Aerosol compositional analysis

Size cut enhances resolution, optically important aerosol analysis, low artifact potential, particle bounce amenable to automated compositional analysis automated versions available large networks under development... 

Zinsmeister, A. R. and Redman, T. C. (1980). A time series analysis of aerosol composition measurements, Atmos. Environ. 14,201-215. 

PLS (partial least squares) multiple regression technique is used to estimate contributions of various polluting sources in ambient aerosol composition. The characteristics and performance of the PLS method are compared to those of chemical mass balance regression model (CMB) and target transformation factor analysis model (TTFA). Results on the Quail Roost Data, a synthetic data set generated as a basis to compare various receptor models, is reported. PLS proves to be especially useful when the elemental compositions of both the polluting sources and the aerosol samples are measured with noise and there is a high correlation in both blocks. 

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. 

Among these techniques are various forms of a statistical method called factor analysis. Several forms of factor analysis have been applied to the problem of aerosol source resolution. These different forms provide several different frameworks in which to examine aerosol composition data and Interpret it in terms of source contributions. 

Hopke, et al. (4) and Gaarenstroom, Perone, and Moyers (7) used the common factor analysis approach in their analyses of the Boston and Tucson area aerosol composition, respectively. In the Boston data, for 90 samples at a variety of sites, six common factors were identified that were interpreted as soil, sea salt, oil-fired power plants, motor vehicles, refuse incineration and an unknown manganese-selenium source. The six factors accounted for about 78 of the system variance. There was also a high unique factor for bromine that was interpreted to be fresh automobile exhaust. Large unique factors for antimony and selenium were found. These factors may possibly represent emission of volatile species whose concentrations do not oovary with other elements emitted by the same source. 

Gatz (8) applied a principal components analysis to aerosol composition data for St. Louis, Mo taken as part of project METROMEX (13-1 t). Nearly 400 filters collected at 12 sites were analyzed for up... 

Sievering and coworkers ( ) have made extensive use of factor analysis in their interpretation of midlake aerosol composition and deposition data for Lake Michigan. 

The second difference is that the correlations between samples are calculated rather than the correlations between elements. In the terminology of Rozett and Peterson ( ), the correlation between elements would be an R analysis while the correlation between samples would be a Q analysis. Thus, the applications of factor analysis discussed above are R analyses. Imbrle and Van Andel ( 6) and Miesch (J 7) have found Q-mode analysis more useful for interpreting geological data. Rozett and Peterson (J ) compared the two methods for mass spectrometric data and concluded that the Q-mode analysis provided more significant informtlon. Thus, a Q-mode analysis on the correlation about the origin matrix for correlations between samples has been made (18,19) for aerosol composition data from Boston and St. Louis. 

It is clear that several forms of factor analysis can be very useful in the interpretation of aerosol composition data. The traditional forms of factor analysis that are widely available permit the identification of sources, the screening of data for noisy results, and the identification of interferences or analytical procedure problems. 

Zinmeister, A.R. and Redman. T.C. (198Q). "A Time Series Analysis of Aerosol Composition Measurements," Atmospheric Environment, 14, 201. 

Jayne, J. T D. C. Leard, X. Zhang, P. Davidovits, K. A. Smith, C. E. Kolb, and D. R. Worsnop, Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles, Aerosol Sci. Techno ., in press (1999). 

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. 

Compositional Analysis of Size-Segregated Aerosol Samples... 

The argument can be made that some lack of precision and accuracy is only to be expected, given the formidable difficulties in accurate collection and compositional analysis of aerosols by size. Thus, while efforts are being... 

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... 

Jayne J. T., Leard D. C., Zhang X., Davidovits P., Smith K. A., Kolb C. E., and Worsnop D. R. (2000) Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol. Sci. Technol. 33, 49-70. 

Dams R (1973) Analysis and interpretation of the inorganic atmospheric aerosol composition. Meded... 

In addition due to inadequate methods for in situ analysis at ambient concentrations analysis of airborne particles in their various forms has been limited. Even if the aerosols are analysed in detail changes have been found to occur in aerosol composition when collected by filters as they tend to react with the filter material itself. Therefore not only is improved identification of sulphate and nitrate speciation reaction mechanisms required but also improved techniques for in situ analysis to facilitate greater understanding of the quantitative aspects of the atmosphere which play a fundamental role in the long range transport of these pollutants. 

Interest in the elemental composition of aerosol particles arises from concerns about health effects and the value of these elements to trace the sources of suspended particles. The following physical analysis methods have been applied for the elemental measurements of aerosol samples. A schematic drawing of an x-ray fluorescence system is presented in Fig. 13.42. 

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