Future applications of active optical remote sensing instruments will include studies relating to tropospheric chemistry, global change, regional photochemical measurement and modeling, air pollution exposure, pollution prevention, and general air quality. Measurement challenges posed by these studies include: (1) detection of multiple gases, (2) sufficient instrument sensitivity, (3) short enough temporal resolution, and (4) appropriate spatial coverage. In order to meet these measurement challenges in a cost-effective manner, remote sensing technologies will need to be further developed in order to provide for real-time, multiple-pollutant, extended spatial coverage monitoring capabilities. Further technological development is needed not only by researchers, but also by industry who must remain in compliance with strict regulations imposed on them to control air pollution emissions.
The FT-IR and OPUV open-path spectrometers measure path-averaged concentrations of multiple gases simultaneously over pathlengths of between 50 to 1000 m. During field measurements, the FT-IR and OPUV monitors direct beams of IR and UV energy towards retromirrors that return each beam to its receiver. Gases that pass through these beams and absorb some of the transmitted energy will appear in the measured absorbance data. The separation between the monitors and the retromirrors is chosen according to the pollutants being observed, expected concentrations, and the physical layout of the monitoring site. Once set up, these systems can provide continuous observations of pollutant fluctuations 24 hours a day. The FT-IR and OPUV spectrometers offer several advantages over conventional point monitors: (1) they are capable of monitoring multiple gases simultaneously and in real time; (2) no canister sampling is required; and (3) data from open-path monitors provide a spatial resolution that is more appropriate for model comparisons than that of point monitors. A description of the FT-IR and OPUV systems follows.
A major advance in the use of FT-IR spectroscopy for gas analysis was the automation of quantitative analysis using multicomponent Classical Least Squares (CLS) [Grant, 1992]. In the CLS use is made of Beer's Law, given by
I(n) = Io(n) exp(-A(n)) with A(n) = a(n)CL
where I and Io are the measured and transmitted intensities, n is the wavenumber (cm-1), A(n) is the absorbance, a(n) is the absorption coefficient, C is the concentration of the absorbing gas, and L is the pathlength of the radiation through the gas. Note that the absorbance is proportional to the concentration-pathlength product, CL, and that the absorption coefficient contains the unique "fingerprint" shapes of the absorption spectra of the different species. The CLS fit is performed between the measured spectrum and a set of reference spectra while simultaneously fitting a linear baseline over the specified wavenumber region. A dry air mixture of one atmosphere total pressure is used to generate each reference spectrum.
FT-IR spectrometers have been used to measure gas concentrations in both the stratosphere and the troposphere [Grant et al., 1992; Kolb, 1991]. In the stratosphere, infrared spectrometers are designed with a fine resolution (0.01 cm-1) because atmospheric pressure is low. However, a lower resolution of between 0.5 cm-1 and 2 cm-1 is commonly used in the troposphere due to pressure broadening effects that result in broadened absorption lines. Infrared spectroscopic techniques in the troposphere are complicated by water vapor concentrations that are much higher than those in the stratosphere. The strong interference of water vapor in the troposphere is overcome by detecting chemical species in narrow bands of the infrared spectrum where water absorption is very weak. The FT-IR can detect over a hundred volatile organic compounds (VOCs) emitted from industry and biogenic VOC emissions such as isoprene and a-pinene.
A scanning-slit technique has been applied to measuring molecular species in the UV relating to atmospheric photochemistry and smog formation [Finlayson-Pitts and Pitts, 1986]. An alternative to the scanning slit approach is to use a grating or prism spectrometer with a photodiode array as a detector [Wahner et al., 1989]. An important advantage of simultaneous measurement of the desired spectral interval (as opposed to scanning) is the elimination of time dependent changes due to atmospheric scintillation effects that occur during the scan. A xenon arc lamp is commonly used as the source and a 1024 element photodiode array as the detector. An instrument resolution of around 0.3 nm is quite adequate for resolving pollutant absorbance peaks. The concentration of each species is determined using a least-squares fit similar to that described for the FT-IR. Note that the fit is carried out over a large portion of the spectral region, instead of at a single pixel, in order to reduce the effect of interfering absorbance features due to other gases.
2. Grant, W.B., Kagann, R.H., and McClenny, W.A., Optical remote measurement of toxic gases, J. Air Waste Manage Assoc., 42, No. 1, pp. 18-30, 1992
3. Kolb, C.E., "Instrumentation for chemical species measurements in the troposphere and stratosphere", Reviews of Geophysics, Supplement, U.S. National Report to International Union of Geodesy and Geophysics, pp. 25-36, 1991.
4. Lamb, B., H. Westberg, and G. Allwine, Isoprene emission fluxes determined by an atmospheric tracer technique, Atmospheric Environment,, Vol. 20, No. 1, 1-8, 1986.
5. National Research Council, Rethinking the Ozone Problem in Urban and Regional Air Pollution; National Academy Press, Washington, D.C., 1991.
6. Wahner, A., R.O. Jakoubek, G.H. Mount, A.R. Ravishankara, and A.L. Schmeltekopf, Remote sensing observations of daytime column NO2 during the airborne Antarctic ozone experiment, August 22 to October 2, 1987, J. Geophys. Res., 94, 16,619-16,632, 1989.
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