Environmental Engineering Reference
In-Depth Information
8.3 COMPOSITION
There are few data on the composition of ambient UFPs. Because they are a very small fraction of the
total ambient particle mass, specialized methods are required to isolate and analyze them. Reported
studies indicate that they are composed, in large part, of organic and elemental carbon together
with trace metals, ammonium, and sulfates [7]. Hughes et al. [8] reported the chemical composition
of particles between 56 and 97 nm in diameter measured at Pasadena, CA. They reported that the
largest fraction is organic compounds, followed by elemental carbon, then trace metals and sulfates.
This is not unexpected since combustion sources are major contributors to ambient UFPs. Detailed
size distributions of ambient sulfates, ammonium, trace metals, and sulfuric acid in ambient particles
smaller than 100 nm were measured by Hazi [9]. Although most of the acid and sulfate mass was
measured in the 0.38 μm midpoint diameter fraction, the ultraine fraction (0.1 μm) was found to
have lower pH. Iron, zinc, and sulfur were the dominant trace elements of the nine measured in the
ultraine fraction. Iron is the most abundant metal measured in UFPs [7,9,10].
Measurements of the number of strong-acid UFPs in New York City and Tuxedo, NY, have been
reported by Cohen et al. [11]. The number was measured by the deposition of ambient particles
less than 100 nm in diameter onto detectors that are coated with a 20 nm thick layer of iron applied
by vapor deposition. The interaction of acid droplets and iron is detected by scanning the detector
surface topography with an atomic force microscope. They report that the fraction of UFPs that
were acidic varied from 10% to 88% for the different seasons and sites. The average concentrations
of acidic particles ranged from about 100-1500 particles cm
-3
over the sampling periods.
Both vehicular and stationary combustion sources contribute UFPs to the atmosphere. Gasoline
and diesel vehicles are major sources. Emission testing indicates that organic carbon comprises
roughly 70%-90% of the carbon emitted from gasoline vehicles and 40%-50% from diesel
vehicles [12]. Sulfur has also been reported as a major component of emissions from combustion
vehicles [13]. A detailed understanding of emissions from mobile sources is documented in [14].
High concentrations of ultraines are found on roadways and people are exposed to them in
vehicles and alongside the roadways. However, because of particle growth, they have very short
residence times, and number concentrations diminish very rapidly with distance from the road. In
some cases, particle number concentrations at 50 m from a busy roadway were ive to eight times
those measured at 100 and 500 m [15].
Recent technological advances provide excellent single-particle data via aerosol time-of-light
mass spectrometry (ATOFMS) instruments [16]. However, data reduction is arduous because a vast
amount of data is collected during a sampling session. Methods for analysis of the data continue
to be developed. These spectrometers are used for continuous single-particle measurements of
size and composition simultaneously [17]. Particles are size selected as they enter the inlet of the
spectrometer. They are then disintegrated by a laser beam, and the resulting ions are driven down
separate positive and negative ion channels for identiication of the charged fragment masses. In
some systems, particle concentrators are being used to increase the concentration at the inlet.
The concentrators have been developed for use in testing the effects of inhalation exposure to
ambient particles. Measurements with these time-of-light instruments have been undertaken in
cities located in different geological areas of the United States to characterize ine and ultraine
urban PM.
8.4 LUNG DEPOSITION
UFPs deposit in the respiratory tract primarily by diffusion, with deposition increasing as particle
size decreases. The total lung deposition of UFPs measured in human volunteers conirms diffusion
as the mechanism [18,19]. When the smallest UFPs are inhaled, they deposit very eficiently in the
nasal and oral passages. The high deposition in the extrathoracic region was determined in human
nasal/oral casts and in human volunteers [20,21].
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