Properties Of Atmospheric Aerosols Research Articles

On the possible role of carbonaceous particles in the control of optical properties of fine atmospheric aerosol

On the possible role of carbonaceous particles in the control of optical properties of fine atmospheric aerosol

Polarimetric properties of aerosol particles

Retrieval algorithms for scattering particles are shown based on photopolarimeric measurements of sky light over the ocean and multiple scattering simulations of the polarization field. Polarized components of the atmospheric constituents have been measured by a photopolarimeter (named PSR1000) with spectral bands set up to correspond to the ADEOS/POLDER. The POLDER is the first sensor on board the satellite to be designed to observe polarization. It is shown that heterogeneous grains are better than homogeneous models to explain polarimetric properties of atmospheric aerosols, and a Maxwell-Garnett mixing rule for small water-soluble (WS) inclusions in an oceanic (OC) matrix is available to interpret the polarization measurements of atmospheric aerosols over the Seto Inland Sea. We also found during our observations that the value of refractive index of the aerosol, i.e., its chemical composition, varies with time and place rather than particle size.

Effects of changes in atmospheric water vapor content on physical properties of atmospheric aerosols at a coastal station

Abstract Observational results are presented on the effects of changes in the atmospheric water vapor content ( W g cm −2 ) on spectral optical depth ( τ pλ ) and the retrieved columnar size distributions of atmospheric aerosols over a natural, coastal environment obtained from simultaneous estimates of these parameters ( τ pλ and W ) using a ground based multiwavelength solar radiometer. It is found that, during meteorologically calm periods (i.e. in the absence of any significant air mass types), the aerosol optical depths increase with increase in W , with the shorter wavelengths being more sensitive to the changes in W , compared to the near infrared (IR) wavelengths. The aerosol (columnar) size distributions, retrieved from τ pλ as a function of W , are found to change from bimodal for very low values of W to unimodal for higher values of W . The mean radius and mode radii of the retrieved size distributions also increase with increase in W .

Atmospheric aerosol and water vapor characteristics over north central Canada during BOREAS

A network of five automated and two handheld solar radiometers was operated during the 1994–1996 Boreal Ecosystem‐Atmosphere Study (BOREAS) in northern Saskatchewan and Manitoba, Canada, in order to characterize the atmospheric aerosol properties. Direct solar measurements were used to measure atmospheric transmission and infer aerosol optical thickness and water vapor column abundance. Near‐Sun sky radiance measurements (solar aureole) were used to estimate the aerosol size distribution. Aerosol conditions were heavily influenced by the presence or absence of forest fires. In 1996, when few fires occurred, conditions were uniform across the region with median aerosol optical thickness (AOT) at 500 nm of 0.12 and 90th percentile values of 0.27 for the May‐October period. During the 1994 and 1995 seasons, numerous fires occurred in the vicinity of the sites. The median AOT values were comparable with the 1996 values, though the 90th percentile values were larger, in general measuring 0.85 (southern 1994 season was 0.43). Median column water vapor measurements for the same 7 month period were in the range from 1.32 to 1.58 cm at both sites, with 1995 being the driest year of observation. Winter median values of AOT and water vapor were typically 0.09 and 0.34 cm, respectively. Size distributions derived from solar almucantar measurements show the predominance of small particles during smoke episodes when compared to that for background conditions. Spectral dependence of the AOT as characterized by the wavelength exponent, α, asymptotes at 1.8 for high optical depths for a 7 month season of cloud‐screened data at the northern young jack pine site. This observed wavelength exponent for boreal biomass burning conditions is within the range of values found during the burning season in a study in Brazil.

