Ultrafine Particles in the Atmosphere

The following sections are included:

• PREFACE

• CONTENTS

Typical size distributions for airborne particles are described and the significance of the ultrafine fraction highlighted. Size distributions may be expressed in terms of either mass (volume), surface area or number, and the interpretation of each is discussed together with appropriate measurement methods. The sources of ultrafine particles in the atmosphere include both primary emissions and secondary particles formed through homogeneous nucleation processes within the atmosphere. Examples of measurements of atmospheric ultrafine particles are given, highlighting situations with high concentrations of primary ultrafine particles and also situations where gas-to-particle conversion through homogeneous nucleation gives rise to bursts of new particle production. Finally, the relationship between particle mass and number within the atmosphere at a polluted site is examined.

Atmospheric ultrafine particles (with diameter less than 0.1 μm) may be responsible for some of the adverse health effects observed due to airpollutant exposure. To date, little is known about the chemical composition of ultrafine particles in the atmosphere of cities. Ultrafine particle samples collected by inertial separation on the lower stages of cascade impactors can be analysed to determine a material balance on the chemical composition of such samples. Measurements of ultrafine particle mass concentration made in seven Southern California cities show that ultrafine particle concentrations in the size range 0.056−0.1 μm aerodynamic diameter average 0.55−1.16 μg m⁻³. The chemical composition of these ultrafine particle samples averages 50% organic compounds, 14% trace metal oxides, 8.7% elemental carbon, 8.2% sulphate, 6.8% nitrate, 3.7% ammonium ion (excluding one outlier), 0.6% sodium and 0.5% chloride. The most abundant catalytic metals measured in the ultrafine particles are Fe, Ti, Cr, Zn, with Ce also present. A source emissions inventory constructed for the South Coast Air Basin that surrounds Los Angeles shows a primary ultrafine particle emissions rate of 13 tonnes per day. Those ultrafine particle primary emissions arise principally from mobile and stationary fuel combustion sources and are estimated to consist of 65% organic compounds, 7% elemental carbon, 7% sulphate, 4% trace elements, with very small quantities of sodium, chloride and nitrate. This information should assist the community of inhalation toxicologists in the design of realistic exposure studies involving ultrafine particles.

Increasing awareness that structures and attributes on a nanometre scale within aerosol particles may play a significant role in determining their behaviour has highlighted the need for suitable single ultrafine particle analysis methods. By adopting technologies developed within complementary disciplines, together with the development of aerosol-specific methods, a basis for characterizing single sub-100 nm (ultrafine) particles and features in terms of size, morphology, topology, composition, structure and physicochemical properties is established. Size, morphology and surface properties are readily characterized in the scanning transmission electron microscope (STEM), while high-resolution transmission electron microscopy (HRTEM) allows structural information on particles and atomic clusters to sub-0.2 nm resolution. Electron energy loss spectroscopy (EELS) and X-ray emission in the STEM allow the chemical analysis of particles and particle regions down to nanometre diameters. Scanning probe microscopy offers the possibility of analysing nanometre-diameter particles under ambient conditions, thus getting away from some of the constraints imposed by electron microscopy. Imaging methods such as atomic force microscopy and near-field scanning optical microscopy (NSOM) offer novel and exciting possibilities for the characterization of specific aerosols. Developments in aerosol mass spectrometry are providing the means for chemically characterizing sizesegregated ultrafine particles down to 10 nm in diameter on-line. By taking a multi-disciplinary approach, the compilation and development of complementary tools allowing both routine and in-depth analysis of individual ultrafine particles is possible.

Internal combustion (IC) engines are a major contributor to the total particulate emissions inventory, especially in urban areas. Recent epidemiological studies suggesting links between fine particles and negative health effects have sparked an increased interest in this subject. While particulate emissions from IC engines have been the focus of research for many years, a great deal of information crucial to our understanding of this subject still remains unknown. In this paper the authors address some of these unknowns, focusing primarily on the process and consequences of aerosol dilution strategy. The thermodynamics of dilution are considered, and the inadequacy of conventional constant-volume sampling dilution tunnels for ultrafine particle characterization are demonstrated using experimental data. Finally, time-resolved data demonstrating the variation in concentration of pollutants in a vehicle moving in traffic are used as an example of the difficulties in setting legislation aimed at controlling exposure to ultrafine particles.

