Aerosol Refractive Index Research Articles

Method for Estimation of Aerosol Parameters Based on Ground Based Atmospheric Polarization Irradiance Measurements

Method for aerosol refractive index estimation with ground based polarization measurement data is proposed. The proposed method uses a dependency of refractive index on p and s polarized down welling solar diffuse irradiance. It is much easy to measure p and s polarized irradiance on the ground with a portable measuring instrument rather than solar direct, diffuse and aureole measurements. Through theoretical and simulation studies, it is found that the proposed method show a good estimation accuracy of refractive index using measured down welling p and s polarized irradiance data with a measuring instrument pointing to the direction which is perpendicular to the sun in the principal plane. Field experimental results also show a validity of the proposed method in comparison to the estimated results from the conventional method with solar direct, diffuse and aureole measurement data.

Comparison of the Accuracy of Aerosol Refractive Index Measurements from Single Particle and Ensemble Techniques

The ability of two techniques, aerosol cavity ring down spectroscopy (A-CRDS) and optical tweezers, to retrieve the refractive index of atmospherically relevant aerosol was compared through analysis of supersaturated sodium nitrate at a range of relative humidities. Accumulation mode particles in the diameter range 300-600 nm were probed using A-CRDS, with optical tweezer measurements performed on coarse mode particles several micrometers in diameter. A correction for doubly charged particles was applied in the A-CRDS measurements. Both techniques were found to retrieve refractive indices in good agreement with previously published results from Tang and Munkelwitz, with a precision of ±0.0012 for the optical tweezers and ±0.02 for the A-CRDS technique. The coarse mode optical tweezer measurements agreed most closely with refractive index predictions made using a mass-weighted linear mixing rule. The uncertainty in the refractive index retrieved by the A-CRDS technique prevented discrimination between predictions using both mass-weighted and volume-weighted linear mixing rules. No efflorescence or kinetic limitations on water transport between the particle and the gas phase were observed at relative humidities down to 14%. The magnitude of the uncertainty in refractive index retrieved using the A-CRDS technique reflects the challenges in determining particle optical properties in the accumulation mode, where the extinction efficiency varies steeply with particle size.

Dust layer effects on the atmospheric radiative budget and heating rate profiles

The effect of mineral aerosol optical properties and vertical distribution on clear-sky, instantaneous and daily-average aerosol direct radiative effects (DREs) and heating rates (HRs) is analyzed in the solar (S, 0.3–4 μm) and terrestrial (T, 4–80 μm) spectral domain, respectively. The used radiative transfer model is based on lidar, sun-sky photometer, and radiosonde measurements. The study focuses on the Sahara dust outbreak of July 16, 2009 which advected dust particles from north-western Africa over south-eastern Italy. Clear-sky, instantaneous aerosol DREs and HRs undergo large changes within few hours, for the variability of the dust aerosol properties and vertical distribution. The daily-average, clear-sky aerosol S-DRE is near −5 Wm−2 and −12 Wm−2 at the top of the atmosphere (ToA) and surface (sfc), respectively. The daily-average aerosol T-DRE offsets the S-DRE by about one third at the ToA and by about one half at the surface. The daily average aerosol HR integrated over the whole aerosol column is 0.5 and −0.3 K day−1 in the S and T domain, respectively. Thus, the all-wave integrated HR is 0.2 K day−1. These results highlight the importance of accounting for the interaction of dust particles with T and S radiation. Sensitivity tests indicate that the uncertainties of the aerosol refractive index, size distribution, and vertical distribution have on average a large impact on aerosol HRs in the S and T domain, respectively. Refractive index and aerosol size distribution uncertainties also have a large impact on S- and T-DREs. The aerosol vertical distribution that has a negligible impact on aerosol S-DREs, is important for aerosol T-DREs. It is also shown that aerosol HRs and DREs in the terrestrial domain are affected by the water vapour vertical distribution.

Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation over Milan

Vertical profiles of aerosol number–size distribution and black carbon (BC) concentration were measured between ground-level and 500 m AGL over Milan. A tethered balloon was fitted with an instrumentation package consisting of the newly-developed micro-Aethalometer (microAeth® Model AE51, Magee Scientific, USA), an optical particle counter, and a portable meteorological station. At the same time, PM 2.5 samples were collected both at ground-level and at a high altitude sampling site, enabling particle chemical composition to be determined. Vertical profiles and PM 2.5 data were collected both within and above the mixing layer. Absorption coefficient ( b abs) profiles were calculated from the Aethalometer data: in order to do so, an optical enhancement factor ( C), accounting for multiple light-scattering within the filter of the new microAeth® Model AE51, was determined for the first time. The value of this parameter C (2.05 ± 0.03 at λ = 880 nm) was calculated by comparing the Aethalometer attenuation coefficient and aerosol optical properties determined from OPC data along vertical profiles. Mie calculations were applied to the OPC number–size distribution data, and the aerosol refractive index was calculated using the effective medium approximation applied to aerosol chemical composition. The results compare well with AERONET data. The BC and b abs profiles showed a sharp decrease at the mixing height (MH), and fairly constant values of b abs and BC were found above the MH, representing 17 ± 2% of those values measured within the mixing layer. The BC fraction of aerosol volume was found to be lower above the MH: 48 ± 8% of the corresponding ground-level values. A statistical mean profile was calculated, both for BC and b abs, to better describe their behaviour; the model enabled us to compute their average behaviour as a function of height, thus laying the foundations for valid parametrizations of vertical profile data which can be useful in both remote sensing and climatic studies.

Influences due to calibration accuracy of skyradiometer which allows aerosol refractive index and size distribution estimations together with influences due to size distribution estimation accuracy on vicarious calibration accuracy of spaceborne based visible and near infrared radiometers

Influences due to calibration accuracy of skyradiometer which allows aerosol refractive index and size distribution estimations together with influences due to size distribution estimation accuracy on vicarious calibration accuracy of spaceborne based visible and near infrared radiometers

Use of a Condensation Particle Counter and an Optical Particle Counter to Assess the Number Concentration of Engineered Nanoparticles

There is a need to evaluate nanoparticle (< 100 nm) exposures in occupational settings. However, portable instruments do not size segregate particles in that size range. A proxy method for determining nanoparticle count concentrations involves subtracting counts made with a condensation particle counter (CPC) from those of an optical particle counter/sizer (OPC), resulting in an estimation of “very fine” particles < 300 nm, where 300 nm is the OPC lower detection limit. However, to determine size distributions from which particles < 100 nm may be estimated, the resulting count of particles < 300 nm can be used as an additional channel of count data in addition to those obtained from the OPC. To test these methods, the very fine number concentrations determined using a CPC and OPC were compared with those from SMPS measurements and were used to verify the accuracy of a very fine particle number concentration determined by an OPC and CPC. Two “size-distribution” methods, weighted-average and log-probit, were applied to reproduce particle size distributions from OPC and CPC data and were then evaluated relative to their ability to accurately estimate the nanoparticle number concentrations. Various engineered nanoparticles were used to create test aerosols, including titanium dioxide (TiO 2 ), silicon dioxide (SiO 2 ), and iron oxide (Fe 2 O 3 ). These materials were chosen because of their different refractive indices and therefore may be measured differently by the OPC. The count-difference method was able to estimate very fine particle number concentrations with an error between 10.9 to 58.4%. In estimating nanoparticle number concentrations using the size-distribution methods, the log-probit method resulted in the lowest percent errors that ranged from –42% to 1023%. Percent error was lower than the instrument manufacturer's indicated level of accuracy when the test aerosol refractive index was similar to that used for OPC calibration standards. Accuracy could be increased if there was an increase in the size resolution for number concentrations measured by the CPC of very fine particles and mitigation of optical effects.

