Aerosol Extinction Spectra Research Articles

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance

In combination with other parameters, the real, n(), and imaginary, k(), components of the complex refractive index, = n + ik, can be used to simulate the optical properties of a material in different forms, e.g., its infrared spectra. Ultimately, such n/k values can be used to generate a database of synthetic reflectance spectra for the different morphologies to which experimental data can be compared. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH4)2SO4, powder and derived the pellets' n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers-Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass, and density (void space), drying, grinding, sieving, and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (<50 µm) particles and pressing at ≥10 tons for at least 30 min were found key to forming highly reflective samples. Comparison of two sets of previous literature n() and k() values obtained from crystalline (NH4)2SO4 both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube shows reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH4)2SO4, also demonstrating promise to measure the optical constants of other materials.

New approach for the determination of aerosol refractive indices – Part II: Experimental set-up and application to amorphous silica particles

This article is the Part II of a work aimed at proposing a new method for determining the optical constants of aerosols. The Part I detailed the theoretical and numerical basis of an algorithm devoted to retrieve the imaginary and the real part of complex refractive indices from extinction spectra of aerosols. This algorithm associates the Mie theory, the single subtractive Kramers-Kronig relation, and an optimal estimation method in an iterative process. This Part II presents the experimental set-up developed to record simultaneously high spectral resolution extinction spectra and size distributions of airborne silica particles. Extinction spectra are measured with a high spectral resolution on a broad spectral range, including both infrared (650−2,500cm−1) and UV-visible (9,000−32,500cm−1) spectral regions. Experimental data were used to retrieve the complex refractive indices of aerosol particles. By associating the numerical procedure presented in the first paper and this experimental set-up, complex refractive indices of silica spherical aerosol particles have been determined under controlled experimental conditions. Additional comparison between experimental and simulated extinction spectra from retrieved complex refractive indices shows that this new methodology provides optical properties representative of the material.

Infrared Optical Constants of Highly Diluted Sulfuric Acid Solution Droplets at Cirrus Temperatures

Complex refractive indices for supercooled sulfuric acid solution droplets in the mid-infrared spectral regime (wavenumber range 6000-800 cm(-1)) have been retrieved for acid concentrations ranging from 33 to 10 wt % H2SO4 at temperatures between 235 and 230 K, from 36 to 15 wt % H2SO4 at temperatures between 225 and 219 K, and from 37 to 20 wt % H2SO4 at temperatures between 211 and 205 K. The optical constants were derived with a Mie inversion technique from measured H2SO4/H2O aerosol extinction spectra that were recorded during controlled expansion cooling experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The new data sets cover a range of atmospherically relevant temperatures and compositions in the binary sulfuric acid/water system for which infrared refractive indices have not been published so far, namely, the regime when supercooled H2SO4/H2O solution droplets at T < 235 K are subjected to an environment that is supersaturated with respect to the ice phase. With increasing ice supersaturation, the H2SO4/H2O aerosol particles will continuously dilute by the uptake of water vapor from the gas phase until freezing of the solution droplets eventually occurs when the acid concentration has dropped below a critical, temperature-dependent threshold value. With the aid of the new measurements, the homogeneous freezing process of supercooled H2SO4/H2O solution droplets at cirrus temperatures can be quantitatively analyzed by means of Fourier transform infrared spectroscopy, thereby overcoming a major drawback from previous studies: the need to use complex refractive indices that were measured at temperatures well above 235 K to deduce the composition of the low-concentrated H2SO4/H2O aerosol particles. As in the case of the complex refractive indices for sulfuric acid solutions with acid concentrations greater than 37 wt % H2SO4, the new low-temperature optical constants for highly diluted droplets also reveal significant temperature-induced spectral variations in comparison with the refractive indices for higher temperatures, which are associated with a change in the equilibrium between sulfate and bisulfate ions.

