Aerosol sampling and measurement techniques

The great diversity of application, the size range of atmospheric aerosol particles, the physical and chemical concentration variations, and the variety of measurement principles available imply many different combinations of application and measurement methods and procedures. Therefore, this chapter is focused on the most important methods in use. The methods applied for atmospheric aerosol sampling include filters and cascade impactors which collect the aerosol particles onto a surface. The collected sample must therefore be evaluated for size and composition. Because accumulation mode aerosols (fine aerosol particles) contain a substantial fraction of liquid material at normal temperatures and humidities, these fine aerosol particles must be sized in situ without precipitation. In some extreme cases, such as in Los Angeles smog, the liquid content may be as high as 75% or 80% of the total mass (Ho et al., 1974). 

Due to their simplicity of construction and use and the relatively sharp cut-off characteristics, cascade impactors have been widely used for the size classification and size-classified chemical analysis of aerosols. Table 6.1 lists the most important integrating sampling methods with their main characteristics. Table 6.2 gives the most important differential, size-resolving methods used to sample and measure atmospherie aerosol particles. The section of the particle size distribution and the modes that dominate the sensitivity of the methods are indicated. The upper and lower size limits are nominal values for the most commonly used forms of the techniques. Cost, complexity of operational requirements, calibration problems, and the demands of the particular evaluation to be used also affect the choice of methods. For example, chemical analysis usually requires that a sample be collected, then taken to the evaluation device. 

Major integral sampling and measurement methods used for measuring atmospheric aerosols 

Method or instrument Effective size limit (pm) Lower Upper Lower concentration limit Measurement principle  

Hi-vol filter and other filters—measures all of the nonvolatile mass in None 25 2 pg/m Filter-weighing  

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Aerosol sampling and measurement techniques AMBIENT AIR FLOW... 

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. 

Actually measuring the composition and phase of PSCs and aerosols in the stratosphere is extremely difficult. Some direct measurements have been made by collecting aerosol samples and subsequently analyzing them using techniques such as X-ray energy-disper-... 

Modified filter sampling methods that are available will measure ambient levels of strong acid in ambient aerosol samples, and these methods do so with acceptable precision and accuracy [as indicated by the balance between measured anions and cations (56, 57)] in the absence of significant levels of particulate weak acids. Additional intercomparisons involving intrinsically different techniques for particulate strong acidity [e.g., IR spectroscopy (48), thermal speciation (38, 45), and filter methods (28)] are needed. Further information on the occurrence of various weak acids in airborne particles is needed, along with further studies of techniques for their specific determination in atmospheric aerosol samples. 

A measurement method based on an ion-selective electrode determination of chloride is suggested. This method is reasonably simple, rugged, and can be run for long times without attention. Several variations of this technique are considered. One variation is a conventional electrode measurement except that the flowing-junction reference electrode can be replaced by a fluoride ion selective electrode as a nonflowing reference. Another method was developed and evaluated for the measurement of very small aerosol samples the measurement of chloride ion concentration is made on 50-/>tL samples. When the major-constituent composition of sea salt is known, the total aerosol weight can be calculated. The small volume allows measurable concentrations to be attained with short sample collection times. This technique is described in detail later. 

In the study of atmospheric aerosols several techniques have been used to determine optical constants from measurements on particulate samples. And there have been many such measurements. Yet under pressure from funding agencies and from those waiting at computer terminals for optical constants of the complicated mixture that is the atmospheric aerosol, comparatively little effort has been expended on evaluating these techniques by applying them to particles of solids with known optical constants. 

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. 

More ambitious attempts at measuring the heterogeneity of the atmospheric aerosol have been undertaken as well. Single-particle analysis by mass spectrometry was demonstrated by Sinha and co-workers (31, 32). In this technique, an aerosol sample is introduced into a vacuum chamber in the form of a particle beam. The particles are injected into a Knudsen cell oven, where they undergo many collisions with the cell wall and are ultimately vaporized and ionized. The ions are then mass-analyzed with a quad-rupole or sector mass spectrometer. So that individual particles can be analyzed, the flux of particles into the Knudsen cell is limited so that coincidence errors are minimized. Ion pulses from individual particles allow the determination of the amount of the species being analyzed in the particular particle. The sensitivity of the technique is limited. For sodium, the detection... 

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

Measurements of the air mobility spectrum seem to add considerable information toward an understanding of aerosol formation and growth at sizes below a few nanometers. Hence, to characterize nucleation mechanisms more precisely, such data should be included in experimental designs. Lagrangian aerosol sampling techniques would also be favored, since this approach can yield data on microphysical evolution without the complicating effects of a changing air mass. Further laboratory studies should be undertaken to quantify the thermodynamic data that define ion properties under tropospheric conditions, at ion sizes and compositions relevant to aerosol nucleation. The sparseness of such data imposes a limitation on our ability to quantify ion-based nucleation mechanisms [19,33],... 

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