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6 Lawrence K. Wang et al.

(d ) and 50%, or the ratios for 50% and 15.87% (d ). This geometric standard
84.13 15.87
deviation (σ ) becomes:
g
σ = d d = d d
g 84.13 50 50 15 87 (5)
.
In Fig. 1, σ is 3.76. Note that the slopes of the curves (σ ) should be similar for all
g g
three methods of expressing the same material.
If the particulate matter is composed of more than one material or if it is a single
substance in different physical structures, it will most likely be bimodal in size distri-
bution. This can be true for material in the stack effluent and mixtures in the free
(ambient) atmosphere. For example, combustion-flue gases contain particulates com-
posed of a large fraction mainly entrained as partially unburned fuel, plus a smaller
fraction consisting of ash. Particulates sampled from a stoker-fired, chain grate boiler
(2) are shown in Fig. 2. Note how this material must be plotted as two intercepting
lines on log probability coordinates.
As shown in Fig. 2, atmospheric particulates are also bimodal in size distribution
(3). These data are plotted as ∆mass/∆log diameter versus the log of diameter to
amplify the bimodal distribution character. In general, atmospheric particulates con-
sist of a submicron group (<1 µm) and a larger group. Although the data shown in Fig.
3 are typical for the United States, similar results are obtained throughout the world,
as reported, for example, in Japan (4) and Australia (5). These authors note that atmo-
spheric sulfates and nitrates dominate the smaller group, which by mass accounts for
40% and include particles with diameters from 0.5 to 1.5 µm, which account for
another 40%.

3. STANDARDS
3.1. Ambient Air Quality Standards
Ambient air is defined as the outside air of a community, in contrast to air confined
to a room or working area. As such, many people are exposed to the local ambient air
24 h a day, 7 d a week. It is on this basis that ambient air quality standards are formu-
lated. The current standards were developed relatively quickly after the numerous
episodes of the 1960s. The feeling of many people were summed up by President
Johnson’s statement in 1967: “If we don’t clean up this mess, we’ll all have to start
wearing gas masks,” and “This country is so rich that we can achieve anything if we
make up our mind what we want to do.” There are those who believe that some require-
ments in the standards disregard costs of control compared with costs of benefits, but
all benefits and costs cannot be accurately assessed. Even so, we would be reluctant to
put dollar values on our own health and life.
The Clean Air Act Amendments of 1970 (Public Law 91-601, signed December 31,
1970) include ambient air standards that consist of two parts: primary standards, which
are intended for general health protection, and the more restrictive secondary standards,
which are for protection against specific adverse effects on health and welfare. “Welfare”
here includes plants, other animals, and materials. The primary standards are effective as
of 1975, and the secondary standards are effective as of June 1, 1977. An abbreviated list
of these 1997 standards for particulate matter and categories of gaseous pollutants is