Light Absorption Measurements Research Articles

Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols

Data on light absorption by atmospheric particles are scarce relative to the need for global characterization. Most of the existing data come from methods that measure the change in light transmission through a filter on which particles are collected. We present a calibration of a recently developed filter-based instrument for continuous measurement of light absorption (model PSAP, Radiance Research, Seattle, WA) that has been incorporated in several measurement programs. This calibration uses a reference absorption determined as the difference between light extinction and light scattering by unaltered (suspended) particles. In addition, we perform the same calibration for two other common filter-based methods: an Integrating Plate and the Hybrid Integrating Plate System. For each method, we assess the responses to both particulate light scattering and particulate light absorption. We find that each of the instruments exhibits a significant response to nonabsorbing aerosols and overestimates absorption at...

Storage of marine particulate samples for light𠅀absorption measurements

Spectral absorption studies of marine particulates by spectrophotometry usually require the concentrating of material, and it is often desirable to store concentrated samples prior to analysis. The effects of various storage methods for particles collected on glass-fiber filters were examined using natural marine particles. Sensitivity to storage effects differed for particles collected from different environments, but results consistently indicated that refrigeration for more than a few hours and freezing at -10°C are inadequate for preserving absorption by phytoplankton pigments in the visible region of the spectrum. Storage at -80°C and at liquid-nitrogen temperatures was found to be satisfactory, with liquid nitrogen providing the most reliable results for the longest periods evaluated (up to 1 yr). Increases in absorption were observed at ultraviolet wavelengths under all storage conditions, which suggests that optical properties in this region of the spectrum cannot be reliably assessed unless samples are analyzed immediately after collection.

Photoacoustic spectrometer for measuring light absorption by aerosol: instrument description

A photoacoustic spectrometer has been developed to measure in situ light absorption by aerosol. The measured quantity is the sound pressure produced in an acoustic resonator caused by light absorption. The current lower detection limit for light absorption is 0.4 Mm -1 which corresponds to an elemental carbon mass density of ≈40 ng m -3 assuming an efficiency for light absorption of 10 m 2 g -1. Calibration is performed using simple theory for the instrument along with use of a calibrated microphone and laser. The acoustic resonator is operated in the plane wave mode, which has a quality factor of ≈80, a resonance frequency of ≈500 Hz, and a photoacoustic coefficient of 12.8 Pa (W m -1) -1. The equivalent noise bandwidth of the resonator is ≈5 Hz. Coherent acoustic noise was supressed through the use of acoustic notch filters and laser beam ports at pressure nodes of the resonator. The relatively low-quality factor made it possible to use phase-sensitive detection having an equivalent noise bandwidth of ≈7.5 mHz. This was achieved by vector time averaging the microphone signal for ≈8 min. Two compact, efficient lasers were used during instrument evaluation performed in the Northern Front Range Air Quality Study (Colorado, 1996/97). One was a laser diode pumped, frequency doubled, solid state laser, and the other was a laser diode. Laser wavelengths were 532 nm and 685 nm, and corresponding average powers were 60 and 87 nW. Some examples are provided for light absorption measurements using the photoacoustic instrument and a nearby aethalometer.

Spatial Distribution and Characteristics of Ozone Generation with Glow Discharge using a Double Discharge Method

For the purpose of investigating the effect of a wide discharge gap (4 mm) on ozone generation, a UV light absorption measurement technique has been used to determine the spatial distribution of ozone concentration. A glow discharge at atmospheric pressure was produced by a double discharge method with a pair of metal-to-metal electrodes. Efficient and uniform ozone generation has been accomplished. The obtained ozone concentration near the cathode is about 1.25 times higher than that at the center of the main gap. The rate constant κ1 for O+ O2+ M\\stackrelκ1→ O3+ M yields the values of (2.0–2.7)×10-34 cm6 molecule-2 s-1 from the ozone growth curve measured using the wavelength of 254 nm.

Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil

Black carbon mass absorption efficiencies of smoke particles were measured for various types of biomass fires during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment using thermal evolution measurements for black carbon and optical absorption methods. The obtained results range between 5.2 and 19.3 m2 g−1 with an average value of 12.1±4.0 m2 g−1. Particle size distributions and optical properties were also measured to provide a full set of physical parameters for modeling calculations. Mie theory was used to model the optical properties of the particles assuming both internal and external mixtures coupling the modeling calculations with the experimental results obtained during the campaign. For internal mixing, a particle model with a layered structure consisting of an absorbing black carbon core, surrounded by a nonabsorbing shell, was assumed. Also, for internal mixing, a discrete dipole approximation code was used to simulate packed soot clusters commonly found in electron microscopy photographs of filters collected during the experiment. The modeled results for layered spheres and packed clusters explain black carbon mass absorption coefficients up to values of about 25 m2 g−1, but measurements show even higher values which were correlated with the chemical composition and characteristics of the structure of the particles. Unrealistic high values of black carbon absorption efficiencies were linked to high concentrations of K, which influence the volatilization of black carbon (BC) at lower temperatures than usual, possibly causing artifacts in the determination of BC by thermal technique. The modeling results are compared with nephelometer and light absorption measurements.

Photoacoustic and filter measurements related to aerosol light absorption during the Northern Front Range Air Quality Study (Colorado 1996/1997)

A new photoacoustic instrument for the measurement of aerosol light absorption was collocated with conventional aerosol instrumentation during the 1996–1997 winter intensive monitoring period of the Northern Front Range Air Quality Study. Measurements of the light absorption efficiency for black carbon were 5 m2/g at 685 nm and 10 m2/g at 532 nm, and for elemental carbon, they were 3.6 m2/g at 685 nm. We show that these values together with previous photoacoustic measurements of aerosol light absorption shed some light on the wavelength dependence of absorption efficiency for carbonaceous aerosol in the visible and near‐visible region. Integrating plate type filter measurements of aerosol light absorption result in far larger values than those measured with the photoacoustic instrument. We demonstrate that a recently published correction technique [Horvath, 1997] can yield improved agreement.

Phase measurement of light absorption and scatter in human tissue

Analog and digital technologies are presented for precise measurement of propagation delay of photons from source and detector placed on portions of the human body. The goal of the apparatus design is to quantify absorption (μa) and scattering (μs′) induced by biological pigments and biological structures, respectively. Body tissues are highly scattering with a mean distance between scatterers of less than a mm (at 700–850 nm). Significant absorption is mainly due to 5%–10% of the tissue volume occupied by blood. Measurement of μa and μs′ is done by both time and frequency domain equipment. This article focuses upon frequency domain equipment because of its simplicity, reduced noise bandwidth, versatility, and the strong analogy to very high frequency/ultrahigh frequency communication devices, particularly those using phase modulation. Comparisons are made of homodyne and heterodyne systems together with evaluation of single and multiple side band systems, with particular emphasis on methods for multiplexed optical and radio frequencies by frequency encoding or time-sharing technologies. The applications of these phase modulation systems to quantitative brain and muscle blood oximetry, functional activity of the forebrain, and other important problems of medical science, are presented.

On the characterisation and gas sensing properties of Cu(II) tetra(alkylamino carbonyl) phthalocyanine LB films

Langmuir–Blodgett (LB) films of a substituted phthalocyanine containing the amidic group (CONH) instead of the ester group (COO) have been studied. Floating films were spread onto the water surface from chloroform solutions and were transferred onto hydrophilic quartz substrates using the vertical lifting method. Linearly polarised light absorbance measurements were performed at room temperature in the 400–800 nm spectral range. The average orientation of the phthalocyanine molecular rings in the LB film, with respect to the normal to the substrate and dipping direction, has been evaluated. Dynamic response characteristics of the electrical conductance of the LB films to different NO 2 concentrations, carried out in dry air and at room temperature, have shown a high sensitivity to concentrations of nitrogen dioxide smaller than 50 ppm.

Study of the optical properties of TeO2-PbO-TiO2 glass system

We describe the preparation and some optical properties of high refractive index TeO2-PbO-TiO2 glass system. Highly homogeneous glasses were obtained by agitating the mixture during the melting process in an alumina crucible. The characterization was done by X-ray diffraction, Raman scattering, light absorption and linear refractive index measurements. The results show a change in the glass structure as the PbO content increases: the TeO4 trigonal bipyramids characteristics of TeO2 glasses transform into TeO3 trigonal pyramids. However, the measured refractive indices are almost independent of the glass composition. We show that third-order nonlinear optical susceptibilities calculated from the measured refractive indices using Lines' theoretical model are also independent of the glass composition.