The formation of new atmospheric particles by gas-to-particle conversion leads to enhanced concentrations of nanoparticles. We have studied the formation and growth of new particles in urban Atlanta and in the remote atmosphere in locations ranging from the North Pole to Mauna Loa, Tasmania and the South Pole. Key to this work was our development of new measurement techniques for freshly formed nucleation mode particles between 3 and 10 nm. In this paper we show that measured aerosol size distributions in the 3–10 nm diameter range often increase with decreasing size down to our minimum detectable size of 3 nm, presumably because nucleation was occurring during the measurement. Furthermore, we show that the Atlanta nucleation mode size distributions are consistent with a collision-controlled nucleation process in which accommodation coefficients for all collisions between condensing molecules and molecular clusters and between molecular clusters are assumed to be equal to one, and in which evaporation from molecular clusters is neglected, as would be expected for a highly supersaturated vapour.

While much of the suspended particulate matter found in the ambient air in urban areas has been emitted directly into the atmosphere, some has been formed there by photochemical reactions from gaseous precursor species. Two major components of this secondary particulate matter have been selected for detailed study in the United Kingdom context. These are particulate sulphate, formed from the precursor, sulphur dioxide, and secondary organic aerosols, formed from oxidation of terpenes and aromatic hydrocarbons. A Lagrangian dispersion model has been used to describe the emissions, transport and transformation of SO₂ into particulate sulphate. The origins of the particulate sulphate are delineated in two separate pollution episodes which occurred during 1996. A photochemical trajectory model is used to describe the formation of secondary organic aerosols and to assess the relative contributions from natural biogenic and man-made precursor sources during conditions typical of photochemical pollution episodes.

Soot formation and oxidation will be analysed with respect to the most important processes, namely particle inception, coagulation and surface growth. Time-scales of surface growth are estimated for premixed and diffusion flames and compared with time-scales for coagulation. It turns out that characteristic time-scales for soot formation and coagulation are similar and about one order of magnitude larger than the characteristic time-scales for combustion reactions and much smaller than the time-scales of molecular transport.

Coagulation processes will be discussed in detail and a detailed chemistry approach for surface growth will be presented. The detailed information will be put into a soot model that reproduces a number of phenomena in sooting premixed hydrocarbon flames, for example:

(i) the dependence of surface growth and oxidation rates on the chemical ‘environment’ of soot particles; and

(ii) the fraction of soot formed by particle inception and surface growth reactions and addition of polyacrylic aromatic hydrocarbon (PAH).

The ‘fine structure’ of soot is not resolved by this approach, and, furthermore, the predictions depend sensitively on information about the kinetics of growth of PAH-like structures, the detailed processes occurring on the surface of soot particles, and, most importantly, the pressure dependence of all these processes.

Inhaled ultrafine particles are increasingly being recognized as a potential threat to health. Aerosols in workplace environments may come from a wide variety of sources, depending on the type of activity and processes taking place. Some activities and processes are acknowledged as being ‘dusty’, where aerosol is generated from the mechanical handling and attrition of solid or liquid material, and are not considered to be plausible sources of ultrafine particles. However, hot processes, involving the vaporization of material, and inevitable subsequent cooling, do have the potential to generate significant number concentrations of ultrafine particles. However, consideration of the physical conditions required for the generation of particles in the range below 100 nm suggests that those conditions are not easily met in workplaces. More generally, the conditions are such that particles grow out of this range, either by continuing condensation (as happens at high vapour concentrations) or by agglomeration between smaller particles (as happens at high number concentrations). Not much is known about ultrafine particles in actual workplaces, mainly because our view has been obscured for the past few decades by the fact that most occupational aerosol standards have been based on the mass concentration of airborne particulate matter. Now that a new awareness has set in, it is expected that new research will address the problem.

Most current aerosol standards are expressed in terms of the mass concentration of particulate matter conforming to a particle size fraction, where the latter is based on knowledge of how particle size relates to where particles deposit in the human respiratory tract and any subsequent effects. At present no such basis exists for ultrafine particles, but one is needed before progress can be achieved towards meaningful standards for occupational ultrafine aerosols. It is expected that, for ultrafine particles, such a standard may, in the future, be expressed in terms of the number concentration of particles less than a certain size, that size to be determined on the basis of the physical and chemical nature of the particle at that size, human physiology and toxicology.

Within the last 20 years, advances in characterization methods, particularly in the field of high-resolution electron microscopy, have made it possible to probe the surface and internal structure of sub-100 nm particles, or nanoparticles. Such studies have indicated conclusively that surface-energy considerations in metal nanoparticles cause these particles to adopt structures which only approximate to close packing but are terminated by close-packed faces. In oxides, where stoichiometry must be maintained, the adoption of low-index crystallographic faces almost invariably necessitates the introduction of cation or anion vacancies, and both have been observed. In such cases, the structure at the edges of the particles differs greatly from that of bulk phases, and it seems highly probable that the physical and chemical properties of these particles are also different. In certain cases it appears that new structural types, found only in nanoparticulate form, may exist. The significance of these findings, particularly as regards their relevance to particulate pollutants in the atmosphere, may be of great interest.

