Aerosol composition particles

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

Lannefors H, Hansson HC, Granat L. 1983. Background aerosol composition in southern Sweden — Fourteen micro and macro constituents measured in seven particle size intervals at one site during one year. Atmos Environ 17 87-101. 

Lundgren, D. A. Atmospheric aerosol composition and concentration as a function of particle size and of time. J. Air Pollut. Control Assoc. 20 603-608, 1970. 

Novakov, T., P. K. Mudler, A. E. Aloocer, and J. W. Otvos. Chemical composition of Pasadena aerosol by particle size and time of day. III. Chemical states of nitrogen and sulfur by photoelectron spectroscopy. J. Colloid Interface Sci. 39 225-234, 1972. 

Among the multivariate statistical techniques that have been used as source-receptor models, factor analysis is the most widely employed. The basic objective of factor analysis is to allow the variation within a set of data to determine the number of independent causalities, i.e. sources of particles. It also permits the combination of the measured variables into new axes for the system that can be related to specific particle sources. The principles of factor analysis are reviewed and the principal components method is illustrated by the reanalysis of aerosol composition results from Charleston, West Virginia. An alternative approach to factor analysis. Target Transformation Factor Analysis, is introduced and its application to a subset of particle composition data from the Regional Air Pollution Study (RAPS) of St. Louis, Missouri is presented. 

The aerosol composition distribution. Figure 3, shows pronounced variation with particle size. The distribution has a distinct break in the 0.1 to 0.3 um size range. The larger particles account for the vast majority of the aerosol mass. 

It is of interest to compare the aerosol composition of polluted air on 16-17 March with results of the preceding week. This comparison is given in Table I. Element ratios are presented relative to excess fine particle potassium, Kjj, i.e. the fine mode which has been resolved from total K as shown in Figure 4. The fine modes Fe and Mn j were obtained in a similar way. 

This is supported by studies of the aerosol composition in forested areas. For example, Kavouras et al. (1998) identified cis- and tram-pinonic acids as well as pinonaldehyde and nopinone in particles in a forest in Portugal. The diurnal variations of the pinonic acids and formic acid were similar, peaking in the afternoon as expected if they were formed by the reaction of O, with a-pinene. On the other hand, the concentrations of pinonaldehyde, expected from the oxidation of a-pinene by OH, O, and NO, and nopinone, from the oxidation of j3-pinene, were the smallest in the after-... 

In short, the direct effects of aerosol particles in terms of backscattering solar radiation out to space and hence leading to cooling are reasonably well understood qualitatively and provided the aerosol composition, concentrations, and size distribution are known, their contribution can be treated quantitatively as well. However, major uncertainties exist in our knowledge of the physical and chemical properties, as well as the geographical and temporal variations, of aerosol particles and it is these uncertainties that primarily limit the ability to accurately quantify the direct effects at present. 

Table I. Average Aerosol Composition for Fine and Coarse Particles at a Rural, Forested Location (Great Smoky Mountains, Tennessee) and an Urban Location (Houston, Texas) ...

The understanding of four aspects of the lower stratosphere must be improved. The first is the processes of heterogeneous conversion. Does it occur in three hours or in three days Which type of aerosols or particles are most favorable for conversion and what are the conditions required for their formation How do the particles form, and what are their compositions and structures What are the most important mechanisms for denitrification These issues can be settled only by a combination of laboratory and stratospheric measurements. 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

On-line measurements of the sulfur content of atmospheric aerosols have been made by removing gaseous sulfur species from the aerosol and then analyzing the particles for sulfur with a flame photometric detector (24) or by using an electrostatic precipitator to chop the aerosol particles from the gas so that the sulfur content could be measured by the difference in flame photometric detector response with and without particles present. These and similar methods could be extended to the analysis of size-classified samples to provide on-line size-resolved aerosol composition data, although the analytical methods would have to be extremely sensitive to achieve the size resolution possible in size distribution analysis. 