Non-separation assay for glycohemoglobin

The determination of glycohemoglobin [HbA1c, HbA1, or total glycohemoglobin (GHb)] has become an established procedure in the management of diabetes mellitus. Here, we describe the development of a simple, fluorescence, non-separation assay for the percentage of GHb (%GHb). The fluorescence of an eosin-boronic acid derivative when it was mixed with hemolysates of unwashed erythrocytes was quenched in proportion to the percentage of glycohemoglobin. Measurement of the fluorescence intensity gave an estimate of GHb in the sample, and measurement of light absorbance gave an estimate of total hemoglobin. A combination of the two measurements gave the assay response. Comparison with HPLC (Menarini-Arkray HA-8140 fully automated analyzer) for the percentage of HbA1 (%HbA1) gave %GHb(NETRIA) = 1.1(SD +/-0.03)%HbA1 +0.6(SD +/-0.3), S(y/x) = 0.821, r = 0.972, n = 80; comparison for HbA1c gave %GHb(NETRIA) = 1.3(SD +/-0.04)%HbA1c + 1.8(SD +/-0.3), S(y/x) = 0.813, r = 0.973, n = 80. Precision, estimated as the percentage of the CV of the %GHb assay results, was <2% (intraassay, range 5-22% GHb) and <4.2% (interassay, range 4-16% GHb). Dilution of a high-percentage GHb sample lysate showed that the assay was linear, and addition of glucose (60 mmol/L), bilirubin (250 micromol/L), and triglycerides (14 mmol/L) to low, medium, and high %GHb samples showed no clinical interference in assay results.

Use of a Simple Light Absorbance Assay to Measure Lymphocyte Proliferation

The proliferative response of human lymphocytes to stimuli such as foreign histocompatibility antigens or mitogens is generally assessed by measuring the amount of tritiated thymidine which the cells incorporate in culture. In this paper, the possibility of assessing lymphocyte proliferation and viability by an empirical assay, using measurement of light absorbance on ELISA reader in the yellow wave length (450 nm/air-550 nm/air), has been studied. The correlation of these measurements with a colormetric viability assay using MTS/PMS, with tritiated thymidine incorporation and with trypan blue exclusion viability counting, was determined. The results showed that the light absorbance assay correlate well with cell proliferation during 48–120 hours culture period and with cell viability after a 72 hours culture period. The MTS/PMS colormetric assay as well as trypan blue exclusion cell counting confirmed that the light absorbance assay was not merely caused by dead cells. This data confirm that the light absorbance assay is sufficiently sensitive to low levels of proliferation to allow detection of such responses at least as effectively as thymidine incorporation. The light absorbance assay procedure avoids the expense, time and hazards associated with scintillation counting, and is simple to perform without the necessity for reagents and preparative steps required by other assays.

Enhancement of intersubband transition energies in GaAs quantum wells by Si delta doping of high concentration

Intersubband light absorption measurements have been performed on a series of GaAs/AlAs quantum wells with heavy delta doping of Si atoms in the range 9.0×1011–6.5×1012 cm−2. The increase of intersubband transition energy by as much as 38 meV has been observed, and attributed to the deepening of the V-shaped potential by the Si layer and to the depolarization effects.

Effect of Hydration on Opacity in the Bovine Corneal Opacity and Permeability (BCOP) Assay

AbstractThe bovine corneal opacity and permeability (BCOP) assay has been proposed an an in vitro method for predicting the irritancy or toxicity of chemical substances. A loss of corneal transparency, which reduces visual acuity, can be the result of an increase in corneal hydration or direct damage to corneal tissues. The BCOP assay as currently practiced does not differentiate between these mechanisms. The purpose of this study was to determine the effect of increased hydration on transparency of the bovine cornea and to measure the hydration levels of corneas treated with compounds and substances known, from previous BCOP studies, to cause corneal opacity. Corneas were treated according to the BCOP protocol and corneal opacity was determined by measurement of light absorbance at 570 nm (A570). Corneal hydration was determined by measurement of wet and dry weights. Hydration of the intact cornea was 3.86 mg H2O/mg dry weight and A570 = 0.048. After removal of the epithelium and incubation in MEM, corne...