Formation and properties of secondary atmospheric aerosol: from the laboratory to the supercomputer

Formation and properties of secondary atmospheric aerosol: from the laboratory to the supercomputer

Sensitivity of multiangle remote sensing observations to aerosol sphericity

Multiangle, multispectral remote sensing observations, such as those anticipated from the Earth Observing System (EOS) multiangle imaging spectroradiometer (MISR), can distinguish spherical from nonspherical particles over calm ocean for mineral‐dust‐like particles with the range of sizes and column amounts expected under natural conditions. The ability to make such distinctions is critical if remote sensing of atmospheric aerosol properties is to provide significant new contributions to our understanding of the global‐scale, clear‐sky solar radiation balance. According to theoretical simulations the measurements can retrieve column optical depth for nonspherical particles to an accuracy of at least 0.05 or 10%, whichever is larger. In addition, three to four distinct size groups between 0.1 and 2.0 microns effective radius can be identified at most latitudes.

Aerosol measurements in Siberia

The purpose of this study was to measure and document the properties of atmospheric aerosol in Novosibirsk region (South-Western part of Siberia). The measured properties of atmospheric aerosols were: aerosol mass and number concentrations, light scattering, aerosol size distribution, and ion composition. The results of the three-year study (1992–1994) were evaluated for obtaining the most general description of Siberian atmospheric aerosol. It was shown, that concentration of submicron aerosol is higher in winter, compared with summer. In winter, the main aerosol mass is in submicron aerosols, in summer, it is in large particles. In winter, the main part (by mass) of aerosol particles is contributed by soluble inorganic compounds of condensation origin and may be ash. In summer, the majority of aerosols are organic, biogenic and insoluble erosion particles. In summer, in Siberia, an anthropogenic influence on global fraction of atmospheric aerosol is apparent only close to the sources of pollution. In winter, much larger areas may be polluted by an industrial center.

Effect of physical properties of atmospheric aerosols on path radiance

Using a set of simplified radiative transfer equations, the effects of the changes in the physical properties (such as columnar size distribution, optical depth and refractive index) of atmospheric aerosols on the upwelling atmospheric path radiance are investigated for a nadir viewing remote sensor at various solar illumination angles in the context of developing operational methods for correcting radiances received by remote sensing satellites for atmospheric effects. It is seen that at low solar illumination angles and high haze conditions (aerosol optical depths > 0.5) the path radiance depends strongly on the nature of the aerosol size distribution, whereas for low to moderate values of aerosol optical depths ( < 0.5), the path radiance increases nearly linearly with optical depths. Spectral variations indicate a large increase in path radiance with decrease in wavelength.

Physicochemical properties of atmospheric aerosol at South UIST

Four field campaigns over the period November 1993 to August 1994 were undertaken on the island of South Uist, off the northwest coast of Scotland as part of the BMCAPE project. Measurements were made of the concentration and chemical composition of aerosol particles, utilising a variety of instruments. Sulphur and nitrogen gas species were also measured throughout these campaigns, together with appropriate meteorological parameters. A variety of air mass types were encountered during the campaigns and the relationship between the physical and chemical aerosol properties are discussed in terms of air mass histories and season factors.

Activation properties of ambient aerosol in the Netherlands

A cloud chamber has been used to study the cloud activation of ambient aerosol in The Netherlands. The large dimensions and throughput of the chamber allowed unperturbed collection of aerosol and droplets with cascade impactors and on-line measurements with cloud monitors (FSSP) inside the facility. The study provided maxima for the number of man-made aerosol acting as cloud nuclei in marine clouds in The Netherlands. Emphasis was given to the investigation of cloud formation in marine air, since sensitivity studies had shown that such clouds are most effectively influenced by the (extra) anthropogenic aerosol particles. For this reason the supersaturations in the study were low (on average 0.12%), similar to those in actual marine stratus. The effect of the anthropogenic aerosols on cloud formation was determined by comparing the number of droplets formed in “clean” arctic marine air to the number of droplets formed in “polluted” marine air (air which had travelled over the U.K.). Air masses with the total aerosol number concentration of the order of 100 cm −3 were considered as “clean” marine air. Air masses with higher aerosol concentrations were divided into “moderately” and “heavily” polluted with total aerosol concentrations of the order of 1000 and 10,000 cm −3, respectively. In the clean marine air all potential cloud nuclei (particles larger than the threshold size of the smallest reference particles that were activated at a given supersaturation) were activated and the number of cloud droplets formed was on average 45 cm −3. In the moderately polluted air 72% of potential cloud nuclei were activated and the average droplet number was 190 cm −3. The difference in the actual cloud droplet number and the number of potential cloud nuclei could be explained by the presence of water-insoluble particles which do not activate. In the heavily polluted air the average droplet concentration was around 320 cm −3. which is, on average, 24% of the number of potential cloud nuclei.