Infrared Spectroscopic Properties of Sodium Bromide Aerosols

The infrared extinction spectra of aqueous NaBr aerosols at ambient temperature have been measured as a function of relative humidity. Submicron-sized aerosol particles atomized from aqueous NaBr solutions at various concentrations are dried and/or mixed with nitrogen at different humidities and spectroscopically monitored as they flow through an infrared absorption cell. Estimated dry particle median diameters range from 0.24 to 0.15 microm, as calculated from Mie extinction theory. Measured deliquescence and efflorescence relative humidities (35-40% and 25-30%, respectively) are in accordance with previously reported ones. Our results show that NaBr particles take up water only moderately over the deliquescence point, with a significant increase at relative humidities above 70%. The effect of particle size onto water uptake properties has been studied, indicating that smaller particles take up lower amounts of water, and only increase their size significantly at relative humidities near saturation. Particle composition and diameter growth factors have been calculated from spectral data and are shown to be consistent with those predicted from thermodynamic data and Kohler theory. Band centers of liquid water in NaBr aerosols relative to pure water are blue-shifted up to 50 cm-1 at low humidities. Particle structure and phase, together with atmospheric implications, are also discussed.

Applying open-path Fourier transform infrared spectroscopy for measuring aerosols

This paper examines the feasibility of using Open-Path Fourier Transform Infrared Spectroscopy (OP-FTIR) to measure aerosols. The extinction spectra of water, ammonium nitrate, and ammonium sulfate aerosols were first simulated with various particle size distributions (geometric mean ranged from 2 to 10 μ m; geometric standard deviation ranged from 1.1 to 2.5) based on the Mie theory. An optimization procedure was developed to retrieve the geometric mean and standard deviation of the aerosols size distributions from the spectra, assuming that the complex refractive index is known. To test sensitivity, we also added 4%, 7%, and 10% noise levels to the spectra and compared the reconstruction results. In the experimental study, water aerosols were generated by a two-fluid nozzle inside a cylindrical chamber (3325 cm3). The extinction spectrum was collected with a modified FTIR and the size distribution information was retrieved following the same optimization procedure as the one used in the simulation study. The optimization procedure developed in this study reconstructed the size distribution reasonably well for particles with known refractive index (i.e. homogeneous or internally mixed aerosols). The results were robust with the added noise levels up to 10%, after removing inaccurate estimates with the use of the censoring criteria for reconstructed GSD < 1.3, reconstructed GM < 2.5 μ m and GSD < 1.5, and reconstructed GM > 10 μ m. With regard to externally mixed aerosols, the reconstructed results were sensitive to the noise within the measuring systems, although most ambient aerosols were internally mixed. The reconstructed size distribution in the chamber experiment had a GM of 3.85 μ m and GSD of 1.70. The simulation results were applied to support this reconstruction result. We conclude that OP-FTIR can be used to measure aerosols and screen for the right region for a more detailed aerosol measurement campaign.

Choosing a suitable analytical model for aerosol extinction spectra in the retrieval of UV/visible satellite occultation measurements

In the retrieval of satellite occultation experiments the optical extinction by atmospheric gases can be adequately modeled as the product of an extinction cross section with the number density. For aerosols the situation is more complex. Usually, the extinction spectrum is modeled by some analytical function of wavelength, controlled by a small number of parameters. However, a consensus about which function to use does not exist. The goal of this paper is to find out which analytical function is most suitable and how many parameters are necessary to capture the background aerosol extinction behavior. To do this, we first retrieve atmospheric constituent altitude profiles from a set of simulated spectra, measured by a virtual instrument of which the characteristics are based on the ones from the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument. Several aerosol extinction models are used in the retrieval. The effect of the aerosol model choice on the retrieval error and bias is then calculated by comparing the retrieved profiles with the ones that were used in the simulation. The results show that the use of a second‐degree polynomial leads to good retrievals of aerosol extinction coefficients, while simpler models already give very acceptable results for the other species.

Frequency Dependent Complex Refractive Indices of Supercooled Liquid Water and Ice Determined from Aerosol Extinction Spectra

Complex refractive indices of supercooled liquid water at 240, 253, 263, and 273 K, and ice at 200, 210, and 235 K in the mid infrared from 460 to 4000 cm(-1) are reported. The results were obtained from the extinction spectra of small (micron-size) aerosol particles, recorded using the cryogenic flow tube technique. An improved iterative procedure for retrieving complex refractive indices from extinction measurements is described. The refractive indices of ice determined in the present study are in good agreement with data reported earlier. The temperature region and range of states covered in the present work are relevant to the study of upper tropospheric and stratospheric aerosols and clouds.

