Aerosol Particles in the Polluted and Global Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 111

cluster becomes a stable new particle and can grow
further.
Homogeneous nucleation occurs without the aid
of an existing surface. Another type of nucleation is
heterogeneous nucleation ,which occurs when a clus-
ter forms on a preexisting particle surface. Thus, it does
not result in new particles. If background aerosol par-
ticles are present, a nucleating gas is more likely to
nucleate heterogeneously than homogeneously. Thus,
homogeneous nucleation is likely to occur either in rel-
atively clean air or if concentrations of a nucleating
gas become very high. In addition, gases that nucleate
must be condensable. Conditions for condensation are
discussed in Section 5.3.2.1. Homogeneous or hetero-
geneous nucleation must occur before a particle can
grow by condensation or vapor deposition, processes
discussed later in the chapter.
Homogeneous and heterogeneous nucleation are
homomolecular, binary, or ternary. Homomolecular
nucleation occurs when molecules of only one gas
species nucleate, binary nucleation occurs when
molecules of two gas species nucleate, and ternary
nucleation occurs when molecules of three gas species
nucleate.
Some gases that undergo homomolecular nucleation
include high-molecular-weight organic gas oxidation
products of toluene, xylene, alkylbenzenes, alkanes,
alkenes, and terpenes. In addition, the chemical reac-
tion of ammonia gas plus hydrochloric acid gas pro-
duces ammonium chloride gas, which can homoge-
neously nucleate to form solid ammonium chloride
if concentrations are sufficiently high and the rela-
tive humidity is sufficiently low (Seinfeld and Pandis,
2006). Homogenous nucleation of water vapor does
not occur under typical atmospheric conditions. Water
nucleation is always heterogeneous. Indeed, a cloud
drop in the atmosphere forms when water nucleate het-
erogeneously on one aerosol particles, and additional
water then rapidly condenses on the embryo. Aerosol
particles that can become cloud drops following hetero-
geneous nucleation and condensation are called cloud
condensation nuclei (CCN).
Sulfuric acid and water are the most common gases
that undergo binary nucleation. Newly formed homo-
geneously nucleated sulfuric acid-water particles are
typically 1 to 3 nm in diameter. In the remote atmo-
sphere (e.g., over the ocean), homogenous nucleation
events can produce more than 10 4 particles cm − 3 in this
size range over a short period.
If ammonia gas is present, it can combine with sul-
furic acid and water in a ternary nucleation event.

Table 5.5. Metals present in fly ash of different
industrial origin

Source Metals present in fly ash

Smelters Fe, Cd, Zn

Oil-fired power plants V, Ni, Fe

Coal-fired power plants Fe, Zn, Pb, V, Mn, Cr, Cu,

Ni, As, Co, Cd, Sb, Hg

Municipal waste
incineration

Zn, Fe, Hg, Pb, Sn, As, Cd,
Co, Cu, Mn,Ni,Sb

Open hearth furnaces at
steel mills

Fe, Zn, Cr, Cu, Mn, Ni, Pb

Sources: Henry and Knapp (1980); Schroeder et al. (1987);
Pooley and Mille (1999); Ghio and Samet (1999).

are emitted simultaneously. Table 5.5 lists some metals
present in fly ash of different industrial origin.
Of the metals emitted into the air industrially, iron
is by far the most abundant. Lead, a criteria air pollu-
tant, is emitted industrially from lead ore smelting, lead
acid battery manufacturing, lead ore crushing, and solid
waste disposal.

5.2.1.7. Miscellaneous Sources
Additional particle types in the air include tire rubber
particles, pollen, spores, bacteria, viruses, plant debris,
and meteoric debris. Tire rubber particles are emitted
due to the constant erosion of a tire at the tire-road inter-
face. Such particles are generally larger than 2.5 min

diameter. Pollens , spores , bacteria , viruses , and plant
debris are biological particles lifted by the wind. They
often serve as sites on which cloud drops and ice crystals
form. A stratosphere source of new particles is meteoric
debris .Mostmeteorites disintegrate before they fall to
an altitude of 80 km. Those that reach the stratosphere
contain iron (Fe), titanium (Ti), and aluminum (Al),
among other elements. The net contribution of mete-
orites to particles in the stratosphere is small (Sheridan
et al., 1994).

5.2.2. Homogeneous Nucleation
Aside from emissions, homogeneous nucleation is the
only source of new particles in the air. Homogeneous
nucleation is a process by which gas molecules aggre-
gate to form clusters that change phase to a liquid
or solid. If the radius of the cluster reaches a criti-
cal size (generally fifteen to twenty molecules), the