Titan haze distribution and optical properties retrieved from recent observations

The VIMS instrument onboard Cassini observed the north polar region of Titan at 113° phase angle, 28 December 2006. On this spectral image, a vast polar cloud can be seen northward to 62°N, and elsewhere, the haze appears as the dominant source of scattering. Because the surface does not appear in the wavelength range between 0.3 and 4.9 μ m , this spectro-image is ideal to study airborn scatterers both in methane bands and windows. In this work, we study this image, along with another image taken at 13° phase angle. This image probe both the atmosphere and the surface from pole to pole. First, we characterise the spatial distribution of the haze layer above 100 km between 80°S and 70°N. We find a north south asymmetry with a haze opacity increasing by a factor 3 from the south pole to the equator, then a constant value up to about 30°N and a decrease of a factor 2 between 30°N and about 60°N. Beyond 60°N, we can see the influence of the north polar cloud, even in the band, but no polar haze accumulation. The fact that the north polar region is still in the polar night is a possible explanation. No haze accumulation is observed in the southern polar region. Secondly, we partly identify the origin of spectral features in the 2.8 - μ m methane window, which are found to be due to deuterated methane ( CH 3 D ) . This allows the analyse of this window and to retrieve the opacity of scatterers layer below 80 km (presumably made of aerosols and condensate droplets) between 35°N and 60°N. Finally, we constrained the values and the spectral behaviour of the imaginary part of the aerosol refractive index in the range between 0.3 and 4 μ m . To do so, we studied the 2.8 - μ m window with the image taken at 113° phase angle. To complete the analysis, we studied the transmission through the haze layer in the 3.4 - μ m band observed in solar occultation mode with VIMS, and we analysed the single scattering albedo retrieved with DISR instrument between 0.4 and 1.6 μ m . The imaginary part of the refractive index that we find for Titan aerosols follows Khare et al. (Khare, B.N. et al. [1984]. Icarus 60, 127–137) optical constant up to 0.8 μ m and becomes constant beyond this wavelength at least up to 1.6 μ m . It also has a prominent peak at 3.4 μ m and a secondary peak at 3 μ m , which indicates material rich in C – H bonds, with much less N – H bonds than in Khare et al. (1984) tholins.

Error analysis and sensitivity analysis of the estimation method of aerosol refractive index and size distribution with ground based polarized diffuse irradiance measurement data for vicarious calibration of solar reflection channels of mission instruments onboard satellites

Error budget analysis and sensitivity analysis of the method for aerosol refractive index and size distribution estimation with ground based polarization measurement data for vicarious calibration of solar reflection channels of mission instruments onboard satellites are performed. The proposed method uses a relation between aerosol refractive index and p and s polarized downwelling solar diffuse irradiance. It is much easy to measure p and s polarized irradiance on the ground with a portable measuring instrument rather than solar direct, diffuse and aureole measurements. Through theoretical and simulation studies, it is found that the proposed method shows an acceptable estimation accuracy of refractive index and size distribution using measured downwelling p and s polarized irradiance data with a measuring instrument. It found that Degree of Polarization : DP is most sensitive to imaginary part refractive index followed by real part of refractive index, aerosol optical depth, surface reflectance and Junge parameter. It is also found that 5% of error of DP measurement is acceptable for less than 3% of estimation accuracy of aerosol refractive index and size distribution. The instrument is pointing to the several directions from the center of perpendicular to the sun in the principal plane. Field experimental results also show a validity of the proposed method in comparison to the estimated results from the conventional method with solar direct, diffuse and aureole measurement data.

Determination of particulate matter vertical columns using satellite observations

Abstract. A new technique to retrieve the particulate matter vertical columns from spaceborne observations is described. The method is based on the measurements of the spectral aerosol optical thickness (AOT). The spectral slope of the derived aerosol optical thickness is used to infer the size of particles, which is needed (along with the absolute value of AOT) to determine corresponding vertical columns. The technique is applied to the case of a cloudless atmosphere over Germany and results are compared with ground-based observations. Several assumptions are made in the retrieval process such as the prescribed phase function, single scattering albedo, the refractive index of aerosol, and the half-width of the size distribution.

