The high health risk associated with the inhalation of airborne particles has been recognized and documented (see e.g. Brown et al., 2002; Pope et al., 1995). Many epidemiological studies have shown associations between exposure to particulate matter in the air and increases in morbidity and mortality, Dockery et al. (1993). There is a growing recognition that health risks associated with airborne particles are influenced by size. Some studies indicate that nanoparticles have increased specific toxicity relative to larger particles composed of the same materials (Donaldson et al., 1998; Ferin, 1994; Ferin et al., 1992; Oberdorster et al., 1995). Size-resolved sampling of the total aerosol is therefore necessary if health risks associated with inhalation of airborne particles are to be properly assessed.
Size-resolved sampling of airborne particles requires various techniques to be employed (John, 2001). A significant benefit would therefore arise from a sampler that could reliably collect size-resolved samples across the entire size range which is considered to be relevant to health effects.
Cascade impactors have been employed for more than half a century to fractionate aerosol particles according to their aerodynamic diameter (Marple et al., 2001
). Usually, a cascade impactor enables aerosol particles to be collected onto 5–10 successive impactor stages with decreasing cut-off diameters. For instance, in the May cascade impactor (May, 1945
), at 20 dm3
flow rate, particles are separated into seven fractions (defined as diameters at which the collection efficiency is 50%) with aerodynamic diameters ranging from ~20 to 0.25 μm. Other cascade impactors collect similar size ranges.
However, particles smaller than ~0.25 μm are also required to be size selectively collected to properly assess health risk. Since this is not possible using standard impactors, low-pressure cascade impactors have been developed to enable particles considerably <0.25 μm to be fractionated. Although low-pressure impactors can extend the range of size-selective sampling to particles with sizes of 20–30 nm, they cannot be accepted as ideal instruments for sampling particles in the nanosize range due to the following reasons: (i) The high pressure drop in low-pressure cascade impactors corrupts size distributions and causes condensation of water as well as other atmospheric constituents on substrates, e.g. Hart and Pankow (1994) have estimated that the gas–particle mass exchange for polycyclic aromatic hydrocarbons could cause errors in measurements of up to 40%. Moreover, large mass changes were directly observed by Moor et al. (1998) in experiments where atmospheric aerosol particles collected onto substrates of a cascade impactor were exposed to conditions with lowered partial pressures of semi-volatile compounds. (ii) Low pressure may cause volatilization of some constituents and change the chemical composition of substances—especially organic substances. (iii) High flow velocity in a low-pressure impactor causes bouncing. Rotating substrates are used to increase the uniformity and to reduce bouncing. This increases the cost and weight of impactors. (iv) Impactors fractionate particles according to aerodynamic equivalent diameter. In nanosize ranges, deposition of particles in the respiratory tract depends primarily on diffusion. For this reason, mobility equivalent diameter is a better measure of nanoparticle sizes when evaluating health effects.
Therefore, there is a need for a wide-range aerosol sampling instrument that enables more reliable data on the size-resolved chemical composition of aerosols.
The operation of all cascade impactors is based upon the inertial deposition of aerosol particles.
Diffusivity, like inertial deposition, is influenced by the size of particles and can be used to derive size distributions. In the past, diffusion batteries were successfully employed to obtain aerosol particles size distributions in the nanoparticle range (e.g. Fuchs et al., 1962
)—e.g. Sinclair and Hoopes (1975)
developed the screen diffusion battery.
If a collector based on diffusion could be combined with a cascade impactor, many of the disadvantages of the low-pressure impactor could be overcome, providing a means for fractionating aerosol particles with a wide range of sizes.
In addition, human health risk is influenced by deposition of airborne particles at specific sites in the respiratory tract. The efficiency of deposition in the respiratory system was subject to experimental research and modelling, e.g. Chamberlain (1985)
, Hinds (1999)
, Lippmann (1995)
and Yeh et al. (1996)
. It is well established that the deposition efficiency in the respiratory tract is influenced by the size of particles and forms a ‘V-curve’ comprising two branches caused by two main mechanisms of particle deposition: inertial deposition for larger particles with diameters >200 nm and diffusion for smaller particles (diameter
150 nm) e.g. Wilson et al. (1985)
, Schiller et al. (1988)
, ICRP (1994)
and Jaques and Kim (2000)
Using both diffusion and impactor collectors correctly accounts for both processes, without implication as to where in the respiratory tract deposition occurs.