Retrieval of aerosol refractive index from extinction spectra with a damped harmonic-oscillator band model

A new method for the retrieval of the spectral refractive indices of micrometer-sized particles from infrared aerosol extinction spectra has been developed. With this method we use a classical damped harmonic-oscillator model of molecular absorption in conjunction with Mie scattering to model extinction spectra, which we then fit to the measurements using a numerical optimal estimation algorithm. The main advantage of this method over the more traditional Kramers-Kronig approach is that it allows the full complex refractive-index spectra, along with the parameters of the particle size distribution, to be retrieved from a single extinction spectrum. The retrieval scheme has been extensively characterized and has been found to provide refractive indices with a maximum uncertainty of approximately 10% (with a minimum of approximately 0.1%). Comparison of refractive indices calculated from measurements of a ternary solution of HNO3, H2SO4, and H2O with those published in J. Phys. Chem. A 104, 783 (2000) show similar differences as found by other authors.

Infrared complex refractive indices for nopinone

Complex refractive indices for nopinone have been inverted from the mid‐infrared spectra of laboratory generated aerosols. The particles were formed via homogeneous nucleation, injected into a laminar flow cell, and analyzed by Fourier‐transform infrared spectroscopy. The inversion was initiated by using a thin‐film spectrum of nopinone to obtain an estimate of the imaginary component of the frequency‐dependent refractive indices. The corresponding real components were then determined through the Kramers‐Kronig transformation. The refractive indices were then used in a weighted Mie scattering calculation to fit nopinone aerosol extinction spectra that showed evidence of scattering. The inversion results were averaged together to produce the data set reported here from 750 to 5000 cm−1. The nopinone refractive indices are also compared to those reported for carvone, a biosynthesized keto‐monoterpene.

Infrared Optical Constants for Carvone from the Mie Inversion of Aerosol Extinction Spectra

Frequency-dependent, complex refractive indices for carvone in the mid-infrared from 750 to 5000 cm-1 have been inverted from the Fourier transform extinction spectra of laboratory-generated aerosols recorded at room temperature. The refractive indices obtained in this analysis are in good qualitative agreement with the results of a previous thin-film study of carvone. However, a full quantitative assessment of this agreement cannot be made as tabulated data for the previous study are no longer available. The refractive indices, or optical constants, reported here add to the small number of frequency-dependent refractive index data sets for other organic compounds that are available in the literature. Such data can be used to elucidate the optical properties of a substance, which are of critical importance in the interpretation of remote sensing data and in the evaluation of how atmospheric particulate matter consisting of organic compounds may affect climate change.

The Nitric Acid Hydrates: Ab Initio Molecular Study, and RAIR Spectra of the Solids

The molecular complexes formed by nitric acid with one, two, and three water molecules have been investigated by high level ab initio calculations. The B3LYP/aug-cc-pVTZ method and basis set have been used to derive equilibrium geometries and binding energies and to predict the infrared spectra of these complexes. It has been found that the complexes become stable through the formation of several hydrogen bonds. The main one is established between the acid, which acts as a donor, and the first water molecule, but further bonds are created between successive water molecules, with a weaker bond making a closed structure between the last water unit and the acid. Films of nitric acid mono-, di-, and trihydrates (NAM, NAD, and NAT) have been prepared by deposition on a cold surface and annealing to induce crystallization. RAIR spectra have been taken of these films. The spectra are compared to previous investigations, and the assignments of the spectra and the structure of the crystals are discussed. A global good agreement is obtained with previous transmission and RAIR spectra of thin films and extinction spectra of aerosols.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. 1. Analysis of aerosol extinction spectra from the AMON and SALOMON balloonborne spectrometers.

Aerosol extinction coefficients have been derived in the 375-700-nm spectral domain from measurement in the stratosphere since 1992, at night, at mid- and high latitudes from 15 to 40 km, by two balloonborne spectrometers, Absorption par les Minoritaires Ozone et NO(chi) (AMON) and Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NO(chi) (SALOMON). Log-normal size distributions associated with the Mie-computed extinction spectra that best fit the measurements permit calculation of integrated properties of the distributions. Although measured extinction spectra that correspond to background aerosols can be reproduced by the Mie scattering model by use of monomodal log-normal size distributions, each flight reveals some large discrepancies between measurement and theory at several altitudes. The agreement between measured and Mie-calculated extinction spectra is significantly improved by use of bimodal log-normal distributions. Nevertheless, neither monomodal nor bimodal distributions permit correct reproduction of some of the measured extinction shapes, especially for the 26 February 1997 AMON flight, which exhibited spectral behavior attributed to particles from a polar stratospheric cloud event.

