Optical characteristics of atmospheric aerosols

Techniques have been developed to make point measurements of particle size, chemical nature, scattering and absorption extinction coefficients. These measurements have been shown to be sufficient to describe the optical or visual effects of trace materials in urban or rural air. These techniques and methods of analysis are described in this document. Conclusions include: scattering extinction and fine particle mass, absorption extinction and graphitic carbon are highly correlated. SO²⁻₄ is usually the dominant scattering species and it occurs both in acid and neutral salt (with NH⁺₄) forms. The role of organic carbon, especially in rural atmospheres appears small.     

The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant

Instrumented aircraft have been used to determine the rate of oxidation of sulfur dioxide to sulfate aerosols within the dispersing plume of a coal-fired power plant. Measurements have been made in the stack and from 10 to 105 km downwind of a 2600-MW power plant. Sampling experiments were conducted during the winter of 1975–1976 and in the fall of 1976. Average temperatures and relative humidities varied from −5 to 17°C and 22 to 71% respectively. Data were also collected on sulfate production at night.An average of 1.1% and a maximum of 4.3% of the plume sulfur was in the form of sulfate. Most of the atmospheric oxidation that was observed appeared to occur in the immediate vicinity of the power plant. The average oxidation rate, beyond 10km, was found to be 0.2% h⁻¹. Analyses of individual particles showed that the sulfur found in plume particles is usually associated with submicron fly ash particles.     

Application of an SO2-denuder for continuous measurement of sulfur in submicrometric aerosols

A method is described for the continuous measurement of total sulfur in submicrometric aerosols suspended in air containing sulfur dioxide. The aerocolloid is passed through a tube coated internally with lead dioxide. The gaseous sulfur dioxide diffuses to the surface of the tube and reacts irreversibly to form lead sulfate. The aerosol is not significantly removed in the tube. The total sulfur in the aerosol is determined by a hydrogen-air flame photometric detector. A sulfur balance has been demonstrated for the sulfur dioxide-ozone-olefin reaction system, which produces aerosols containing sulfur.     

Oxidation of sulfur dioxide in aqueous ammonium sulfate aerosols containing manganese as a catalyst

The pseudo-first order reaction rate coefficient for the oxidation of SO₂ in air-borne water droplets containing (NH₄)₂SO₄ and MnSO₄ H₂O has been evaluated in a laminar gas flow apparatus. The sulfate and bisulfate formed in the reaction were detected using ³⁵S as a radioactive tracer. Droplet diameters ranged from 1 to 10 μm. The gas phase SO₂ concentrations ranged from 0.1 to 1.0 ppm. Initial measurements yielded a value of the rate coefficient as defined in Equation (2) of about 0.04s⁻¹ for the Mn²⁺ catalyzed reaction system at a manganese concentration of about 0.1 mol dm⁻³, at a temperature of 25°C, and at a relative humidity of 90%.     

Chemical characteristics of atmospheric aerosols over southwest coast of India

Ambient aerosol samples, collected from Mangalore region in the southwest coast of India during the period of late winter (February and March) to early summer (April and May), have been analysed for water-soluble ionic species. Their abundance pattern is dominated by HCO₃⁻, SO₄²⁻, Na⁺, Cl⁻, with minor contribution from NO₃⁻, Ca²⁺, NH₄⁺, K⁺ and Mg²⁺ indicating the contribution from not only sea salt, but also from anthropogenic and dust sources; with pronounced seasonal variability. The suspended particulate matter concentration varied from 35 to 160 μg m⁻³, with consistently higher values during the late winter. Back trajectory analysis suggests the origin of the air masses shifting from Indo-Gangetic Plains (during late winter) to those from the Arabian Sea and the area around Persian Gulf during April–May. Air masses passing over Northern India (Indo-Gangetic Plains) impart characteristic contribution of ionic species from fossil fuel combustion, biomass burning and eolian dust as asserted by the factor analysis. A detailed study on characterisation of aerosols from south Asian region is rather sparse but essential for modelling the effect of tropospheric aerosols on climate.     

Removal characteristics of sulfuric acid aerosols from coal-fired power plants

With increasing attention on sulfuric acid emission, investigations on the removal characteristics of sulfuric acid aerosols by the limestone gypsum wet flue gas desulfurization (WFGD) system and the wet electrostatic precipitator (WESP) were carried out in two coal-fired power plants, and the effects of the WFGD scrubber type and the flue gas characteristics were discussed. The results showed that it was necessary to install the WESP device after desulfurization, as the WFGD system was inefficient to remove sulfuric acid aerosols from the flue gas. The removal efficiency of sulfuric acid aerosols in the WFGD system with double scrubbers ranged from 50% to 65%, which was higher than that with a single scrubber, ranging from 30% to 40%. Furthermore, the removal efficiency of WESP on the sulfuric acid aerosols was from 47.9% to 52.4%. With increased concentrations of SO₃ and particles in the flue gas, the removal efficiencies of the WFGD and the WESP on the sulfuric acid aerosols were increased.

