Aerosol Particles in the Polluted and Global Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 105

asea spray drop and forces chloride to evaporate as
hydrochloric acid [HCl(g)]. Some sea spray drops lose
all their chloride in the presence of nitric or sulfuric acid.
The size of a sea spray drop is also affected by dehy-
dration. Dehydration (loss of water) occurs when water
from a drop evaporates due to a decrease in the relative
humidity between the air just above the ocean surface
and that a few meters higher. Dehydration increases the
concentration of solute in a drop.

rocks are subjected to high pressure by soil or other
rocks lying above. Stresses also arise when rocks freeze
and then thaw, or when saline solutions enter cracks
and cause rocks to disintegrate or fracture. Salts have
higher thermal expansion coefficients than do rocks;
thus, when temperatures increase, salts within rock
fractures expand, forcing the rock to open and break
apart. One source of salt for rock disintegration is sea
spray transported from the oceans. Another source is
desert salt (from deposits) transported by winds. Physi-
cal weathering of rocks on the Earth's surface can occur
by their constant exposure to winds or running water.
Another process that causes some rocks to break
down and others to reform is chemical weathering .
Reaction 3.20, which showed calcium carbonate break-
ing down upon reaction with dissolved carbonic acid,
wasachemical weathering reaction. Another chemical
weathering reaction is

5.2.1.2. Soil Dust and Fugitive Dust Emissions
Soil is the natural, unconsolidated mineral and organic
matter lying above bedrock on the surface of the Earth.
A mineral is a natural, homogeneous, inorganic, solid
substance with a characteristic chemical composition,
crystalline structure, color, and hardness. Minerals in
soil originate from the breakdown of rocks. Organic
matter originates from the decay of dead plants and ani-
mals. Rocks are consolidated or unconsolidated aggre-
gates of minerals or biological debris.
Three generic rock families exist: sedimentary,
igneous, and metamorphic. Sedimentary rocks cover
about 75 percent of the Earth's surface; they form pri-
marily on land and on the floors of lakes and seas by
the slow layer-by-layer deposition and cementation of
carbonates, sulfates, chlorides, and shell and existing
rock fragments. An example of a sedimentary rock is
chalk ,made of skeletons of microorganisms. Igneous
rocks (“rocks from fire” in Greek) form by the cool-
ing of magma, which is a molten, silica-rich mixture
of liquid and crystals. Igneous rocks can form either
on the Earth's surface following a volcanic eruption or
beneath the surface when magma is present. Granite is
one example of an igneous rock. Metamorphic rocks
(rocks that “change in form”) result from the struc-
tural transformation of existing rocks due to high tem-
peratures and pressures in the Earth's interior. During
metamorphosis, no rock melting or change in chemical
composition occurs; elements merely rearrange them-
selves to form new mineral structures that are stable
in the new environment. Marble is an example of a
metamorphic rock.

Ca 2 + SO 2 −

4

CaSO 4 -2H 2 O(s) + + 2H 2 O(aq)

(5.1)

Calcium sulfate Calcium Sulfate Liquid

dihydrate (gypsum) ion ion water

which occurs when gypsum dissolves in water. Alterna-
tively, when high sulfate and calcium ion concentrations
are present in surface or groundwater, Reaction 5.1 can
proceed to the left, producing gypsum. Gypsum forms
when sulfate-containing particles from the air dissolve
in stream water containing calcium ions. Crystalline
gypsum can also form when sea spray drops, which
contain calcium, collide with volcanic aerosol particles,
which contain sulfate.

5.2.1.2.2. Types of Minerals in Soil Dust. Table
2.2 shows the most abundant elements in the Earth's
continental crust. Soil dust particles, emitted from the
top layer of the crust, contain minerals made of these
elements and organic matter. Minerals in soil include
quartz, feldspars, hematite, calcite, dolomite, gypsum,
epsomite, kaolinite, illite, smectite, vermiculite, and
chlorite.
Pure quartz [SiO 2 (s)] is a clear, colorless mineral
that is resistant to chemical weathering. The Greeks
believed that it was frozen water. Its name origi-
nates from the Saxon word querkluftertz ,which means
“cross-veined ore.”
Feldspars ,which comprise at least 50 percent of
the rocks on the Earth's surface, are by far the most
abundant minerals on Earth. The name feldspar orig-
inates from the Swedish words feld (“field”) and
spar (the name of a mineral commonly found over-
lying granite). Two common types of feldspars are

5.2.1.2.1. Breakdown of Rocks to Soil Material.
Breakdown of rocks to soil material occurs primarily
by physical weathering. Physical weathering is the
disintegration of rocks and minerals by processes that
do not involve chemical reactions. Disintegration may
occur when a stress is applied to a rock, causing it
to break into blocks or sheets of different size and,
ultimately, into fine soil minerals. Stresses arise when