Ternary H2SO4/HNO3/H2O Optical Constants: New Measurements from Aerosol Spectroscopy under Stratospheric Conditions

We present here the first measurements of the infrared complex refractive indices for supercooled ternary solutions of H2SO4, HNO3, and H2O under stratospheric conditions. The data sets were retrieved directly from the infrared extinction spectra of laboratory-generated aerosols. Comparisons of these results with a previously reported empirical mixing rule model1 reveal significant differences, which we trace to errors in the binary acid/water thin film refractive indices used in the model. In addition, the model does not properly account for the concentrations of ionic and neutral species in the ternary solutions. The experimentally determined optical constants reported here will be useful for determining polar stratospheric cloud aerosol properties from infrared spectroscopic remote sensing measurements.

Infrared complex refractive indices of supercooled liquid HNO3/H2O aerosols

Complex refractive indices have been retrieved for the first time directly from the Fourier transform infrared extinction spectra of supercooled liquid HNO3/H2O aerosols at 220 K. Aerosol compositions were determined by simultaneously monitoring equilibrium HNO3 and H2O vapor pressures with a tunable diode laser. The new optical constants are compared with previous results from room temperature bulk solutions and &lt;160 K amorphous solid thin films, and the differences are explained primarily in terms of the temperature‐dependent dissociation of molecular HNO3 into its constituent ions. Mie scattering analysis of aerosol extinction spectra suggests that using either the room temperature liquid or the amorphous solid refractive indices may result in inaccurate particle size determination. These new data sets are directly applicable to the characterization of polar stratospheric clouds through spectroscopic remote sensing.

A new approach to retrieval of aerosol size distributions and integral properties from SAGE II aerosol extinction spectra

A new approach to the retrieval of aerosol size distributions and integral properties from Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction spectra is proposed. This method assumes that the aerosol size distribution can be approximated by a histogram of number density as a function of particle size. The retrieved number density in each bin is expressed as a linear combination of the SAGE II aerosol extinctions at four or fewer wavelengths. The coefficients in the weighted linear combination are obtained by minimizing the retrieval error averaged for a set of testing size distributions. The same method has been applied to retrieve aerosol surface area and volume densities from SAGE II aerosol extinctions. The retrieval accuracy was studied by calculating the aerosol surface area and volume densities for six aerosol size distributions retrieved by using the proposed linear minimizing error method. In general, the retrieval error increases with the decreasing number of wavelength‐dependent extinction measurements available for retrieval. Besides accuracy, the proposed method has the advantage of not requiring an initial guess or a weighting function. Furthermore, it is very simple and fast. Another advantage is that the proposed method can still be applied to the case when the number of reliable aerosol extinction is less than four. The formulas presented in this paper can be used easily by investigators to retrieve surface area and volume densities from SAGE II aerosol extinction spectra at different altitudes for a variety of purposes. The proposed new method can also be extended to retrieve properties from other remote sensing systems. The application of the proposed method for instrument design is discussed.

Infrared and visible Fourier-transform spectra of sulfuric-acid–water aerosols at 230 and 294 K

The extinction spectra of aqueous sulfuric acid aerosols fully covering the mid-IR to visible regions from 750 to 23,000 cm(-1) (13.9-0.4 microm) have been measured in the laboratory with a Fourier-transform spectrometer. Both large and small aerosol particles with compositions of approximately 60-70-wt. % H(2)SO(4) were generated and their spectra recorded at 230 and 294 K. The spectra were fitted to a model incorporating room-temperature refractive-index data [Appl. Opt. 14, 208 (1975)] and Mie theory calculations to characterize the composition and size distributions of the aerosol samples.

Isosbestic point: An application for aerosol spectrometry

When only two substances, in chemical equilibrium with each other, are responsible for all absorption in a wavelength region, there must exist at least one wavelength in the spectrum of this region where the absorption coefficient will be independent of the ratio of the concentrations of the two substances. This wavelength is called as the isosbestic point, or point of equal absorbance. Such points can also exist in the extinction spectra where at a particular wavelength the extinction coefficient becomes independent of solution concentration. Mie theory has been applied to compute such extinction coefficients. This paper shows the occurrence of isosbestic points in the extinction spectra of liquid droplet aerosols of sizes ranging from 0.1 μm to 10 μm and comprising several concentrations each of hydrochloric, nitric, sulfuric, and orthophosphoric acids in water. Such isosbestic points can serve as reference wavelengths for aerosol spectrometry and also for estimation of liquid content in haze, fog, and nonraining clouds.

