Measurement of Ambient Diesel Aerosol

W. Addy Majewski, Heinz Burtscher

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

Abstract: Measurement of ambient aerosols typically involves collecting particulates on a sampling filter, followed by a gravimetric analysis. The sampling involves particle size classification in accordance with sampling conventions. In many occupational health applications, elemental carbon (EC) analysis can provide a specific marker of diesel exposure. Several specialized diesel particulate sampling and analysis techniques have been developed in underground mining.

Aerosol Sampling and Analysis

Overview

A number of measurement methods have been developed for the determination of ambient levels of suspended particulate matter (PM), as well as gaseous pollutants, for the purpose of air quality surveys in ambient (i.e., environmental) and occupational settings. In both cases the purpose of the measurement may be either (1) determining compliance with ambient air quality or occupational health standards or (2) air quality research. There are many directions of such air quality research, for instance evaluating health effects of pollution, studying the chemical composition and physical properties of air pollution, and apportioning the pollution constituents to their emitting sources. A substantial amount of research is also performed to evaluate the measurement methods themselves, correlate results obtained from different methods and to improve and to develop new air sampling and measurement techniques.

As it was the case with PM measurement in engine emissions, methods for ambient particulate measurement can be classified as (1) collecting or (2) in-situ techniques. Traditionally, ambient air measurements have been dominated by collecting methods, which involve the two following steps:

  1. Sampling (collecting) of the particulate matter on a filter, and
  2. Analysis of the collected material.

Methods for sampling and analysis, often developed independently, are combined in order to determine concentrations of measured compounds. In general, this sampling/analysis approach is similar to that used in measurement of engine emissions. However, while the methods used for engine emission measurement are very similar worldwide, a wide range of different methods have been developed for different ambient air sampling applications. One has to remember that in the case of complex air pollutants which are not well-defined chemical species, such as diesel particulates or ambient particulate matter, the emission is defined by the measurement method. A consensus had been reached for particulate sampling and analysis in the engine tailpipe emission testing, where the PM definition covers all material collected on the filter at a standard temperature and similar dilution ratios, including solids (inorganic carbon and ash), SOF, and sulfates, as determined gravimetrically. That substance is commonly referred to as total particulate matter (TPM). There is no single sampling method to measure ambient exposures to diesel particulates. As a consequence ambient particulates measured using various methods may be different, both in terms of quality (i.e., chemical composition and physical properties) and quantity (e.g., mass). In fact, different occupational health regulatory authorities have formulated exposure limits for diesel particulates using different definitions, such as “elemental carbon” or “total carbon”. Making comparisons between data obtained using different methods or formulating correlations between exposure figures and engine emissions requires that a correlation between methods be established.

Another important difference between engine emission and ambient sampling is that engine sampling produces a sample composed of “pure” diesel particulate matter, whereas ambient air samples contain a mixture of particulates from a number of man-made and natural sources, which may include varying proportions of diesel and/or other combustion particulates. Determination of diesel particulate matter exposures based on a measurement of total suspended particulates requires apportioning a certain fraction of the measured particulate exposure to the diesel source. This is not an easy task; in essence, accurate discrimination between diesel and non-diesel particulates based exclusively on measurement is not possible. Many methods of diesel exposure estimation, especially those used in environmental settings, utilize various forms of complex mathematical modeling based on a number of assumptions in addition to or in lieu of measurement.

In most occupational settings, the total suspended particulates can be traced back to fewer sources than in the case of ambient air studies and their composition is known or can be deducted from the processes in use. Diesel particulates are also subject to increasing number of occupational air quality standards (in contrast to ambient air standards which regulate the total suspended particulates such as PM10 or PM2.5). For these reasons, a number of “diesel specific” sampling and analysis methods have been developed for use in occupational health air surveys. Most of these methods attempt to differentiate between diesel and other-source particulates based on two characteristics of diesel particles: (1) high proportion of elemental carbon and (2) high proportion of fine particles. The elemental carbon (EC) content of total particulates can be determined through analysis, while fine particle fractions may be obtained through size-selective sampling techniques. The accuracy of measurement depends on the particular application. For instance, EC may be a good surrogate for diesel particulates in non-coal mines, where no other significant sources contribute to inorganic carbon emissions. The same method is not likely to yield precise results in coal mines, due to interferences from coal dust, or in any environment with significant concentration of non-diesel combustion aerosols.

With few exceptions, current measurement methods for ambient aerosols are dominated by the use of collection on sampling filters followed by mass (gravimetric) analysis. In addition to the gravimetric analysis, samples may be subjected to chemical analysis for specific elements or compounds. There is a great deal of commonality between methods used in general environments and in the workplace. However, the mass concentrations involved are typically an order of magnitude greater in occupational settings compared to the general environment [703]. Sampling and analysis of ambient aerosols have become a complex area of knowledge. Sophisticated sampling methods have been developed, which utilize inertial, gravitational, centrifugal, and thermal mechanisms of particle collection. There is an increasing trend in using direct-reading, in-situ techniques utilizing such principles as optical particle detection. Detailed discussion of measurement methods and instruments for ambient aerosols, as well as the underlying science can be found in the literature [2419][706][717].

Most sampling methods developed specifically for diesel aerosols, which are derived from general aerosol sampling techniques, are also based on filter collection and gravimetric determination of the particulate mass. Gravimetric measurement of diesel particulates was the basis for nearly all health studies in the past. There is a growing body of literature suggesting that toxic response for some insoluble materials may be associated with surface area and/or particle number, rather than with particle mass [2737][2738][2739]. It is believed that these alternative metrics may be particularly appropriate for the characterization of combustion aerosols, which are composed of much finer particles than general ambient aerosols. It can be expected that increasing number of particle size and number related methods and instruments will be used for ambient particulate measurement.

Biologically Relevant Sampling

Ambient air aerosol sampling is ultimately concerned with measuring those aspects of aerosols that lead to specific health effects. The biological effects resulting from deposition of an aerosol in the respiratory tract depend on the dose received and on the response of the human organism. Sampling methods are thus designed to measure a “potential dose” by sampling particle fractions in relevance to their deposition in various areas of the respiratory system. The human respiratory system, a size-selective aerosol sampler on its own, can be divided into three zones [703]:

The respirable aerosol is a subfraction of the thoracic aerosol, which in turn is a subfraction of the inhalable aerosol. Figure 1 shows the deposition efficiency in the different sections of the human respiratory system for the particle size range that is of interest for diesel particles.

[SVG image]

Figure 1. Particle Deposition in Human Respiratory Tract (ICRP Model) and Typical Diesel Size Distribution

Deposition of the diesel particles mainly occurs in the alveolar region. For particles larger than 1 µm deposition efficiency in the head airways (nose) increases rapidly.

Sampling standards have been developed which describe penetration characteristics of aerosol particles through the respiratory system as a function of the particle aerodynamic diameter. International workplace sampling conventions [710] and the PM10 and PM2.5 ambient conventions are illustrated in Figure 2.

[chart]

Figure 2. PM Sampling Conventions

These sampling conventions provide a basis for estimating the aerosol concentration potentially available to cause harm within the respective areas of the respiratory system. As seen in the chart, the probability of a particle entering the mouth and nose (inhalable particles) is about 50% for particles of 100 µm aerodynamic diameter, and increases to nearly 100% for particles below a few µm. It should be noted that nearly all diesel particulates have diameters below 1 µm and, thus, the diesel aerosol is almost totally respirable. The above sampling conventions are underlying many of industrial hygiene aerosol sampling methods and instruments.

###