Refractive Index Of Aerosol Particles Research Articles

Multi-wavelength radiometry of aerosols designed for more accurate night sky brightness predictions

Scattering by aerosols and gases cause a certain fraction of artificial light emitted upwards is redirected to the ground. Of all atmospheric constituents just the aerosols are most important modulators of night-sky brightness under cloudless conditions. Unlike most of the previous we highlight a crucial role of solar radiometry for determining the atmospheric optical depth before night-time observation is to be made. Aerosol optical depth at visible wavelengths extracted from the data measured provides then the information on size distribution or mean refractive index of aerosol particles that in turn are both necessary to make night sky brightness prediction more accurate. Therefore, combining daytime and night-time radiometry we can achieve accuracy much higher than ever before. This is due to significantly reduced uncertainty in aerosol properties.The aerosol data are retrieved from a new portable multi-wavelength optical analyzer that operates Ocean Optics spectrometer. The equipment provides the radiance data from 350 nm to 1000 nm with spectral resolution of 1 nm. Due to high sun radiance levels we use a system of mirrors each reducing the signal to about 4%, while keeping the integration time short. The minimum integration time of 3 ms allows for detection of direct sunlight. The system developed is sensitive to small changes in the aerosol system, while showing a good detection limit even under low turbidity conditions. The system performance is demonstrated in field experiment conducted shortly after front passage when most of aerosol particles is effectively removed by rain.

On Possible Changes in the Physical Characteristics of the Aerosol in the Deep Layers of the Atmosphere of Saturn

This study is dedicated to determining the values of the physical parameters of cloud particles in the deep layers of Saturn’s atmosphere using the data from remote measurements of the solar radiation field diffusely reflected by the giant planet. In the previous studies, using the effective optical depth method, from the spectral measurements of the geometric albedo of Saturn in 1993 in the wavelength range of 300–1000 nm, a pressure dependence of the aerosol scattering component of the optical depth, i.e., a change in its value with altitude in the atmosphere, was obtained. The analysis of the initial data was performed in the long-wave part of the spectrum in the methane absorption lines of different levels of absorption with the centers at wavelengths of λ = 619, 727, 842, 864, and 887 nm. At certain altitude levels in the deep layers of the atmosphere of the giant planet, the indicated dependence shows the features that possibly reflect changes in the physical characteristics of aerosol. Therefore, the aim of this work was to determine the possible values of the physical parameters of aerosol particles in the deep layers of Saturn’s atmosphere at altitudinal levels with the features noted above. As a result, an increase in the effective radius of cloud particles was observed in the transition from the outer to the deeper layers of the atmosphere: from 1.4 μm in the upper part of atmosphere to 1.83 μm in the altitudinal region with a pressure of 1.0–1.25 bar and up to 2.2–2.4 μm in the region with a pressure of 1.5–2.0 bar. In the latter segment, a decrease by 3.5% was revealed in the real part of the refractive index of aerosol particles. A possible reason for this decrease is the change in the phase state of aerosol particles in the lower and warmer atmospheric layers of Saturn due to the presence of ammonium hydroxide in their composition at a sufficient high concentration.

Comparing aerosol refractive indices retrieved from full distribution and size- and mass-selected measurements

Refractive index retrievals (also termed inverse Mie methods or optical closure) have seen considerable use as a method to extract the refractive index of aerosol particles from measured optical properties. Retrievals of an aerosol refractive index use one of two primary methods: 1) measurements of the extinction, absorption and/or scattering cross-sections or efficiencies of size- (and mass-) selected particles for mass-mobility refractive index retrievals (MM-RIR) or 2) measurements of aerosol size distributions and a combination of the extinction, absorption and/or scattering coefficients for full distribution refractive index retrievals (FD-RIR). These two methods were compared in this study using pure and mixtures of ammonium sulfate (AS) and nigrosin aerosol, which constitute a non-absorbing and absorbing material, respectively. The results indicate that the retrieved complex refractive index values are correlated to the amount of nigrosin in the aerosol but can be highly variable with differences in the real and imaginary components that range between –0.002 and 0.216 and –0.013 and 0.086; the average and standard deviation of the differences are 0.046 ± 0.046 and 0.023 ± 0.033, respectively. Forward calculation of the optical properties yielded average absolute values of the relative deviation of ≈ 15% and ≈ 26% for FD-RIR data using the MM-RIR values and contrariwise. The range of retrieved refractive indices were used to calculate the normalized global average aerosol radiative forcing of a model accumulation mode remote continental aerosol. Deviations using the refractive indices of the pure materials range from 9% to 32% for AS and 27% to 45% for nigrosin. For mixtures of nigrosin and AS, deviations were all > 100% and not always able to capture the correct direction of the forcing; i.e., positive versus negative.