Vicarious Calibration of ASTER/VNIR Onboard Terra Satellite Based on Measurements and Characterization of Aerosol Refractive Index and Size Distribution

Vicarious Calibration of ASTER/VNIR Onboard Terra Satellite Based on Measurements and Characterization of Aerosol Refractive Index and Size Distribution

Verification of satellite-derived aerosol optical thickness over land with AERONET data

The Meteorological Satellite Center (MSC) of the Japan Meteorological Agency began retrieving aerosol optical thickness over land in March 2007 using the AVHRR/NOAA visible channel. Exact nonspherical scattering with realistic particle shape distribution is fully considered. We compared the MSC optical thickness with AERONET version 1.5 optical thickness at nine AERONET stations in northeastern Asia. Although we found statistically significant correlations at all stations, the dispersions were large, especially at southern stations. We thus examined the samples with large errors at each station to investigate the causes of the errors. The following suggestions were obtained:(1) The MSC optical thickness shows overestimation for severe Yellow Sand events and underestimation for very thick gray haze. This suggests that the absorption property of aerosol particles used in our theoretical calculation is unrealistic. A revision of the complex refractive index of aerosol used in making the look-up table may be required.(2) The influence of small cumuli or cloud edges is seen in the medium range of optical thickness (< ∼2). When cloud-contaminated samples are removed by reference to MODIS high-resolution images, the correlations between the MSC and AERONET optical thickness are notably improved and are as high as 0.6 at the most of the stations. However, systematic underestimations were found at many stations.(3) These underestimations are likely to be caused by the aerosol-free assumption used in the land surface reflectance modelling because the samples on adjacent ocean grids showed excellent correlations and no systematic bias. This is confirmed by the fact that the monthly minimums of optical thickness at AERONET stations show notable positive values.

Synergetic technique combining elastic backscatter lidar data and sunphotometer AERONET inversion for retrieval by layer of aerosol optical and microphysical properties

We present a so-called lidar and almucantar (LidAlm) algorithm that combines information provided by standard elastic backscatter lidar (i.e., calibrated attenuated backscatter coefficient profile at one or two wavelengths) and sunphotometer AERONET inversion of almucantar like measurements (i.e., column-integrated aerosol size distribution and refractive index). The purpose of the LidAlm technique is to characterize the atmospheric column by its different aerosol layers. These layers may be distinct or partially mixed, and they may contain different aerosol species (e.g., urban, desert, or biomass burning aerosols). The LidAlm synergetic technique provides the extinction and backscatter coefficient profiles, particle size distributions, and backscatter-to-extinction ratios for each aerosol layer. We present the LidAlm procedure and sensitivity studies. The applications are illustrated with examples of actual atmospheric conditions encountered in the Paris area.

Seasonal statistical characteristics of aerosol optical properties at a site near a dust region in China

A ground‐based sky radiometer was used to measure direct and diffuse solar irradiances at Dunhuang, China from January 1999 to March 2001. The aerosol optical thickness (AOT), Angstrom exponent (α), volume size distributions, single scattering albedo and refractive index of aerosols were simultaneously retrieved using the “SKYRAD” inversion code and their seasonal variations and statistical characteristics were studied. The results reveal that during the study period, the AOT at Dunhuang varied seasonally, with the maximum AOT occurring in the spring and the minimum AOT occurring in the fall. The variation in α showed an opposite pattern, with a minimum in the spring and a maximum in the fall. A simple exponential function can express the relationship between AOT and α. The frequency distributions of AOT and α approximately follow a lognormal probability distribution and a normal probability distribution, respectively. The aerosol volume size distributions can be characterized by the sum of two lognormals distributions, and represent an accumulation mode with a radius of about 0.25 um, and a coarse mode with a radius of about 7.7 um. A pseudomode with a radius of about 1.69 um located between the accumulation mode and coarse mode is present in the springtime. The single scattering albedo showed a slight increasing trend with wavelength in spring, summer and autumn and a decreasing trend with wavelength in winter. The maximum value of the real part of the refractive index occurs at 400 nm, and the minimum value occurs at 875 nm for all seasons. Two Gaussian models were developed to describe the frequency distribution of the real part of the aerosol refractive index; the results indicate that the differences between the fits are greatest in the spring. The statistical characteristics of the frequency distributions of aerosol properties might provide a way to identify and estimate aerosol optical properties in areas located near dust regions.

