Transmissometer System Research Articles

Additive Manufacturing of a Terahertz Back-to-Back Horn Antenna for Use in Life Sciences

A 3-D-printed, 0.3-THz, back-to-back, smooth-walled, horn is presented. It was made from “material” as mating symmetric halves, with internal surfaces metallized with gold plate. The junction of the back-to-back horns hosts a 1-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> microfluidic aperture for feeding analytes undergoing dielectric properties’ characterization of typically solvated proteins. The configuration allows for enhanced beam–sample interaction and efficient detection than would be conventionally achieved with either traditional quasi-optical (QO) or cavity resonator circuits. The horn pair was designed for 0.3–032-THz operation. It can, however, work over a wider domain of frequencies, namely, 0.2–0.32 THz, if restrictions on cross-polarization and matching are relaxed. Initial measurements of the horn pair have shown that there is an increase of 2.5 dB in the signal strength when the horn apertures are equidistant from the focusing mirrors of a defocused QO transmissometer system.

Multi-aperture laser transmissometer system for long-path aerosol extinction rate measurement.

We present the theory, design, simulation, and experimental evaluations of a new laser transmissometer system for aerosol extinction rate measurement over long paths. The transmitter emits an ON/OFF modulated Gaussian beam that does not require strict collimation. The receiver uses multiple point detectors to sample the sub-aperture irradiance of the arriving beam. The sparse detector arrangement makes our transmissometer system immune to turbulence-induced beam distortion and beam wander caused by the atmospheric channel. Turbulence effects often cause spatial discrepancies in beam propagation and lead to miscalculation of true power loss when using the conventional approach of measuring the total beam power directly with a large-aperture optical concentrator. Our transmissometer system, on the other hand, combines the readouts from distributed detectors to rule out turbulence-induced temporal power fluctuations. As a result, we show through both simulation and field experiments that our transmissometer system works accurately with turbulence strength Cn2 up to 10-12 m-2/3 over a typical 1-km atmospheric channel. In application, our turbulence- and weather-resistant laser transmissometer system has significant advantages for the measurement and study of aerosol concentration, absorption, and scattering properties, which are crucial for directed energy systems, ground-level free-space optical communication systems, environmental monitoring, and weather forecasting.

Monitoring Automotive Particulate Matter Emissions with LiDAR: A Review

Automotive particulate matter (PM) causes deleterious effects on health and visibility. Physical and chemical properties of PM also influence climate change. Roadside remote sensing of automotive emissions is a valuable option for assessing the contribution of individual vehicles to the total PM burden. LiDAR represents a unique approach that allows measuring PM emissions from in-use vehicles with high sensitivity. This publication reviews vehicle emission remote sensing measurements using ultraviolet LiDAR and transmissometer systems. The paper discusses the measurement theory and documents examples of how these techniques provide a unique perspective for exhaust emissions of individual and groups of vehicles.

A case study of real-world tailpipe emissions for school buses using a 20% biodiesel blend

Numerous laboratory studies report carbon monoxide, hydrocarbon, and particulate matter emission reductions with a slight nitrogen oxides emission increase from engines operating with biodiesel and biodiesel blends as compared to using petroleum diesel. We conducted a field study on a fleet of school buses to evaluate the effects of biodiesel use on gaseous and particulate matter fuel-based emission factors under real-world conditions. The field experiment was carried out in two phases during winter 2004. In January (phase I), emissions from approximately 200 school buses operating on petroleum diesel were measured. Immediately after the end of the first phase measurement period, the buses were switched to a 20% biodiesel blend. Emission factors were measured again in March 2004 (phase II) and compared with the January emission factors. To measure gaseous emission factors we used a commercial gaseous remote sensor. Particulate matter emission factors were determined with a combination of the gaseous remote sensor, a Lidar (light detection and ranging), and transmissometer system developed at the Desert Research Institute of Reno, NV, U.S.A. Particulate matter emissions from school buses significantly increased (up to a factor of 1.8) after the switch from petroleum diesel to a 20% biodiesel blend. The fuel used during this campaign was provided by a local distributor and was independently analyzed at the end of the on-road experiment. The analysis found high concentrations of free glycerin and reduced flash points in the B 100 parent fuel. Both measures indicate improper separation and processing of the biodiesel product during production. The biodiesel fuels used in the school buses were not in compliance with the U.S.A. ASTM D6751 biodiesel standard that was finalized in December of 2001. The U.S.A. National Biodiesel Board has formed a voluntary National Biodiesel Accreditation Program for producers and marketers of biodiesel to ensure product quality and compliance with the ASTM standard. The results of our study underline the importance of the program since potential emission benefits from biodiesel may be reduced or even reversed without appropriate fuel quality control on real-world fuels.