A comparison of values of the imaginary part of refractive index of aerosol particles in the latitudinal belts of Northern hemisphere of Saturn

A comparison of values of the imaginary part of refractive index of aerosol particles in the latitudinal belts of Northern hemisphere of Saturn

On the possibility of changing of radius and imaginary part of the refractive index of aerosol particles in the deep layers of Jovian’s atmosphere

On the possibility of changing of radius and imaginary part of the refractive index of aerosol particles in the deep layers of Jovian’s atmosphere

On the possibility of determining the imaginary part of the complex refractive index of aerosol particles in an individual altitudinal cloud layer of Jupiter’s atmosphere

We suggest the method for determining the imaginary part n i of the complex refractive index of aerosol particles forming a cloud layer at a specified altitude in the atmosphere of a giant planet. From the data of spectral measurements of the geometric albedo of Jupiter (carried out in 1993), the value of n i was calculated for the whole atmospheric column and the pressure range of 0.52 to 0.78 bar in the cloud layer presumably composed of ammon i um hydrosulfides. The values of n i obtained for the cloud layer and the whole atmospheric column substantially differ and amount to 0.00098 and 0.00012, respectively.

On the possibility of separation of aerosol and methane absorption in the long-wavelength spectral range for giant planets

To determine the aerosol absorption component of the effective optical depth τeffκ, a special method has been developed. It is based on the implausible effect of the decrease of the aerosol scattering component τeffa in dependence on the atmospheric pressure P, if τeffκ = 0. It is the elimination of this effect by selecting the values of τeffκ that allows the latter to be determined. This method was tested with the data of observations of the North Equatorial Belt (NEB) of Jupiter’s disk in the methane absorption bands at λ = 619 and 727 nm. This allowed us to determine the values of τeffκ and the imaginary part of the refractive index of aerosol particles (ni = 0.00063, 0.00065, 0.0007, and 0.00069 at λ = 605.5, 631.3, 714.7, and 741.4 nm, respectively) and to correctly specify the characteristic of the vertical structure of clouds of the planet.

Aspect ratio as a function of particle radius: Inversion of extinction and scattering data

The sun and sky photometry made concurrently or, alternatively, simultaneous measurements of extinction and scattering data both represent a valuable tool for gathering the information on aerosol particles. Most typically the size distribution and/or refractive index of aerosol particles can be inferred from multispectral and/or multiangle optical data. Extraction of size-dependent aspect ratio of aerosol particles from optical data is a highly non-trivial task since the kernel of the particular integral equation is a non-linear function of the sought solution. The iterative solution to this problem is introduced and demonstrated on synthetically generated data. It is shown that retrieval of size-dependent aspect ratio is possible even for complex morphologies, most typically for irregularly shaped dust particles.

Experimental investigation on optical constants of aerosol particles

Based on the experimental infrared spectral transmittances, an inverse model for determining the optical constants of aerosol particles is presented in this paper. Combined with the Mie theory and Kramers–Kronig (K–K) relations, the complex refractive indices of the aerosol particles are retrieved by the spectral transmittance distribution of a cloud of aerosol particles in potassium bromide (KBr) pellets. Particular attention is given to the unique value problems of the complex refractive index. According to calculation and analysis, the uniqueness range of the complex refractive index is obtained. The complex refractive indices of SiO 2, Fe 2O 3, CaSO 4, Al 2O 3 particles or their mixtures with different volume fractions are obtained. At the same time, the retrieved effective refractive indices of mixed particles are compared with those from effective medium theory. Good agreement is obtained between the inverse simulation results and the results from effective medium theory. Finally, the complex refractive index of aerosol particles collected in China is determined by the inverse model used in the present work.