Measurement of light absorption by aquatic particles retained on filters: determination of the optical pathlength amplification by the ‘transmittance-reflectance’ method

The optical pathlength amplification (p factor), induced by multiple scattering in glass-fibre filters used for light absorption measurements of aquatic particles, has been measured by the trans- mittance-reflectance (T-R) method for algal species with cell diameter in the range from 1 to 5 um. The p factors obtained showed a much lower dispersion than that observed in similar measurements carried out by the standard transmittance method, suggesting that this could be due, at least in part, to the incomplete account of the scattering losses taken by the latter method. An experimental test has verified that the (3 factor expression derived from measurements carried out by the T-R method on an algal culture is also applicable to detrital particles. Introduction The light-transmission measurement of particles retained on glass-fibre filters (Yentsch, 1962) is now the most frequently used method for determination of the light absorption spectrum of particles sampled from aquatic environments. The measurement is normally performed by dual-beam spectrophotometers, where the light transmitted through the particle-retaining filter is compared with the light transmitted through an unused 'reference' filter. In the following, this procedure is referred to as the 'standard transmittance' method. The experi- mental raw data are expressed in terms of the dimensionless optical density (OD), also called absorbance, which is equal to the base 10 logarithm of the inverse transmittance. The spectral range in these measurements covers the visible region, and it often extends to the UV and near-IR. The use of glass-fibre filters offers one major advantage and one major disadvantage. The advantage is that the filtration allows one to concentrate particles that are naturally too dilute to measure with standard spectrophoto- meters. The disadvantage is that the particles are embedded within a highly scat- tering medium, which causes an increase in the optical pathlength, resulting in a considerable enhancement of the measured optical density (often referred to as the p factor, following the terminology of Butler (1962)). Conversion of the optical density of the filter-retained particles, OD{, to the optical density of the particle suspension with the same geometrical pathlength, ODSUS, is carried out using empirical relationships. The uncertainty of these relationships probably represents the largest error source affecting the light-transmission measurements of particles retained on glass-fibre filters. Once ODSUS corresponding to a given measured ODf is determined, the volume absorption coefficient of the particle suspension, a (in m1), is obtained from the equation: a = 2.3 ODS11S/X, where X is the ratio of the filtered volume to the

Nonlinear optical crosslinked polymers and interpenetrating polymer networks containing azo-benzothiazole chromophore groups

Two kinds of crosslinked polymers and one kind of interpenetrating polymer network (IPN) containing phenylazo-benzothiazole (BT) chromophore groups were synthesized. Crosslinked polyurethane was obtained by the reaction between (2-hydroxypropyl acrylate-BT-1 methacrylate) copolymer and phenol-capped NCO-terminated BT-2, whereas crosslinked epoxy-based polymer was formed by reaction between epoxy-containing BT and BT-containing aniline. IPNs based on crosslinked polyurethane/epoxy-based polymer were obtained by carrying out the above two reactions simultaneously. The crosslinked polymers and IPNs were characterized by gel content, IR spectra and differential scanning calorimetry. The crosslinked polyurethane and the epoxy-based polymer exhibit glass transition temperatures at 140 and 178°C, respectively, whereas the IPN shows two T g at 142 and 170°C. Thin, transparent poled films of the crosslinked polymers and IPN were prepared by spin-coating, followed by thermal curing and corona poling at 160°C for 1 h. The second-order non-linear optical properties of the poled films were studied by a visible light absorbance measurement according to the one-dimensional rigid oriented gas model. Long-term stability of the dipole alignment at 120°C was demonstrated for the three polymers.

Oxidative stress in the Dahl hypertensive rat.

Enhanced production of oxygen free radicals may play a role in hypertension by affecting vascular smooth muscle contraction, resistance to blood flow, and organ damage. The aim of this study was to determine whether oxygen free radicals are involved in the development of salt-induced hypertension. Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were fed either a high salt (6.0% NaCl) or low salt (0.3% NaCl) diet for 4 weeks. The high salt diet caused the development of severe hypertension in Dahl-S animals and had no effect on blood pressure in Dahl-R animals. A tetranitroblue tetrazolium dye was used to detect superoxide radicals in microvessels of the mesentery. Light absorption measurements revealed enhanced staining along the endothelium of arterioles and venules in hypertensive Dahl-S animals, with significantly lower values in normotensive animals. In addition, a Clark electrochemical electrode was used to measure hydrogen peroxide levels in fresh plasma. Hypertensive Dahl-S animals had a higher plasma hydrogen peroxide concentration compared with their normotensive counterparts (2.81+/-0.43 versus 2.10+/-0.41 micromol/L), while no difference was detected between high- and low salt-treated Dahl-R animals (1.70+/-0.35 versus 1.56+/-0.51 micromol/L). The plasma hydrogen peroxide levels of all groups correlated with mean arterial pressure (r=.77). These findings demonstrate an enhanced production of oxygen free radicals in the microvasculature of hypertensive Dahl-S rats.