Implications: Investigations on removal of sulfuric acid aerosols by the WFGD and the WESP in the power plants were aimed at the control of sulfuric acid emission. The results showed that the improvement of the WFGD system was beneficial for the reduction of sulfuric acid emission, while the WESP system was essential to control the final sulfuric acid aerosol concentration.     

Optical characteristics of southeast Asia's regional aerosols and their sources

The dominant optical characteristics of Southeast Asia (SEA)'s regional aerosols were determined from the cluster analysis of the 26 AERONET aerosol inversion products, including aerosol light scattering/absorption indicators and aerosol size/shape parameters retrieved from 2003 to 2007. The data sets were acquired from four stations: Bac Giang in Vietnam and Mukdahan, Pimai, and Silpakorn University in Thailand. The cluster analysis showed agreement among the aerosol optical characteristics, land cover/uses, season as the surrogate of the prevailing winds, and observations from the literature. The results of this study showed that during the northeast prevailing winds from mid-September to December, the high aerosol exposure events were most frequently observed over the upwind station and less often over the downwind stations. This aerosol exhibited a single scattering albedo (SSA) of approximately 0.95 (440 nm), a relatively low refractive index, and a larger fine-mode size, suggesting it had the characteristics of urban/industrial aerosols reported in the literature. These aerosol sources were upwind from Bac Giang, probably in eastern China. From January to April, the aerosol exhibited a lower SSA of approximately 0.90, a higher refractive index, and a smaller fine-mode size, suggesting biomass burning smoke reported in the literature. The SEA urban aerosol exhibited a mean SSA of approximately 0.90 (440 nm) or lower, and the coarse-mode aerosol, possibly road dust or soil dust, played a role from October to January when seasonal winds are strongest. The results from a canonical discriminant function analysis suggest that the dominant SEA aerosol clusters tended to be separated by a canonical function positively correlated with SSA, the fine-mode asymmetry factor, and the overall fine-mode size and negatively correlated with the refractive index.     

Optical,chemical and physical properties of aerosols over the antarctic ice sheet

Multiwavelength optical measurements of atmospheric extinction and sky brightness in the solar aureole were made at Ross Island and at South Pole Station, Antarctica, to derive the column-integrated size distribution and mass loading of suspended aerosol. In addition, the composition and particle size distribution near the surface at South Pole, the Ross Ice Shelf and from the summit of Mt Erebus volcano were derived by electron microscopic analysis and X-ray spectrometry.The aerosol over the ice sheet has a bimodal size distribution with modal radii in dn/dr at < 0.05 μm and at 0.14 ± 0.02 μm. The number concentration and mass loading for the large mode are 40cm⁻³ and 300 ng m⁻³, respectively, while the number concentration of particles in the smaller mode is ~ 10³ cm⁻³. The particles over the polar ice sheet are dominated by ammonium sulfate and hydrated droplets of sulfuric acid (60–80% by mass) although trace amounts of oceanic and crustal-derived material have also been detected. Aerosols on the Ross Ice Shelf are relatively large and consist mainly of crustal material, presumably from the exposed land within 20 km distance. Submicron particles collected at the fuming vent of Mt Erebus volcano contained 34% Cl, 22% K and only 18%S.The sulfur aerosol residing over the ice cap shows evidence of being produced by nucleation of trace sulfurbearing gases; the most likely source is organic compounds, possibly CS₂ and COS, released by the oceans. These compounds have long lifetimes (3 and 20, respectively), hence they can be transported to the central polar ice cap and converted to sulfates. It is estimated that downwelling air from the stratosphere is probably not an important source for sulfate particles. The rate of removal of particles to the snow and ice surface of the continentis −300 × 10⁻¹⁶g cm⁻²s⁻¹; ninety percent of the removal is by nucleation of snow flakes and ice crystals.     