On the comparison of the average arrival directions of air masses in the Moscow region versus the average Hanel parameters and average particle refractive indices

In January–July 2008, based on spectropolarimetric measurements, we obtained a time series of diurnally averaged values of the Hanel parameter, the aerosol particle refractive index, and the total aerosol particle volume. It is shown that when the air masses change upon the passage of atmospheric fronts, the latter two quantities vary synchronously and in antiphase. For the entire observation series, we obtained an array of two-day back trajectories for the Zvenigorod Scientific Station (ZSS). We analyzed the relation between the air mass arrival direction and the character of the variations of the Hanel parameter χ and particle refractive index n. It is shown that on average, minimum n values are characteristic of aerosol-rich air masses arriving in the Moscow region from the south, namely, from the Rostov region, the Caucasus, and Turkey. Maximum n values are recorded in maritime Arctic masses arriving from the north and north-northwest, in particular, from the North Atlantic and Barents Sea. Maximum average χ values (≈0.6) were observed in continental masses, having arrived from near-Caspian regions, and minimum values (about 0.35) were observed in Arctic masses. For air masses in all western bearings, the average χ value turned out to be approximately constant at about 0.45. There is no relation between the n and χ variations for both synchronously measured separate values and for average values.

Aerosol properties from MIRAI research cruise in the tropical pacific ocean

Aerosol parameters are observed with a photopolarimeter (PSR-1000 device) and by SeaWiFS sensor during two research cruises conducted by R/V MIRAI in the equatorial Pacific regions. Observation of aerosol particles has shown the background characteristics of aerosol particles. The aerosol optical thickness calculated from PSR-1000 data coincide well with those from SeaWiFS data at 0.865 μm and shows difference at 0.443 μm. It is also shown that the data with low aerosol optical thickness has an ambiguity in the calculation of the Angstrom exponent for various sets of wavelengths. The refractive index of aerosol particles is retrieved by combining data from direct solar measurements and those from polarization measurements of scattered sun-light.

Refractive index of aerosol particles over the Amazon tropical forest during LBA-EUSTACH 1999

Optical properties of aerosol particles were characterized during two field campaigns at a remote rainforest site in Rondônia, Brazil, as part of the project European Studies on Trace Gases and Atmospheric Chemistry, a contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). The measurements included background (wet season), biomass burning (dry season), and transition period conditions. Optical measurements of light scattering and absorption were combined with data on number/size distributions in a new iterative method, which retrieves the effective imaginary refractive index of the particles at a wavelength of 545 nm . For ambient relative humidities lower than 80%, background aerosols exhibited an average refractive index of 1.42−0.006 i. Biomass burning aerosols displayed a much larger imaginary part, with an average refractive index of 1.41−0.013 i. Other climate-relevant parameters were estimated from Mie calculations. These include single-scattering albedos of 0.93±0.03 and 0.90±0.03 (at ambient humidity), asymmetry parameters of 0.63±0.02 and 0.70±0.03, and backscatter ratios of 0.12±0.01 and 0.08±0.01 for background and biomass burning aerosols, respectively.

Case Studies of Aerosol Retrievals over the Ocean from Multiangle, Multispectral Photopolarimetric Remote Sensing Data

To evaluate the global effects of aerosols on the direct radiative balance, tropospheric chemistry, and cloud properties of the earth’s atmosphere requires high-precision remote sensing that is sensitive to the aerosol optical thickness, size distribution, refractive index, and number density. This study uses the multiangle 0.41-, 0.55-, 0.865-, and 2.25-mm channel data from the airborne Research Scanning Polarimeter to retrieve aerosol properties over the Pacific Ocean. It is shown that such photopolarimetric data are highly sensitive to the size distribution and refractive index of aerosol particles, which reduces the nonuniqueness in aerosol retrievals using such data as compared with less comprehensive datasets. Moreover, it is found that polarized reflectances obtained at the shorter wavelengths (0.41 and 0.55 mm) are significantly less sensitive to the contribution of the ocean’s upwelling light than total reflectance measurements, providing a natural tool for the separation between the estimation of oceanic and atmospheric scattering properties.