Atmospheric nuclei and related aerosol fields over the Atlantic: Clean subsiding air and continental pollution during ASTEX

Meteorological conditions in the Atlantic Stratocumulus Transition Experiment (ASTEX) region favored advection of clean air from the central Atlantic during the early part of the experiment that was replaced by polluted air of European origin during the latter part of the experiment. Marked differences in the aerosol size distribution, composition, and state of mixing existed in these air masses. Pronounced differences in their vertical structure also demonstrated that surface measurements often do not represent average boundary layer or column concentrations. Clean subsiding air from the free troposphere had concentrations of condensation nuclei that significantly exceeded concentrations in the boundary layer and had very low mass concentrations and volatility consistent with homogeneous nucleation aloft, supporting the hypothesis that these nuclei can provide a source for “new” nuclei into the marine surface layer. This finding was in contrast to polluted air characterized by high concentrations of aged aerosol but having no evidence for significant recent nuclei formation. Particles in polluted air consisted of more than 90% volatile mass (mostly sulfate) and a refractory residual remaining at 300°C. The refractory mass varied with the concurrently measured light absorption coefficient, associated with combustion‐derived soot. In spite of 2 orders of magnitude more volatile sulfate in polluted air, most of the particle number in polluted air remained after volatilization at 300°C, in contrast to less than 10% in clean air. This finding suggests that the number of particles in polluted air may reflect the combustion process producing the primary soot aerosol more than the SO2 emissions responsible for much of the sulfate aerosol mass. The accumulation mode mass mean diameter was found to increase with total accumulation mode mass, suggesting that a size parameterization based upon the more commonly measured sulfate mass could be included in current aerosol models.

Photon correlation spectroscopy applied to characterisation of denaturation and thermal stability of human albumin

Photon correlation spectroscopy and light absorption measurements have been applied for characterisation of denaturation kinetics and thermal stability of human albumin in solution. The hydrodynamic size of the molecules has been studied as a function of pH, and the denaturation rate of ten different lots of 5% (w/v) human albumin solution has been measured at various temperatures. In the native (pH 7) state, the hydrodynamic molecular diameter was found to 6.3 nm. The molecular size was relatively stable between pH 10 and 5, but increased with decreasing pH to approximately 20 nm at pH 3. The denaturation rate, measured as change in hydrodynamic diameter per min, was strongly dependent on temperature and increased 3-fold per degree in the 73–75°C range. The investigated lots of albumin solution showed large variations in stability at 74°C, with denaturation rates ranging from 10 to 100 nm min −1. The observed thermal stability for the lots investigated was ranked identically with both the employed techniques. In an effort to explain the observed lot to lot variations in denaturation rate, a broad chemical characterisation including determination of free SH content, fatty acid content and composition and metal content, was performed. However, lot to lot variations in these parameters was not found to fully elucidate the observed variations in thermal stability.

Experimental calibration for aerosol light absorption measurements using the integrating plate method—Summary of the data

The determination of the light absorption coefficient by means of the integrating plate method is a very simple and reproducible procedure frequently used in various configurations. In this work it was calibrated with laboratory generated aerosol of different absorption and scattering properties generated from internal and external mixtures of carbon and a transparent substance. The light absorption coefficient was determined by an independent method which can be considered free of systematic errors.The light absorption coefficient determined by the integrating plate method was always higher than the value obtained by the reference method. It was strongly dependent on the ratio of light absorption coefficient to extinction coefficient of the aerosol. For weakly absorbing aerosols the integrating plate method gave excessively high values. For example, when absorption contributed 5% to the extinction, the absorption coefficient determined by the integrating plate method was too high by a factor of 2.8. Calibration factors for a variety of aerosols are given.The deviations can be explained by considering the transmission, scattering and absorption of light in the slab of deposited particles. Radiative transfer calculations show the same trend as the experimental values. It turns out that this deviation is inherent to all methods which determine the light absorption coefficient by transmission with simultaneous integration of the scattered light.

Systematic deviations of light absorption measurements by filter transmission methods

Systematic deviations of light absorption measurements by filter transmission methods