Visibility impairment during Yellow Sand periods in the urban atmosphere of Kwangju, Korea

For continuous monitoring of atmospheric visibility in the city of Kwangju, Korea, a transmissometer system consisting of a transmitter and a receiver was installed at a distance of 1.91 km across the downtown Kwangju, Korea. At the transmitter site a nephelometer and an aethalometer were also installed to measure the scattering and absorption coefficients of the atmosphere, respectively. Unusually high number of Yellow Sand events had occurred in the Northeast Asia during the spring of 2000. Visibility in Kwangju under such conditions was greatly impaired over large area for a few days. In order to investigate the effects of Yellow Sand on visibility impairment, chemical and elemental analyses of aerosol samples were performed along with the optical measurement of visibility. Hourly averaged visual range decreased from 61.7 to 1.9 km when hourly averaged concentration of PM10 varied from 32.9 to 601.8 μg/m 3 during Yellow Sand periods. Fine particulate (<2.5 μm) concentrations were relatively lower than coarse particulate matters. Results of the data analyses show that mineral dusts originated from continental sources were simultaneously transported along with anthropogenic sulfate aerosols and marine aerosols. Total light extinction coefficient, b ext, proposed by the IMPROVE network showed poor correlation with b ext measured by transmissometer. Coarse mass scattering efficiency was classified into three categories; E NHSOc, E mineral, and E sea-salt, which were determined as ammonium sulfate combined with nss-sulfate of 1.0, sea-salt of 0.4, and mineral of 0.77 m 2/g, respectively. Mass fraction of NHSOc, sea-salt, and mineral dust was 0.20, 0.14, and 0.66, respectively.

Forwardscattering corrections for optical extinction measurements in aerosol media 2: Polydispersions

This paper, second of two parts, presents a parametric study of the forwardscattering corrections for experimentally measured optical extinction coefficients in polydisperse particulate media, since some forward scattered light invariably enters, along with the direct beam, into the finite aperture of the detector. Forwardscattering corrections are computed by two methods: (1) using the exact Mie theory, and (2) the approximate Rayleigh diffraction formula for spherical particles. A parametric study of the dependence of the corrections on mode radii, real and imaginary parts of the complex refractive index, and half-angle of the detector's view cone has been carried out for three different size distribution functions of the modified Gamma type. In addition, a study has been carried out to investigate the range of these parameters in which the approximate formulation is valid. The agreement is especially good for small-view cone angles and large particles, which improves significantly for slightly absorbing aerosol particles. Also discussed is the dependence of these corrections on the experimental design of the transmissometer systems.

Measurement of Opacity and Particulate Emissions with an On-Stack Transmissometer

An on-stack transmissometer system which is designed to provide a precision measurement of the opacity of visible emissions is described. The sources of error in opacity measurements with regard to recent EPA emission monitoring requirements and planned specifications are discussed. Sources of error are voltage changes, temperature changes, light source and detector aging and effects of ambient light. Other major operational errors are caused by alignment drift and soiling drift. The methods employed to minimize these errors achieve an accuracy of ±3% of span and a maintenance free operational period of 3 months. The relationships between optical density, opacity and transmittance are described. The instrument measurement can be correlated with dust loading provided the particle size distribution is constant. Examples are given of correlations obtained between optical density and particulate concentration in the gas on various types of emission sources and the observed error margins are summarized.

A Single-ended Atmospheric Transmissometer

It has been shown experimentally that a measurement of the Raman shifted component of the laser backscatter from atmospheric nitrogen will give a direct determination of transmission as a function of range. This type of single-ended device, when operating in a radar-like mode, can satisfy the need to accurately measure atmospheric transmission. A major difficulty in the interpretation of pulsed lidar backscatter data is that unless a priori information is available concerning the relationship between the volume backscattering coefficient and the attenuation coefficient, the received intensity cannot be easily evaluated as transmittance. The backscatter coefficient for Raman scattering, however, depends only on the Raman cross-section of the specific molecule used and the number density of that molecule. In the lower atmosphere the density of atmospheric nitrogen is constant. A measurement of Raman scattering from nitrogen will therefore give a direct determination of transmission as a function of range. Experiments were conducted over a 1/4-mile range and produced consistent results for transmissions down to as low as 2 percent when compared with simultaneous double-ended reference transmissometer data. The laser Raman transmissometer system is now computer controlled and produces real time data displays.

Turbidity Generation and Distribution in Tampa Bay Monitored with Towable Optical Transmissometer: ABSTRACT

Turbidity in Tampa Bay was monitored for 6 months using a towable 1-m optical transmissometer system. The system provided for continuous chart readout and allowed short-term water-mass migrations to be mapped quantitatively. Transmission readings ranged from 80% in the open Gulf of Mexico to zero in Hillsborough Bay. Five major sources of turbidity were identified: (1) resuspended bottom sediment, present both naturally over shoals and caused artificially by dredging and other developmental activities, most important; (2) suspended phytoplankton (seston) important locally, especially in Hillsborough Bay, where the pollutional nutrient input is excessive and promotes plankton blooms; (3) sewage outfalls and miscellaneous inputs related to urbanization; (4) river-borne tripton; and (5) surf-generated fine debris. End_of_Article - Last_Page 1694------------