Page 371 - Corrosion Engineering Principles and Practice

340 C h a p t e r 9 A t m o s p h e r i c C o r r o s i o n 341

been identified as one of the most important air pollutants that
contribute to the corrosion of metals.
Less recognized as corrosion promoters, are the nitrogen oxides
(NO ), which are also products of combustion. A major source of NO in
x
x
urban areas is the exhaust fumes from vehicles. Sulfur dioxide, NO and
x
airborne aerosol particles can react with moisture and UV light to form
new chemicals that can be transported as aerosols. A good example of
this is the summertime haze over many large cities. Up to 50 percent of
this haze is a combination of sulfuric and nitric acids.

9.3.3 Deposition of Aerosol Particles
The behavior of aerosol particles in outdoor atmospheres is explained
by laws that govern their formation, movement, and capture. These
particles are present throughout the planetary boundary layer and
their concentrations depend on a multitude of factors including
location, time of day or year, atmospheric conditions, presence of
local sources, altitude, and wind velocity.
The highest concentrations are usually found in urban areas,
3
reaching up to 10 and 10 particles per cm , with particle size ranging
8
9
from around 100 mm to a few nanometer. Size is normally used to
classify aerosol because it is the most readily measured property and
other properties can be inferred from size information [7]. The highest
mass fraction of particles in an aerosol is characterized by particles
having a diameter in the range of 8 to 80 mm [8].
Some studies have indicated that there is a strong correlation
between wind speed and the deposition and capture of aerosols. In
such a study of saline winds in Spain a very good correlation was
found between chloride deposition rates and wind speeds above a
−1
threshold of 3 m s or 11 km h [9].
−1
Aerosols can either be produced by ejection into the atmosphere,
or by physical and chemical processes within the atmosphere (called
primary and secondary aerosol production, respectively). Examples
of primary aerosols are sea spray and wind-blown dust. Secondary
aerosols are produced by atmospheric gases reacting and condensing,
or by cooling vapor condensation. Once an aerosol is suspended in
the atmosphere, it can be altered, removed, or destroyed.
Aerosol particles do not stay in the atmosphere indefinitely, and
average lifetimes are of the order of a few days to a week, depending
on their size and location. Aerosol particles have a finite mass and are
subject to the influence of gravity, wind resistance, droplet dry-out,
and possibilities of impingement on a solid surface. Studies of the
migration of aerosols inland of a sea coast have shown that typically
the majority of the aerosol particles are deposited close to the shoreline
(typically 400 to 600 m) and consist of large particles (>10mm diameter),
which have a short residence time and are controlled primarily by
gravitational forces [8;9].