An examination of the physical and optical properties of aerosols collected in the IMPROVE program

The Interagency Monitoring of Protected Visual Environments (IMPROVE) protocols for reconstructing the ambient light extinction coefficient (b_ext) from measured aerosol species are the basis for evaluating compliance under the Regional Haze Rule. Aerosol mass composition and optical properties have been measured as part of the IMPROVE program since 1988, providing a long-term data set of aerosol properties at 38 sites around the US. This data set is used to evaluate assumptions made in calculating reconstructed mass and b_ext by applying statistical analysis techniques. In particular, the molecular weight to carbon weight ratio used to compute particulate organic matter is investigated. An annual average value of 1.7±0.2 for the IMPROVE sites, compared to the value of 1.4 currently assumed in the IMPROVE algorithm, is derived. Regression analysis also indicates that fine soil mass concentrations are underestimated by roughly 20% on average. Finally, aerosol mass scattering and extinction efficiencies assumed in the IMPROVE reconstructed b_ext protocol are examined. Fine mode (D_p<2.5 μm) mass scattering efficiencies have a functional dependence on mass concentrations at many sites, and use of a mass-concentration-dependent adjustment factor to refine the assumed efficiencies provides for closer agreement between measured and reconstructed b_ext.     

Concentration characteristics of bromine and iodine in aerosols in Shanghai,China

Aerosol samples (TSP and PM₁₀) during each season were collected at a national monitoring point in Shanghai in 2008. Halogens (Br, I) were determined in samples along with sodium (Na) by ICP-MS and ICP-OES after microwave digestion. In this report we focused on the concentration characteristics of halogen elements Br and I and their seasonal distributions. The mean annual concentrations of total Br and I were 24 ng m^?3 and 12 ng m^?3 for TSP, 21 ng m^?3 and 9 ng m^?3 for PM₁₀, respectively. Concentrations of Br and I in TSP and PM₁₀ were lowest in summer but an increase occurred in autumn and winter. Water-soluble Br and I accounted for about 32% of the total Br and I in aerosols, and about 68% of Br and I was non soluble which may be non-soluble organic species. These non-soluble organic species are present in aerosols in the possible binding forms as mineral dust, natural organic matter, and adsorption to black carbon or mineral material such as iron oxides. Soluble Br and I in PM₁₀ extracted by a dilute acid solution (HNO₃ + H₂SO₄) increased by 22% and 18%, respectively, compared with water-soluble Br and I. A positive correlation with Na and sea water enrichment factors for Br and I indicated that bromine and iodine in aerosols originated mostly from marine sources in Shanghai.     

The absorption of Hydrogen chloride by aqueous aerosols

A general gas/droplet interaction model has been used to examine the absorption of gaseous hydrogen chloride in aqueous aerosols formed in the presence of typical ambient concentrations of soluble and insoluble particulate over the relative humidity range 60–99 % and temperature range 273–283 K. Absorption of hydrogen chloride in aqueous aerosols formed on inert fugitive ash from a point source has also been simulated using simple puff and constant angle dispersion models. For ambient atmospheres it is predicted that appreciable uptake of hydrogen chloride occurs only for relative humidities approaching 99%. Equilibration times increase with particle size and gas concentration and lie in the range < 1–600 s. For diluting point source emissions, it is predicted that conditions close to equilibrium exist throughout most of the dispersion period. Hydrogen chloride absorption increases with initial plume water content and ambient relative humidity and decreases with ambient temperature. Substantial temporary removal of gaseous hydrogen chloride is predicted under most conditions simulated but the period of uptake is significant only for slowly diluting plumes at ambient relative humidities > 95%. Initial plume particulate diameters > 5 μm are required for enhanced deposition and substantial permanent removal of gaseous HCl. Absorption in aqueous ambient and plume aerosols is thus unlikely to constitute a general highly efficient process for the removal of hydrogen chloride from the atmosphere.     

The impaction of aerosols on a nuclepore filter

The flow field approaching a nuclepore filter is determined in the limit of zero Reynolds number. Thus the trajectories of particles in the flow are computed accurately. From these trajectories the collection efficiency due to impaction on the face of a filter is calculated. The calculations account explicitly for the finite size of the particles. The data are fitted to a formula which is valid for Stokes numbers less than about 100 and for filters of porosity between 0.04 and 0.36.     

Physicochemical and toxicological characteristics of urban aerosols during a recent Indonesian biomass burning episode

Air particulate matter (PM) samples were collected in Singapore from 21 to 29 October 2010. During this time period, a severe regional smoke haze episode lasted for a few days (21–23 October). Physicochemical and toxicological characteristics of both haze and non-haze aerosols were evaluated. The average mass concentration of PM_2.5 (PM with aerodynamic diameter of ≤2.5 μm) increased by a factor of 4 during the smoke haze period (107.2 μg/m³) as compared to that during the non-smoke haze period (27.0 μg/m³). The PM_2.5 samples were analyzed for 16 priority polycyclic aromatic hydrocarbons (PAHs) listed by the United States Environmental Protection Agency and 10 transition metals. Out of the seven PAHs known as potential or suspected carcinogens, five were found in significantly higher levels in smoke haze aerosols as compared to those in the background air. Metal concentrations were also found to be higher in haze aerosols. Additionally, the toxicological profile of the PM_2.5 samples was evaluated using a human epithelial lung cell line (A549). Cell viability and death counts were measured after a direct exposure of PM_2.5 samples to A459 cells for a period of 48 h. The percentage of metabolically active cells decreased significantly following a direct exposure to PM samples collected during the haze period. To provide further insights into the toxicological characteristics of the aerosol particles, glutathione levels, as an indirect measure of oxidative stress and caspase-3/7 levels as a measure of apoptotic death, were also evaluated.     