Theoretical study on the change of the particle extinction coefficient during the aerosol dynamic processes

In this study, the effects of aerosol dynamics on the aerosol optical properties were analytically investigated using an approximated expression for the overall extinction coefficient ( b ext ) , which was valid for a wide range of particle size. The processes of both coagulation and condensation in the continuum region were considered using the moment method, with an overall extinction coefficient obtained from the harmonic mean between the overall particle extinction coefficient of the Rayleigh scattering and geometric scattering regions. The derived analytical expressions were compared with the asymptotic expressions of the extinction coefficient, which showed that the obtained expressions ultimately approached the asymptotic expressions. Subsequently, this study showed a simple and straightforward method for estimating the changes in the extinction coefficient and visibility of polydispersed aerosols during aerosol dynamic processes. The accuracy of the approximated overall coefficient was dependent on a given aerosol refractive index and size parameter. Usually, the error was less than 8% for an average size parameter range of 0–80.

Absorption properties of urban/suburban aerosols in China

The broadband diffuse radiation method is improved to retrieve the aerosol refractive index imaginary part (AIP) and broadband (400–1000 nm mean) single scattering albedo (SSA). In this method, four sets of SSA selection criteria are proposed for quality control. The method is used to retrieve AIP, SSA and absorptive optical thickness (AbOT) from routine bourly-exposed pyrheliometer and paranometer measurements over 11 sites (meteorological observatories) in China during 1998–2003. Apart from one suburban site (Ejin Qi), the other urban sites are all located around big or medium cities. As shown in the retrieval results, annual mean SSA during 1998–2003 changes from 0.941 (Wuhan) to 0.849 (Lanzhou), and AIP from 0.0054 to 0.0203. The 11-site average annual mean SSA and AIP are 0.898 and 0.0119, respectively. SSA during winter is smaller for most sites. There is an evidently positive correlation between SSA and aerosol optical thickness (AOT) for all sites. There is also a positive correlation between SSA and relative humidity for most sites, but a negative correlation for a few sites, such as Kashi and Urumqi in Northwest China.

Observations of smoke‐influenced aerosol during the Yosemite Aerosol Characterization Study: 2. Aerosol scattering and absorbing properties

Direct observations and size‐distribution‐based estimates of light‐scattering coefficients (bsp) are presented for the Yosemite Aerosol Characterization Study (YACS), which took place during the summer of 2002, an active fire year. Uncertainty in the estimates of bsp were found to be sensitive to the choice of the aerosol refractive indices, which were retrieved by aligning optical particle counter measurements to differential mobility analyzer observations and, in addition, were calculated from composition measurements. Aerosol composition during the study was dominated by organic carbon, with highest levels observed during periods impacted by biomass burning smoke influenced hazes. As a result, estimates of the aerosol refractive index from composition measurements were highly sensitive to the assumed properties of organic carbon. Retrieved and calculated refractive indices were in agreement for reasonable assumptions for properties of organic carbon. Excellent agreement (within 6%) was found between measured bsp and size‐distribution derived bsp if the imaginary component of the refractive index, determined from composition measurements and assuming all species were internally mixed, was included in the Mie calculation. Under the internally mixed assumption, aerosols sampled during the study were modeled as weakly absorbing, with computed dry single scattering albedo ranging from 0.86 to 0.94. Calculated light absorption coefficients yielded a study‐averaged dry aerosol mass absorption efficiency of 0.37 ± 0.05 m2g−1.

In situ ship cruise measurements of mass concentration and size distribution of aerosols over Bay of Bengal and their radiative impacts