The main advantage of such a combination is clearer in a comparison with data on nanoparticles collected using low-pressure cascade impactors. Deposition of nanoparticles in the respiratory tract is mainly controlled by diffusion and, therefore, calculation of the deposited dose from exposure requires knowledge of the diffusivity equivalent diameter Hinds (1999)
Some low-pressure cascade impactors are claimed to collect particles with sizes down to 20–30 nm. However, low-pressure cascade impactors are based upon inertial deposition and, therefore, provide data on aerodynamic equivalent diameters. This creates uncertainties with interpretation of data in the nanorange particularly with respect to non-spherical particles of unknown density (e.g. agglomerates, fractals, soot particles, etc). Such particles are commonly present in aerosol samples.
It is quite difficult to calculate mobility equivalent diameters from aerodynamic equivalent diameters, even when the shape and density are known. For many practical cases, information regarding the shape and density is not available, making it very difficult to obtain robust information on deposition from this data. Uncertainties associated with this factor can cause significant errors in health risk evaluations.
However, combining a diffusion collector with an impactor requires a number of issues to be addressed.
In existing diffusion batteries, particles are collected onto nets. However, diffusion batteries such as these cannot be directly employed for size-resolved sampling because they have a V-shaped dependence between collection efficiency and the size of particles (e.g. Kirsh and Stechkina, 1978
). This means that there is not a one-to-one relationship between size and deposition efficiency. For example, the same collection efficiency corresponds to two different sizes, . Mass size distributions cannot, therefore, be easily retrieved from the mass deposited onto the substrates. (Note: this is not a problem when obtaining number concentration size distributions because fine and ultrafine particles are usually present in much greater numbers than coarse particles and the number of coarse particles can be neglected, but it is not possible to neglect the mass).
In addition to the above, using existing diffusion batteries, like the TSI model 3040/3041, for mass analysis is difficult for practical reasons because of the great number of screens (55/56) and sections (10 covering two decades of sizes) deployed. For mass measurements, the mass of particles collected on a section needs to be sufficient to be detected by an appropriate analytical technique [usually atomic absorption spectroscopy (AAS), gas chromatography–mass spectrometry (MS) or inductively coupled plasma MS]. The diffusion particle collector should therefore be designed to maximize the mass deposited on screens by employing the smallest possible number of sections and screens.
Usually, diffusion batteries, such as those described in the literature or those commercially available, operate at flow rates of ~5 dm3
(e.g. Cheng et al., 1980
) However, cascade impactors are often operated at higher flow rates of ~20 dm3
to minimize measurement errors, particularly if mass measurements are being made. In order to make a combined instrument, it is therefore desirable to develop a diffusion particle collector which operates at a flow rate that would make it compatible with high-flow-rate cascade impactors.
However, diffusion theory has only been developed for low flow rates (Fuchs, 1964
; Kirsh and Stechkina, 1978
; Cheng and Yeh, 1980
) and it is not clear if it can be extended to high-flow-rate conditions. This is important because developing a diffusion unit without a theoretical foundation would be very difficult, especially when deposition kernels are required for data reduction. Obtaining size distributions with diffusion batteries is an ‘ill-defined’ problem that requires knowledge of kernel functions (e.g. Lesnic et al., 1995
). Therefore, the performance of a diffusion collector working at 20 dm3
has to be compared with theoretical predictions to be sure that the theory is still applicable and information obtained on particle sizes is robust.
Cascade impactors are often used to obtain size distributions without employing data inversion techniques. This approximation works well in many practical cases when size distributions are wider than the sharpness of the deposition efficiency on impactor stages. Developing a diffusion collector that also does not require data inversion techniques would have significant advantages in terms of ease of use. Understanding the resulting trade off between resolution and ease of use is therefore an important issue in developing a useful instrument.
Even if a diffusion collector has been developed with sufficient resolution, the question remains: Is the mass in the nanosize region sufficient to be detected using widely available techniques? The mass to number ratio rapidly decreases when reducing particle sizes by a factor of 103 per decade. Consequently, between 1 and 100 nm, the aerosol mass concentration is too small to be readily detected using most common techniques. Thus, it is necessary to demonstrate that enough mass is collected, within a reasonable time period, to allow mass determination.
In this paper, we address the following issues: (i) we discuss how a diffusion deposition collector can be designed and coupled with an inertial deposition unit within a single apparatus for the size-selective sampling of aerosol particles over a wide aerosol size range; (ii) we analyze if sufficient mass of particles can be accumulated in the nanosize region to be detected and (iii) we investigate if the diffusion deposition collector can be operated without data inversion techniques in a way similar to a cascade impactor.