Water vapor is the most important atmospheric greenhouse gas, but its variability and distribution, particularly the
vertical profile, are not well known due to a lack of reliable long-term observations in the upper troposphere and
stratosphere. Additional design and testing is necessary to extend Water Vapor Sensor System (WVSS) sensitivity to
water vapor from a threshold of 100 ppmv to 2.8 ppmv to support operational and climate applications. Laser
photoacoustic spectroscopy (LPAS) technique can extend the sensitivity to this level without extending absorption
chamber path or using expensive laser emitting at stronger absorption line. A laser photoacoustic spectroscopy sensor
based on inexpensive telecommunication style packaged, fiber-coupled near IR distributed feedback (DFB) laser diodes
was developed to quantify concentrations of water vapor (H2O), CO2, and methane in ambient air. The LPAS sensor
assembled in a compact package was designed for airborne, real-time measurements of atmospheric components. A
resonant photoacoustic cell is used to increase the photoacoustic signal, electrical modulation is applied to replace
mechanical chopper, and wavelength modulation spectroscopy is used to minimize the interfering background signal
from window absorption in the sample cell. The minimum detection sensitivities (1σ) of 5 ppm at 1.39 μm (5 mW) for
water vapor, 6 ppm at 1.6 μm (15 mW) for CO2, and 3 ppm at 1.6 μm (15 mW) for methane, are reported.
US EPA's Clean Air Act lists 187 hazardous air pollutants (HAP) or airborne toxics that are considered especially
harmful to health, and hence the measurement of their concentration is of great importance. Numerous sensor systems
have been reported for measuring these toxic gases and vapors. However, most of these sensors are specific to a single
gas or able to measure only a few of them. Thus a sensor capable of measuring many of the toxic gases simultaneously is
desirable. Laser photoacoustic spectroscopy (LPAS) sensors have the potential for true broadband measurement when
used in conjunction with one or more widely tunable laser sources. An LPAS gas analyzer equipped with a continuous
wave, room temperature IR Quantum Cascade Laser tunable over the wavelength range of 9.4 μm to 9.7 μm was used
for continuous real-time measurements of multiple gases/chemical components. An external cavity grating tuner was
used to generate several (75) narrow line output wavelengths to conduct photoacoustic absorption measurements of gas
mixtures. We have measured various HAPs such as Benzene, Formaldehyde, and Acetaldehyde in the presence of
atmospheric interferents water vapor, and carbon dioxide. Using the preliminary spectral pattern recognition algorithm,
we have shown our ability to measure all these chemical compounds simultaneously in under 3 minutes. Sensitivity
levels of a few part-per-billion (ppb) were achieved with several of the measured compounds with the preliminary
laboratory system.
A compact ultraviolet lidar stand-off sensor was recently developed and field-tested for detection of bio warfare (BW) agent stimulant aerosols and interferents. It employed a low-power (~5mW), continuous-wave, 375nm semiconductor ultraviolet optical source (SUVOS) laser diode that was modulated at high-speed with a pseudo-random (PR) code to provide range-resolved lidar detection of both aerosol elastic scattering and fluorescence. The sensor incorporated a 150mm diameter receiver telescope and 3 photon-counting detection channels centered at 375nm, 440nm, and 550nm. Aerosol elastic and fluorescence lidar profiles were obtained by correlating the signal photon-counts with the PR code. Tests of the lidar were performed first with simulants released in the Large Aerosol Chamber at Edgewood Chemical and Biological Center, MD at a lidar range of only 7.5m. The second phase of testing was done at Dugway Proving Ground, UT. Here the lidar was continuously scanned (± 13°) in a horizontal plane to detect downwind simulant and interferent aerosol disseminations at ranges of several hundred meters. Preliminary analyses of these tests show that the lidar detected fluorescence from the BW simulants at ranges up to 100m, and elastic scattering from aerosols up to 350m.
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