Simultaneous measurements of surface level aerosol mass concentrations, their size distribution, and aerosol optical depth (AOD) were made during a ship cruise study conducted over the Bay of Bengal (BoB) between 19 and 28 February 2003, when the prevailing surface level wind flow is predominantly from the continent toward the ocean, using a ten‐stage QCM cascade impactor and Microtops Sun photometer. On all cruise days, air parcels at different altitude levels were coming either from west or from northwest directions, crossing a significant portion of the Indian subcontinent before finally reaching over BoB. Average value of surface level aerosol mass concentration is found to be around 50, 37, and 13 μg/m3 for coarse mode (&gt;1 μm), accumulation mode (between 1 μm and 0.1 μm), and nucleation mode (&lt;0.1 μm) particles, respectively. Size distribution of aerosols measured during the cruise showed the presence of four distinct modes, all of which could be fitted using lognormal distribution. Mode radii for the distributions lie in the range of 0.025–0.036 μm for mode 1, between 0.15 and 0.165 for mode 2, between 0.39 and 0.55 for mode 3, and between 2.2 and 3.5 for mode 4. Over the study region, daily mean AOD values at 380 nm were in the range of 0.34 to 0.75 while those at 1020 nm varied from 0.09 to 0.25. The mean value of Angstrom wavelength exponent α is found to be 1.19 ± 0.12. Regression analysis for the scatterplots between AOD values and surface mass concentrations showed good correlation between them over the entire cruise region. Aerosol optical depths, as well as extinction coefficients calculated from surface level aerosol number concentrations, show higher values over northern and coastal areas of BoB. An estimate of aerosol scale height has been made from the ratio of columnar AOD values and surface extinction coefficients. Columnar aerosol size distributions were derived using King's inversion technique, and the results are found to be less sensitive to the choice of aerosol refractive index values used. Independently, aerosol scale height is also estimated by taking the ratio of derived columnar number distribution and measured surface level aerosol number concentrations. AOD values observed over BoB are used in estimating the aerosol radiative forcing over this region. The Average value of the net direct radiative forcing for the region is found to be in the range of −30.6 to −26.6 W/m2 at the surface and −8.4 to −10.6 W/m2 at the TOA. Magnitude of LW forcing is found to be about 10% of its SW counterpart, at both surface and TOA. Results of a sensitivity test showed that the magnitude of LW aerosol forcing at the surface level decreases with the increase in column water vapor.

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.

The HITRAN molecular spectroscopic database: edition of 2000 including updates through 2001

This paper describes the status circa 2001, of the HITRAN compilation that comprises the public edition available through 2001. The HITRAN compilation consists of several components useful for radiative transfer calculation codes: high-resolution spectroscopic parameters of molecules in the gas phase, absorption cross-sections for molecules with very dense spectral features, aerosol refractive indices, ultraviolet line-by-line parameters and absorption cross-sections, and associated database management software. The line-by-line portion of the database contains spectroscopic parameters for 38 molecules and their isotopologues and isotopomers suitable for calculating atmospheric transmission and radiance properties. Many more molecular species are presented in the infrared cross-section data than in the previous edition, especially the chlorofluorocarbons and their replacement gases. There is now sufficient representation so that quasi-quantitative simulations can be obtained with the standard radiance codes. In addition to the description and justification of new or modified data that have been incorporated since the last edition of HITRAN (1996), future modifications are indicated for cases considered to have a significant impact on remote-sensing experiments.

Determination of Vertical Distributions of Aerosol Optical Parameters by Use of Multi-Wavelength Lidar Data

A lookup table (LUT) method is proposed to derive vertical distributions of aerosol optical parameters in the troposphere from multi-wavelength observations using a Mie-scattering lidar. A LUT is constructed by means of Mie-scattering calculations. For aerosol size distributions, we assume eleven models including the urban and maritime models as two extreme cases. For the aerosol refractive index, the imaginary part is changed between 0.0 and 0.03 in steps of 1.0×10-4, and the real part between 1.40 and 1.60 in steps of 0.01. The observed wavelength dependence of the lidar signal is compared with that of the LUT to determine the aerosol extinction coefficient, size distribution, refractive index, and the extinction-to-backscattering ratio (S1 parameter) for each altitude. We apply the method to analyze data obtained with a four-wavelength lidar (355, 532, 756, and 1064 nm) operated at Chiba University. Compared with the conventional analysis with the Fernald method, the LUT approach has the advantage that it is capable of giving detailed optical parameters simultaneously with the extinction coefficient. Besides, since the analysis starts at the near-end boundary, the effect of detection noise becomes less influential.