Ultrafine particles (less than 0.10 μm in diameter) are ubiquitous in the atmosphere and possess unique physicochemical characteristics that may pose a potential health risk. To help elucidate the potential health risk, we measured respiratory dose of ultrafine particles (0.04, 0.06, 0.08 and 0.10 μm in diameter) in healthy young adults using a novel serial bolusdelivery method. Under normal breathing conditions (i.e. tidal volume of 500 ml and respiratory flow rate of 250 ml s⁻¹), bolus aerosols were delivered sequentially to a lung depth ranging from 50–500 ml in 50 ml increments and deposition was measured for each of ten equal-volume compartments.

Results show that regional deposition varies widely along the depth of the lung regardless of the particle sizes used. Peak deposition was found in the lung regions situated between 150 and 200 ml from the mouth. Sites of peak deposition shifted proximally with a decrease in particle size. Deposition dose per unit surface area was largest in the proximal lung regions and decreased rapidly with an increase in lung depth. Peak surface dose was 5–7 times greater than the average lung dose. The results indicate that local enhancement of dose occurs in normal lungs, and such a dose enhancement may play an important role in the potential health effects of ultrafine aerosols.

There is increased concern about the associations between particulate air pollution and human health. Inhaled and deposited particles play a crucial role in the aetiology of a range of pulmonary diseases. A variety of pulmonary diseases develop from the inhalation and deposition of pathogenic organisms or noxious particles (e.g. viruses, bacteria, spores, pollen, etc.). The inhalation of soot, burned tobacco and paper leads to common pulmonary diseases: chronic bronchitis and lung cancer.

It has been suggested that ultrafine particles might be taken up by cells, including by airway epithelial cells, through a process related to the surface forces exerted on them at the cell membrane–particle interfacial region.

Ultrafine particles (less than 100 nm in diameter) are encountered in ambient air and at the workplace. Normal background levels in the urban atmosphere for ultrafine particles are in the range 1−4 × 10⁴ cm⁻³; however, their mass concentration is normally not greater than 2 μg m⁻³. At the workplace, ultrafine particles occur regularly in metal fumes and polymer fumes, both of which can induce acute inflammatory responses in the lung upon inhalation. Although ultrafine particles occurring at the workplace are not representative, and, therefore, are not relevant for urban atmospheric particles, their use in toxicological studies can give valuable information on principles of the toxicity of ultrafine particles. Studies in rats using ultrafine polymer fumes of polytetrafluoroethylene (PTFE) (count median diameter ca. 18 nm) showed that (i) they induced very high pulmonary toxicity and lethality in rats after 15 min of inhalation at 50 μg m⁻³; (ii) ageing of PTFE fumes resulted in agglomeration to larger particles and loss of toxicity; (iii) repeated pre-exposure for very short periods protected against the toxic and lethal effects of a subsequent 15 min exposure; (iv) rapid translocation of PTFE particles occurred to epithelial, interstitial and endothelial sites. Since one characteristic of urban ultrafine particles is their carbonaceous nature, exposure of rats to laboratory-generated ultrafine carbonaceous (elemental, and organic, carbon) particles was carried out at a concentration of ca. 100 μm⁻³ for 6 h. Modulating factors of responses were prior lowdose inhalation of endotoxin in order to mimic early respiratory tract infections, old age (22-month old rats versus 10-week old rats) and ozone co-exposure. Analysis of results showed that (i) ultrafine carbon particles can induce slight inflammatory responses; (ii) LPS priming and ozone co-exposure increase the responses to ultrafine carbon; (iii) the aged lung is at increased risk for ultrafine particle-induced oxidative stress. Other studies with ultrafine and fine TiO₂ showed that the same mass dose of ultrafine particles has a significantly greater inflammatory potential than fine particles. The increased surface area of ultrafine particles is apparently a most important determinant for their greater biological activity. In addition, the propensity of ultrafine particles to translocate may result in systemic distribution to extrapulmonary tissues.

