Strategic Technology Applications Research Center
The Woodlands, Texas
Active optical remote sensing technologies are helping us understand the threats to public health and the environment. Open path Fourier transform infrared (FT-IR) spectrometers and ultraviolet (OPUV) spectrometers are being used as alarms for accidental releases of toxic gases at industrial plants. A non-dispersive infrared remote sensor was used to determine that about 10% of the motor vehicles are causing most of urban smog in our cities. An ultraviolet differential absorption lidar (DIAL) was used to map the vertical structure of ozone, a major component of urban smog. Ozone accumulation was shown to be a regional phenomena that occurs on geographical scales extending far beyond the well-known urban situation. Thus, high ozone concentrations are found in urban areas, where it is a public health threat, as well as in rural areas, where it causes plant stress. These few examples of current optical remote sensing technologies underscore their potential applications in future environmental management programs.
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.
II. Air Pollution Spectrometry
While other emerging active remote sensing technologies such as tunable diode laser absorption spectrometer (TDLAS) and laser induced fluorescence (LIF) will play a role in future environmental programs [NRC, 1991], the remainder of this article will focus on the FT-IR and OPUV spectrometers (note that a discussion of DIAL systems is given in an accompanying article). Both of these systems have received a great deal of attention from industry and state and federal regulatory agencies, all of which need advanced technology to meet the increased monitoring needs stipulated in the 1990 Clean Air Act Amendments (CAAA). After over four years of testing, the Environmental Protection Agency (EPA) has determined the OPUV to be an acceptable monitoring method for ambient concentrations of O3, SO2, and NO2. This is the first-ever open path federal reference method, and is considered to be a major victory for the remote sensing community. Furthermore, the EPA is midway through the development of a standardized protocol for the FT-IR equipped with a closed cell (rather than using an open path) for monitoring formaldehyde, phenol, and methanol.
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.
III. FT-IR Spectroscopy
Many chemical species of importance to the CAAA and global monitoring studies have absorbance spectra in the two IR atmospheric window regions: 750 to 1200 cm-1 (13.3 to 8.3 mm) and 2100 to 3000 cm-1 (4.2 to 3.3 mm). For example, CO2 and H2O can be readily detected between 756 and 771 cm-1, O3 between 1043 and 1064 cm-1, CO between 2100 and 2177 cm-1, N2O between 2191 and 2224 cm-1, and CH4 between 2840 and 2930 cm-1. The FT-IR uses a Michelson interferometer, which has a large aperture of about 5.0 cm diameter, to create an interferogram. FT-IR systems are designed to transmit the interferogram (a modulated beam) so that it can be differentiated upon reaching the receiver from unmodulated IR energy emitted from the surrounding background. A spectrum is obtained by performing a Fourier transform on the interferogram. Because the FT-IR detects the entire spectral region at once (the multiplex advantage), it can detect several species simultaneously. The source and receiver telescope optics are typically between 12" and 15" in diameter, and the retromirror is typically 24" in diameter. A liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector is used to convert IR energy between 750 and 4000 wavenumbers (cm-1) into an electronic signal.
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.
IV. OPUV Spectroscopy
UV spectroscopic techniques complement IR measurements very well, especially in cases when the IR absorbance features of a particular species is lacking or is masked by another compound. For example, oxides of nitrogen are quickly masked by strong water vapor lines in the IR as the monitoring pathlength increases beyond about 75 m. Strong absorbance features appear between 340 and 380 nm for NO2, between 220 and 230 nm for NO, between 280 and 330 nm for SO2, and between 240 and 300 nm for O3. In addition, the OPUV can detect several aromatic hydrocarbons, such as benzene and toluene, oxides of nitrogen and sulfur, and formaldehyde.
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.
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