Optical characterization of size separated aerosol particles of different composition and morphology with a polar nephelometer

A new technique for the determination of the complex refractive index of aerosol particles has been developed. The particles are deposited on a membrane filter by means of an aerosol inertial spectrometer, in order to have fixed and size-separated particles. The filter is then made transparent by exposure to acetone vapours and the sample inspected with a laser beam. Scattering data are then analysed and the complex refractive index is obtained. Data and results are presented for a number of materials.

Dependence of the power of light scattered by aerosol particles on their refractive index

Calculations are reported of the influence of the real and imaginary parts of the refractive index of aerosol particles on the power of light scattered into a specific solid angle.

The optical constants of atmospheric aerosol particles in the 7.5-12 μm spectral region

Thin films of atmospheric aerosol particles which have been collected by an automatic jet impactor are used to obtain the real and imaginary part of the mean complex refractive index of aerosol particles of various types in the 7.5–12 ?m spectral region. A dispersion analysis of the measured ir-spectra of absorption of the particle films is performed. By applying the Lorentz theory of continuous dielectrics to the particle films a mathematical relation between the optical constants of the particle films and those of the aerosol particles themselves is obtained. The resulting complex refractive indices are applied to Mie computations to indicate the importance of scattering and absorbing aerosol particles to the radiative heat transfer in the longwave window of the atmosphere. DOI: 10.1111/j.2153-3490.1976.tb00675.x

The optical constants of atmospheric aerosol particles in the 7.5-12 pm spectral region

Thin films of atmospheric aerosol particles which have been collected by an automatic jet impactor are used to obtain the real and imaginary part of the mean complex refractive index of aerosol particles of various types in the 7.5–12 ?m spectral region. A dispersion analysis of the measured ir-spectra of absorption of the particle films is performed. By applying the Lorentz theory of continuous dielectrics to the particle films a mathematical relation between the optical constants of the particle films and those of the aerosol particles themselves is obtained. The resulting complex refractive indices are applied to Mie computations to indicate the importance of scattering and absorbing aerosol particles to the radiative heat transfer in the longwave window of the atmosphere. DOI: 10.1111/j.2153-3490.1976.tb00675.x

Calculations about the systematic error of visibility-meters measuring scattered light

Instruments determining the visibility from scattered light give different results, depending on the differing size distribution and refractive index of the atmospheric aerosol particles. Sixteen instruments are tested in a reproduceable way by calculating the differences, which would occur over a great variety of size distributions and refractive indices. Two hundred and thirty seven size distributions found at different times and places around the world are used. All instruments show a small jump in scale reading at the change-over from haze to cloud or fog, which occurs between about 0.6 and 1 km standard visibility. The measured values of the integrating nephelometers vary by a factor of 1.26 and less, around an average value for haze or cloud and fog. The results from backscatter-instruments that measure at a scattering angle of exactly 180° vary by a factor of 1.44 (Lidar: 1.68) and less but only when the refractive index of aerosol particles stays constant. Variations in the refractive index lead to much larger deviations. Backscatter-instruments that do not measure at exactly 180° give a larger range of values around a different average even when the refractive index remains constant. Values derived experimentally often show a smaller spread than those derived from our calculations. The reasons for this discrepancy can only be guessed at. The ratio of the backscattering coefficient to the scattering coefficient for the monostatic Rb-lidar systems lies between 0.013 and 0.036 for haze and between 0.035 and 0.083 for clouds and fog. The values are similar for Nd-lidar systems. The values from backscatter-instruments that do not measure at exactly 180° are up to an order of magnitude smaller for clouds and fog.