Page 44 - Fundamentals of Air Pollution

III. Particulate Matter 23

TABLE 2-3
Conversion Factors between Volume and Mass
Units of Concentration (25°C, 760 mm Hg)
To convert from
3
ppm (vol) to /u.g/m to ppm
3
ju,g/m , (vol), multiply
3
Pollutant multiply by: by (x 10~ ):
Ammonia (NH 3) 695 1.44
Carbon dioxide 1800 0.56
Carbon monoxide 1150 0.87
Chlorine 2900 0.34
Ethylene 1150 0.87
Hydrogen chloride 1490 0.67
Hydrogen fluoride 820 1.22
Hydrogen sulfide 1390 0.72
Methane (carbon) 655 1.53
Nitrogen dioxide 1880 0.53
Nitric oxide 1230 0.81
Ozone 1960 0.51
Peroxyacetylnitrate 4950 0.20
Sulfur dioxide 2620 0.38

exerts its proportionate partial pressure. The particles are aggregates of
many molecules, sometimes of similar molecules, often of dissimilar ones.
They age in the air by several processes. Some particles serve as nuclei upon
which vapors condense. Some particles react chemically with atmospheric
gases or vapors to form different compounds. When two particles collide
in the air, they tend to adhere to each other because of attractive surface
forces, thereby forming progressively larger and larger particles by agglom-
eration. The larger a particle becomes, the greater its weight and the greater
its likelihood of falling to the ground rather than remaining airborne. The
process by which particles fall out of the air to the ground is called sedimenta-
tion. Washout of particles by snowflakes, rain, hail, sleet, mist, or fog is a
common form of agglomeration and sedimentation. Still other particles
leave the air by impaction onto and retention by the solid surfaces of
vegetation, soil, and buildings. The particulate mix in the atmosphere is
dynamic, with continual injection into the air from sources of small particles;
creation of particles in the air by vapor condensation or chemical reaction
among gases and vapors; and removal of particles from the air by agglomer-
ation, sedimentation, or impaction.
Before the advent of humans and their works, there must have been
particles in the air from natural sources. These certainly included all the
particulate forms of condensed water vapor; the condensed and reacted
forms of natural organic vapors; salt particles resulting from the evaporation