Activation properties of ambient aerosol in The Netherlands

Activation properties of ambient aerosol in The Netherlands

Aerosols and global warming

Aerosols play an important role in the radiative balance of the atmosphere. While sulphate aerosols are recognized as the dominant contributor of tropospheric aerosols over and near industrialized regions, smoke aerosols containing soot or elemental carbon are regarded with increasing importance on a global basis. The fate of carbonaceous aerosols is at present poorly understood as a result of various atmospheric processes. This paper examines the effect of morphology on the physical and chemical properties of atmospheric aerosols, in the context of fractal theory. The use of a fractal dimension to describe aggregate morphology enables more accurate modelling of sedimentation and optical characteristics.

Optical properties of atmospheric aerosols from moments of the particle size distribution

The method of moments is a powerful tool for directly calculating the lower‐order moments of an aerosol size distribution without full knowledge of the distribution itself. These moments can be tracked in space and time to represent the effects of nucleation, evaporation and growth, coagulation, and complex flowfield dynamics. This letter applies quadrature methods for efficient estimation of aerosol optical properties directly from a known moment sequence. The approach is demonstrated using particle size distributions obtained from fits to field measurements. Comparison of light‐scattering phase functions and other integral optical properties calculated from the moments with those obtained from the full distributions shows that the lower‐order moments lead to an efficient and accurate parametrization for these properties.

A model for predicting performance of an annular denuder system

A mathematical model for simultaneous gas diffusion, aerosol diffusion and evaporation is presented to predict performance of the annular denuder system. Approximating the aerosol size distribution by a lognormal function and using the statistical method of moment, the evolution of the three aerosol moments and the mass balance equations for the two associated gases in the annular denuder system are obtained. The governing equations were cast into dimensionless form and the performance of the model was validated by the existing data available in the literature. The effects of governing dimensionless groups on the model performance were evaluated in the sensitivity analysis. The model was then applied for sampling nitric acid gas using a typical range of atmospheric aerosol and gas properties to derive the performance equation of the annular denuder system. The methodologies presented herein are helpful in providing design and operational criteria of the annular denuder system for a real sampling of atmospheric reactive gases that minimizes measurement artifacts.

Columnar aerosol properties over oceans by combining surface and aircraft measurements: simulations

We report an algorithm that can be used to invert the radiance exiting the top and bottom of the atmosphere to yield the columnar optical properties of atmospheric aerosol under clear sky conditions over the oceans. The method is an augmentation of a similar algorithm presented by Wang and Gordon [Appl. Opt. 32, 4598 (1993)] that used only sky radiance, and therefore was incapable of retrieving the aerosol phase function at the large scattering angles that are of critical importance in remote sensing of oceanic and atmospheric properties with satellites. Well-known aerosol models were combined with radiative transfer theory to simulate pseudodata for testing of the algorithm. The tests suggest that it should be possible to retrieve the aerosol phase function and the aerosol single-scattering albedo accurately over the visible spectrum at aerosol optical thicknesses as large as 2.0. The algorithm is capable of retrievals with such large optical thicknesses because all significant orders of multiple scattering are included. We believe that combining an algorithm of this type with surface-based and high-altitude aircraft-based radiance measurements could be useful for studying aerosol columnar optical properties over oceans and large lakes. The use of the retrieval method is possible over the ocean because, unlike the land surface, the albedo of the ocean is low and spatially uniform.

