm for O(3). The absorption coefficients are tabulated, and the use of selected laser lines in monitoring ambient concentrations is discussed.