Early results from improved limb atmospheric spectrometer (ILAS) measurements

Improved Limb Atmospheric Spectrometer (ILAS), which was developed by Environment Agency of Japan, was a satellite‐borne sensor to observe chemical and dynamical environment of the stratospheric ozone layer from space. ILAS had been normally operating on board the Advanced Earth Observing Satellite (ADEOS) from the beginning of November 1996 to the end of June 1997 when the sudden failure of ADEOS solar battery system happened. Using the solar occultation technique, the ILAS measured vertical profiles of ozone (03) and ozone‐related species such as nitric acid (HNO3), nitrogen dioxide (NO2), nitrous oxide (N2O), methane (CH4), and water vapor (H2O). It also measured vertical profiles of atmospheric temperature and pressure as well as aerosol extinction coefficient at 780 nm. Extinction coefficients in the infrared wavelength region were also derived and used to infer aerosol types and extinction spectra over the whole infrared channels. ILAS‐derived data will be used for better understanding of the stratospheric ozone layer changes.

Infrared spectroscopy of model tropospheric aerosols as a function of relative humidity: Observation of deliquescence and crystallization

The infrared extinction spectra of model tropospheric aerosols, (NH4)2SO4, NH4HSO4, NaCl, and artificial seawater, have been measured as a function of relative humidity. Experimentally, submicron‐sized aerosol particles are spectroscopically monitored as they flow at atmospheric pressure on a 30‐s timescale through a room temperature infrared absorption cell. By monitoring absorption features due to either constituent ions or water molecules, we infer both the physical phase and, to some degree, the chemical composition of the aerosol particles. It is observed that (1) solid (NH4) SO4 and NaCl aerosol particles exhibit deliquescence at 79±1% and 75±1% relative humidity, respectively, very close to their thermodynamic values; (2) (NH4)2SO4 and NaCl liquid particles exhibit crystallization at relative humidities of 33±2% and 43±2%, respectively, well below their deliquescence points; (3) NH4HSO4 aqueous aerosol particles remain in the liquid state to relative humidities as low as 2%, far below the thermodynamic deliquescence humidity of 39%; and (4) artificial seawater aerosol particles show strong H2O absorption features at low relative humidities, arising either because the particle has not crystallized or because solid hydrates of Mg2+ salts have formed. These observations illustrate the extent to which water will be present in the aerosol condensed phase in both laboratory experiments and in the atmosphere. Specifically, for (NH4)2SO4 and NaCl particles, the water content is expected to be low at relative humidities below the crystallization point, whereas the aerosol particles will be liquid at higher relative humidities. NH4HSO4 and artificial seawater aerosols will both contain significant quantities of water down to very low relative humidities, present either as a liquid or possibly as hydrates of Mg2+ in the case of artificial seawater. By adding gas‐phase D2O to NH4HSO4 and artificial seawater aerosols at low relative humidity, condensed phase D2O features appear in the spectra, indicating facile exchange of water between the gas‐phase and the particles. Conversely, aerosols with low water content, such as solid NaCl, do not exhibit condensed‐phase D2O features in the presence of gas‐phase D2O.

Determination of the layer thickness of submicron coated aerosol particles from optical extinction measurements

This paper describes a method to determine the optical properties of submicron-coated aerosol particles with metallic core. The particles were prepared using aqueous suspensions of colloidal metal particles. After spraying of the suspensions and drying the spray, the residual aerosol particles consisted of a metallic core of well-defined size and a dielectric salt shell of unknown thickness, which formed during the drying process. It was shown that the thickness of the dielectric shell could be determined within an accuracy of 1 nm from evaluation of optical extinction measurements on the aerosol particles in the wavelength range between 300 and 1000 nm. Furthermore, the contribution of salt particles to the aerosol extinction spectra could be resolved from the contribution of the coated particles. For measurement in this spectral region, a White cell with 32 m path length was constructed and combined with a multi-channel spectrometer.