Physical and radiative properties of Arctic atmospheric aerosols

Atmospheric aerosols sampled in the Arctic Basin by impactors have been analyzed for total aerosol and its black carbon fraction. The mass of black carbon aerosol in the lower troposphere is 1% (1 g black carbon in 100 g total aerosol), reduced to one part in 104 in the stratosphere. The aerosol single scatter albedos vary between 1.0 in the stratosphere, currently dominated by high light scattering due to Pinatubo volcanic aerosol, and 0.94 in the lower troposphere. The aerosol has the potential to regionally warm the earth-atmosphere system because of the high surface albedo of snow-covered surfaces in the Arctic.

A PC-based Mie scattering program for theoretical investigations of the optical properties of atmospheric aerosols as a function of composition and relative humidity

Abstract Over the past decade there has been interest in exploring possible relationships between atmospheric visibility (extinction of light) and the chemical form of aerosols in the atmosphere. A user friendly, menu-driven program for the personal computer (AT 286 with math co-processor or higher) has been developed which both calculates the equilibrium composition of the atmospheric aerosol of specified compositions (including particle-bound water as a function of relative humidity) and also applies optical parameters from these end-products to estimating scattering and extinction. Presented here is a description of such a program, with a discussion of the assumptions and approaches implicit in it. The program appears to give results similar to those reported by others.

The (NH&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O system: comparison of deliquescence humidities measured in the field and estimated from laboratory measurements and thermodynamic modeling

Hygroscopicity of atmospheric aerosol particles results in changes of the optical properties and chemical composition of the particles with significant effects on visibility degradation, atmospheric chemistry and possibly the global energy balance. Ambient aerosol, sampled in Riverside, California, USA, was characterized by measuring its light scattering coefficient as a function of controlled relative humidity and thermal treatment. These measurements and results from simultaneous filter samples have been compared with data from laboratory measurements and thermodynamic models for the (NH4)2SO4-Na2SO4-H2O system. Results from field experiments have indicated deliquescence humidities in the 74–79% range, after chemically neutralizing the aerosol with NH3 and preheating the sample to 190 °C. Preheating was used to remove more volatile material from the aerosol particles, such as NH4NO3 which could inhibit the occurrence of deliquescence. The measured deliquescence humidity values were less than expected for pure (NH4)2SO4, but their mean is, at the 95% confidence level, the same with the means of predictions for the deliquescence humidities determined from the phase diagram and thermodynamic modeling for the (NH4)2SO4-Na2SO4-Hs2O system. These results indicate first the importance of mixtures when examining the hygroscopic characteristics of atmospheric aerosol particles, and second the usefulness of in situ optical methods for the assessment of the chemical composition and hygroscopic properties of atmospheric aerosols.

Investigation of optical and radiative properties of atmospheric dust aerosols

Abstract A dust aerosol optical model is proposed as a result of analysis of the data obtained during the U.S.S.R.-U.S. experiment in Tadzhikistan, 1989. The model consists of spectral dependencies of the single scattering albedo, ω, the asymmetry parameter of the scattering phase function, g, and of possible values of the aerosol optical thickness, τ, for background conditions and for dust storms. This model and available published optical models of dust aerosols were used to perform simulations of radiative forcing in the dusty atmosphere. Results obtained are compared with corresponding results for Saharan aerosol. It is shown that atmospheric dust strongly decreases the total radiative balance of the underlying surface and at the same time induces general warming of the underlying surface-atmosphere system due to a decrease in the system albedo over the arid zones. The calculated heating/cooling rates in the atmosphere are analysed together with data from aerological soundings and radiation measurements.

The airborne aerosol inlet workshop: Meeting report

The measurement of atmospheric aerosol properties (i.e. concentration, size, and composition) from an airborne platform is faced by formidable obstacles. The largest problem is one of getting an undisturbed sample into the aerosol sensor sampling volume or onto a filter for subsequent analysis. The importance of such measurements led to the planning of a workshop to address the difficulties and to assess the present state of airborne aerosol sampling technology. The workshop focused on discussions of observational, experimental and theoretical studies of aerosol inlets. The product of this workshop was a summary of the limitations of sampling inlets currently used for airborne applications and a set of recommendations to overcome these limitations and improve the general quality of such measurements.