Aerosol Particles in the Polluted and Global Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 116

surface area concentration (surface area per volume of
air) than do other modes.
Some other condensable gases (with low SVPs)
include high-molecular-weight organic gases, such
as certain products of toluene, xylene, alkylbenzene,
alkane, alkene, and biogenic hydrocarbon oxidation
(Pandis et al., 1992).

in solution. Acidity is measured in terms of pH ,
where

log 10 [H + ]

pH =− (5.4)

[H + ]isthe molarity of H + (moles of H + per liter of
solution). The more acidic a solution, the higher the
molarity of H + and the lower the pH. The pH scale
(Figure 10.3) ranges from less than 0 (highly acidic) to
greater than 14 (highly basic or alkaline). In pure water,
the only source of H + is

5.3.2.1.1. Vapor Deposition/Sublimation. Vapor
deposition is the process by which gas diffuses to an
aerosol particle surface and deposits on the surface as a
solid. Water vapor deposition to ice is the most common
type of vapor deposition in the atmosphere. It occurs in
clouds only at subfreezing temperatures (below 0 ◦ C)
and when the partial pressure of water exceeds the
SVP of water over ice. Figure 5.12b shows the SVP
of water vapor over ice at subfreezing temperatures.
The reverse of water vapor deposition is sublimation ,
the conversion of ice to water vapor. Deposition of
ammonium chloride gas or ammonium nitrate gas to the
solid phase can also occur when the relative humidity
is sufficiently low. At high relative humidity, though,
these gases dissolve in liquid water within aerosol
particles.

H + OH − H 2 O(aq) +

(5.5)

Liquid Hydrogen Hydroxide

water ion ion

where OH − is the hydroxide ion and arrows in
both directions indicate that the reaction is reversible.
Because the product [H + ][OH − ]must equal 10 − 14 mol 2
L − 2 , and [H + ]must equal [OH − ]tobalance charge, the
pH of pure water is 7 ([H + ] 10 − 7 mol L − 1 ).

Acids are substances that, when added to a solu-
tion, dissociate, increasing the molarity of H + .The
more H + added, the stronger the acid and the lower
the pH. Common acids include sulfuric [H 2 SO 4 (aq)],
hydrochloric [HCl(aq)], nitric [HNO 3 (aq)], and car-
bonic [H 2 CO 3 (aq)] acids. When the pH is low (

=

2),
HCl(aq), HNO 3 (aq), and H 2 SO 4 (aq) dissociate read-
ily, whereas H 2 CO 3 (aq) does not. The former acids are
strong acids , and the latter acid is a weak acid .
Bases (alkalis) are substances that, when added
to a solution, remove H + , ncreasing pH. Some
bases include ammonia [NH 3 (aq)] and slaked lime
[Ca(OH) 2 (aq)].
When anions, cations, or certain undissociated
molecules are dissolved in water, the water can bond
to the ion in a process called hydration .Several water
molecules can hydrate to each ion. Hydration, which
increases the liquid water content of aerosol particles,
is important when the relative humidity is less than
100 percent. The higher the sub-100-percent relative
humidity and the greater the quantity of solute in solu-
tion, the greater the liquid water content of aerosol par-
ticles due to hydration. At relative humidities greater
than 100 percent, the volume of water added to a parti-
cle by hydration is small compared with that added by
water vapor condensation.
Next, dissolution and reaction of some strong acids
and a base in aerosol particles are discussed. The dis-
cussion extends to cloud drops and rain in Chapter 10.

<

5.3.2.1.2. Dissolution, Dissociation, and Hydra-
tion. Dissolution is the process by which a gas, sus-
pended over an aerosol particle surface, diffuses to and
dissolves in a liquid on the surface. The liquid in which
the gas dissolves is a solvent .Inaerosol and hydro-
meteor particles, liquid water is most often the solvent.
Any gas, liquid, or solid that dissolves in a solvent is
a solute .Oneor more solutes plus the solvent com-
prise a solution .The ability of a gas to dissolve in
water depends on the solubility of the gas, which is the
maximum amount of a gas that can dissolve in a given
amount of solvent at a given temperature.
In a solution, dissolved molecules may dissociate
(break into simpler components, namely, ions). Pos-
itive ions, such as H + ,Na + ,K + ,Ca 2 + , and Mg 2 + ,
are cations .Negativeions, such as OH − ,Cl − ,NO 3 − ,
HSO 4 − ,SO 4 2 − ,HCO 3 − , and CO 3 2 − ,are anions .The
dissociation process is reversible, meaning that ions
can reform a dissolved molecule. Substances that
undergo partial or complete dissociation in solution are
electrolytes .Thedegree of dissociation of an elec-
trolyte depends on the acidity of the solution, the
strength of the electrolyte, and the concentrations of
ions in solution.
The acidity of a solution is a measure of the con-
centration of hydrogen ions (protons or H + ions)

5.3.2.1.3. Hydrochloric Acid. Gas-phase hydrochlo-
ric acid [HCl(g)] is abundant over the ocean, where it