Page 179 - Fundamentals of Air Pollution

II. Formation of Atmospheric Haze 145

engines. Power plants with advanced control technology still emit substan-
tial numbers and masses of fine particles with diameters <1.0 /am. The
composition of these particles includes soot or carbonaceous material, trace
metals, V 2O 5, and sulfates. In addition, large quantities of NO 2 and SO 2
are released to the atmosphere.

A. Particle Formation in the Atmosphere
The secondary source of fine particles in the atmosphere is gas-to-particle
conversion processes, considered to be the more important source of parti-
cles contributing to atmospheric haze. In gas-to-particle conversion, gas-
eous molecules become transformed to liquid or solid particles. This phase
transformation can occur by three processes: absortion, nucleation, and
condensation. Absorption is the process by which a gas goes into solution
in a liquid phase. Absorption of a specific gas is dependent on the solubility
of the gas in a particular liquid, e.g., SO 2 in liquid H 2O droplets. Nucleation
and condensation are terms associated with aerosol dynamics.
Nucleation is the growth of clusters of molecules that become a thermody-
namically stable nucleus. This process is dependent on the vapor pressure
of the condensable species. The molecular clusters undergo growth when
the saturation ratio, S, is greater than 1, where saturation ratio is defined
as the actual pressure of the gas divided by its equilibrium vapor pressure.
S > 1 is referred to as a supersaturated condition (14).
The size at which a cluster may be thermodynamically stable is influenced
by the Kelvin effect. The equilibrium vapor pressure of a component in-
creases as the droplet size decreases. Vapor pressure is determined by
the energy necessary to separate a single molecule from the surrounding
molecules in the liquid. As the curvature of the droplet's surface increases,
fewer neighboring molecules will be able to bind a particular molecule to
the liquid phase, thus increasing the probility of a molecule escaping the
liquid's surface. Thus, smaller droplets will have a higher equilibrium vapor
pressure. This would affect the minimum size necessary for a thermody-
namically stable cluster, suggesting that components with lower equilib-
rium saturation vapor pressures will form stable clusters at smaller diam-
eters.
Condensation is the result of collisions between a gaseous molecule and
an existing aerosol droplet when supersaturation exists. Condensation oc-
curs at much lower values of supersaturation than nucleation. Thus, when
particles already exist in sufficient quantities, condensation will be the
dominant process occurring to relieve the supersaturated condition of the
vapor-phase material.
A simple model for the formation and growth of an aerosol at ambient
conditions involves the formation of a gas product by the appropriate
chemical oxidation reactions in the gas phase. This product must have a