Chemical characteristics of free tropospheric aerosols over the Japan Sea coast: aircraft-borne measurements

Atmospheric aerosol particulate matter was directly collected in the free troposphere over the Japan Sea coast between 1992 and 1994 using an aircraft-borne nine-stage cascade impactor (particle size range: 0.1–8 μm). The water-soluble components in the aerosol particulate matter were analyzed by ion chromatography. Particulate sulfate and ammonium were detected in most of the samples and their size distributions showed noticeable peaks below the 1 μm particle size range. Water-soluble calcium (Ca²⁺) was detected in half of the samples; the size distribution showed that the maximum particle size was larger than 1 μm. Highly concentrated Ca²⁺ in larger particles was possibly due to transport of Kosa aerosols from the Asian continent in the free troposphere. The concentration of fine particulate sulfate and ammonium tended to increase whenever Ca²⁺ was detected, which suggests possible mixing of Kosa aerosols and non-Kosa aerosols during long-range transport of air masses containing Kosa particles.     

The modality of particle size distributions of environmental aerosols

Knowledge of the distribution of airborne particulate matter into size fractions has become an increasing area of focus when examining the effects of air pollution. While total number and mass concentrations may play an important role in exposure and risk assessment analyses, often an understanding of the particle size distributions provides more information on the type of atmospheric processes resulting in the distributions. The modality of the particle size distribution is one such aspect that has been associated with the aerosol formation mechanisms. The aim of this work is to provide a detailed analysis of the modal characteristics of a large number of particle size spectra collected over a period of three years for a range of ambient aerosol types. Measurements of over 6000 size distributions in the size range 0.016–30 μm were made using a scanning mobility particle sizer and an aerodynamic particle sizer for various ambient aerosols including: traffic influenced, urban, vegetation burning influenced, marine, modified background and suburban. Advanced data analytical procedures were adopted to combine the distributions from the two instruments for the calculation of the volume size distributions to allow clear interpretation of the modal characteristics. It was determined that, while in most cases there is a distinct nuclei mode in the number size distribution, this does not translate to a nuclei mode in the volume size distribution. Furthermore, while many of the number size distributions were different for each aerosol studied, the volume distributions were similar. This finding has serious implications for the setting of mass-based air quality standards.     

The formation and stability of sulfite species in aerosols

Recent epidemiological and animal lexicological studies indicate that reactions between SO₂ and metal containing aerosols result in the formation of respiratory irritants. These studies point out the importance of understanding in detail the chemical species formed by such interactions. Using a combination of thermometric, ESCA and PIXE analysis techniques, it has been demonstrated that both inorganic and organic S(IV) species are stable constituents of aerosols associated with pollution sources containing SO₂ and transition metals or with pollution sources resulting from the combustion of fossil fuels. The data indicate the inorganic sulfite species are present as complexes with Fe(III), Cu(II), Zn(II) and possibly Pb(II). The concentration of these inorganic sulfite species is 10 to 30% of the sulfate concentration in primary aerosols produced by smelters. These inorganic sulfite species tend to be evenly distributed over the various particle sizes. In contrast, the inorganic sulfite in primary aerosols produced by fossil fuel burning sources tends to exist in the <3 μm size range and can vary from a negligible to a major fraction of the sulfur species produced. The factors which control this variability are presently unknown. The principal mode of formation of such species in the ambient atmosphere appears to be via SO₂ absorption. Oxidation of S° or S(-II)species to form inorganic sulfite complexes or oxidation of the sulfite species to sulfate are both extremely slow, with time constants on the order of months. Aerosol samples collected from the plume, stack, or flue lines of coal burning facilities or collected in New York City or rural Utah produce sulfite when hydrolyzed with dilute aqueous acid. It is postulated this sulfite is produced from organic-SO₂ adducts in the sample. These organic S(IV) containing species are predominantly found in the resptrable size range and are present at from 5 to 50% of the sulfate concentration. It is probable that some of these S(IV) species play an important role in the removal of SO₂(g) from the atmosphere to form sulfur containing aerosol species.