Ambient aerosol property measurements

Ambient aerosol property measurements are quite sophisticated source measurements are not as well developed. Certain aerosol property measurements have been made on emissions of many sources. Unfortunately these tests seldom provide a full accounting of samples, sometimes do not report confidence Intervals on the measurements, and often do not fully describe the operating parameters of the source at the time of the test. 

When sources are studied, several things should be done to provide data needed for receptor-model applications. First, particles should be collected In at least two different size fractions corresponding to the division at about 2.5-ym dlam now used In many studies of ambient aerosols. In some cases. It may be desirable to have more size cuts. As noted above, compositions of particles from coal combustion change dramatically below about 0.5-pm dlam (44, 46). Above we Identified a minimum of about twenty elements that should be measured. Also, In order to develop adequate markers for sources that emit carbonaceous particles, measurements of organic compounds and other properties related to carbonaceous particles should be made. 

The microscopic receptor model can include many more aerosol properties than have been used in the chemical mass balance and multivariate models. The data inputs required for this model are the ambient properties measurements and the source properties measurements. To estimate the confidence Interval of the calculated source contributions the uncertainties of the source and receptor measurements are also required. Microscopists generally agree that a list of likely source contributors, their location with respect to the receptor, and windflow during sampling are helpful in confirming their source assignments. 

Existing data on characteristics of particles from various types of sources are inadequate for general use, though they have been used in specific studies with some success. Most of the source tests have been made for purposes other than receptor modeling and complete chemical and microscopical analyses have not been performed. Source operating parameters which might affect the aerosol properties of emissions have not been identified nor measured in ambient sampling and no provision is made for likely transformations of the source material when it comes into equilibrium under ambient conditions. 

It is difficult to extract precise Qd values from ambient air sampling data. Plots of Equation (11) are constructed by measuring Kp on days when temperatures vary (Bidleman et al., 1986 Foreman and Bidleman, 1990 Gustafson and Dickhut, 1997 Pankow, 1991 Yamasaki et al., 1982). Unfortunately, these experimental Kp estimates also reflect the day-to-day variations in relative humidity and aerosol properties. As a result, confidence intervals around Qd are typically large (Bidleman et al., 1986 Pankow, 1991). Moreover, differences in the rates at which gases equilibrate with particles on hot and cold days (Kamens... 

We come now to one of the principal difficulties in the field of aerosol measurements, namely, the determination of chemical composition. The difficulties stem from a number of factors. Aerosols formed under uncontrolled circumstances such as many industrial emissions or the ambient aerosol are often multicomponent. Compo.sitlons differ significantly from particle to particle an individual particle may be a highly concentrated solution droplet containing insoluble matter such as chains of soot particles. The size composition probability density function (Chapter I) can be used to characterize the chemicals and size properties of such systems (but not their morphology). 

While the source-oriented model begins with measurements at the source (i.e., emission rates for the period under study), and estimates ambient concentrations, the receptor-oriented model begins with the actual ambient measurements and estimates the source contributions to them. The receptor model relies on properties of the aerosol which are common to source and receptor and that are unique to specific source types. These properties are composition, size and variability. 

The tracer property method is the simplest. It assumes that each aerosol source type possesses a unique property which is common to no other source type. It works well when a) the concentration of the tracer property in the source material is well known and invariant b) the concentration of the tracer property can be measured accurately and precisely in the ambient sample c) the concentration of the tracer property at the receptor comes only from one source type. 

PERSONAL MONITORING IS A RELATIVELY NEW CONCEPT in community air pollution measurement research (1-3). This fact is not surprising because most air pollution investigations have been directed toward the characterization of the ambient atmosphere, the observation of pollutant trends, the acquisition of data on chemical kinetic parameters and on the physical properties of aerosols, and the determination of compliance to national and other standards (4). Before the late 1970s, research on personal monitors was primarily conducted in industrial settings (5, 6) because American Confer-... 

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