Many ultrafine particles comprised classically of low-toxicity, low-solubility materials such as carbon black and titanium dioxide have been found to have greater toxicity than larger, respirable particles made of the same material. The basis of the increased toxicity of the ultrafine form is not well understood and a programme of research has been carried out in Edinburgh on the toxicology of ultrafines aimed at understanding the mechanism. We used fine and ultrafine carbon black, TiO₂ and latex and showed that there was an approximately 10-fold increase in inflammation with the same mass of ultrafine compared with fine particles. Using latex particles in three sizes—64, 202 and 535 nm—revealed that the smallest particles (64 nm) were profoundly inflammogenic but that the 202 and 535 nm particles had much less activity, suggesting that the cut-off for ultrafine toxicity lies somewhere between 64 and 202 nm. Increased oxidative activity of the ultrafine particle surface was shown using the fluorescent molecule dichlorofluorescein confirming that oxidative stress is a likely process by which the ultrafines have their effects. However, studies with transition-metal chelators and soluble extracts showed that the oxidative stress of ultrafine carbon black is not necessarily due to transition metals. Changes in intracellular Ca²⁺ levels in macrophage-like cells after ultrafine particle exposure suggested one way by which ultrafines might have their pro-inflammogenic effects.

In epidemiological studies associations have been observed consistently and coherently between ambient concentrations of particulate matter and morbidity and mortality. With improvement of measurement techniques, the effects became clearer when smaller particle sizes were considered. Therefore, it seems worthwhile to look at the smallest size fraction available today, namely ultrafine particles (UPs, diameter below 0.1 μm) and to compare their health effects with those of fine particles (FPs, diameter below 2.5 μm). However, there are only few studies available which allow such a comparison.

Four panel studies with asthma patients have been performed in Germany and Finland. A decrease of peak expiratory flow and an increase of daily symptoms and medication use was found for elevated daily particle concentrations, and in three of these studies it was strongest for UPs. One large study on daily mortality is available from Germany. It showed comparable effects of fine and ultrafine particles in all size classes considered. However, FPs showed more immediate effects while UPs showed more delayed effects with a lag of four days between particulate concentrations and mortality. Furthermore, immediate effects were clearer in respiratory cases, whereas delayed effects were clearer in cardiovascular cases.

In total, the limited body of studies suggests that there are health effects, due to both UPs and FPs, which might be independent from each other. If this is confirmed in further investigations, it might have important implications for monitoring and regulation, which until now does not exist for UPs. Data from Germany show that FPs cannot be used as indicator for UPs: the time trends for FPs decreased, while UPs was stable and the smallest size fraction of UPs has continually increased since 1991/92.

There is now a large body of epidemiological evidence associating exposure to ambient particles with short- and long-term effects on health. Most authorities consider that at least some of these associations represent a causal relationship with particles. The size fraction of particles that could potentially harm health is PM₁₀, since only particles less than this size can plausibly reach the small airways and alveoli. Studies of mechanisms and theoretical considerations suggest that the fine (PM_2.5) and ultrafine (PM_0.1) particles are probably more important than larger particles, because of their relatively greater numbers and deeper penetration of the lung. Because of limited population exposure data, there is little direct epidemiological evidence about the effects of ultrafine particles. Indirect evidence falls into three groups. The first comes from studies that have directly compared the coarse (PM_2.5–10) with the fine (PM_2.5) fractions; the findings of these few studies have not been consistent. The second comes from studies of chemical species or measures of particles (sulphates, acid aerosol and black smoke) that reside mainly in the fine fraction; many of these have found associations with adverse health effects. The third group are those few studies that have compared the effects of size/number concentrations with size/mass concentrations; the findings of these have either been inconclusive or have suggested that numbers may be more important than mass.

Inference about the toxic component of particles will depend on all the evidence, especially from toxicology, as well as epidemiology. At present, epidemiological evidence points towards the fine fraction being important, but an effect of the coarse fraction cannot be excluded. Because of a lack of data, epidemiology has little to say about the relative importance of the ultrafine fraction. This is an urgent research need.

Epidemiology is a rather blunt tool for elucidating biological mechanisms that can account for the increased mortality and morbidity associated with population exposures to ambient air particulate matter (PM). However, it has an essential role to play. Recent studies indicate that three readily measurable ambient air PM concentration indices can be significantly associated with one or more elevations of rates of specific disease or dysfunction categories. These three indices, i.e. ultrafine particle number, fine particle mass (PM_2.5) and thoracic coarse mass (PM_10–2.5) differ not only in size range, but also in terms of their sources, deposition patterns, and chemical reactivities, factors that may account for their different associations with human health effects. Further epidemiological studies employing a wider array of air quality and health effects variables should enable us to resolve some of the outstanding questions related to causal relationships for PM components or, at the minimum, to pose some better questions.

The following sections are included:

• INDEX