Niemi JV, Saarikoski S, Tervahattu H, Makela T, Hillamo R, Vehkamaki H, Sogacheva L, Kulmala M (2006) Changes in background aerosol composition in Finland during polluted and clean periods studied by TEM/EDX individual particle analysis. Atmos Chem Phys 6 5049-5066... 

Orsini DA, Ma Y, Sullivan A, Sierau B, Baumann K, Weber RJ (2003) Refinements to the Particle-Into-Liquid-Sampler (PILS) for ground and airborne measurements of water soluble, aerosol composition. Atmos Environ 37 1243-1259... 

Homogeneous Chemical similarity. A homogeneous aerosol is one in which all particles are chemically identical. In an inhomogeneous aerosol different particles have different chemical compositions. 

An example of the information obtainable from measurements of aerosol particle composition as a function of size is given in Figure 10. In this study of the evolution of aerosol composition in an urban air-shed, a multi-stage impactor was used to separate particles by size for the determination of size-dependent composition. The particle mass concentration increases as air moves inland from Santa Catalina Island to Long Beach, Fullerton, and finally Riverside, as do substances such as nitrate. Differences in particle composition as a function of size at a given site and from site to site are evident. 

For many purposes it is important to know the actual chemical species, and determining specia-tion from bulk compositions is a formidable challenge, especially when the aerosol contains particles of different composition, as invariably occurs in ambient air. At best, bulk analysis yields accurate information on the actual species present only for major particle types having simple compositions. Consequently, there has been an increasing interest in the analysis of individual aerosol particles. However, the attendant problems in such analyses are formidable because of the small sizes and, in many cases, the chemical complexity of individual particles. 

There has been growing emphasis on the development and use of on-line measurements, which provide an abundance of data in real time. Short-term variations in aerosol composition can reveal great detail, as shown in Figure 12, which illustrates marked differences in the composition of the aerosol at an urban site over periods as short as tens of minutes. The mass concentration was measured by an oscillating microbalance whose frequency was altered by deposition of material. Ionic species were measured by subjecting the aerosol to supersaturation, which greatly increased particle sizes and permitted... 

Important issue for urban dispersion modelling is the characteristics of the release, e.g., radiochemical composition, density for gases, size distribution for aerosols, etc. For radioactive aerosols the particle size distribution (e.g., number of modes, distribution type, average diameter and standard deviation for each mode, density, and nuclides) varies significantly for different release types and from one nuclide to another. The particle size spectrum could be very broad, e.g. 0.001-200 fim. 

The last system shown in Table 6.3 corresponds to a filter in combination with a method of chemical analysis such as x-ray fluorescence. It provides data on the chemical composition of the entire aerosol. The particles are collected on a filter usually over a period of hours. Concentrations measured by whole sample chemical analysis can be represented by the following expression ... 

The analysis of perturbations to the middle atmosphere must also include natural processes, such as the effects of volcanic eruptions, which produce large quantities of fine particles as well as water vapor and SO2, which eventually produces H2SO4 and sulfate aerosols. The amount of gas injected, the composition and the maximum altitude of injection vary with the intensity of the eruption. Such events can alter the budgets of some atmospheric constituents and are clearly reflected in the middle atmospheric aerosol content. Particles also provide sites for surface reactions to occur. Such heterogeneous reactions may activate chlorine and enhance the depletion of ozone by industrially manufactured halocarbons. 

The concentration of atmospheric aerosols varies considerably in space and time. This variability of the aerosol concentration field is determined by meteorology and the emissions of aerosols and their precursors. For example, the annual average concentration of PM2.5 in North America varies by more than an order of magnitude as one moves from the clean remote to the polluted urban areas of Mexico City and southern California (Figure 8.24). Sulfate dominates the fine aerosol composition in the eastern United States, while organics are major contributors to the aerosol mass everywhere. Nitrates are major components of the PM2.5 in the western United States. The EC makes a relatively small contribution to the particle mass in many areas, but because of its ability to absorb light and its toxicity, it is an important component of atmospheric particulate matter. 

---