Page 71 - Fundamentals of Air Pollution 3E

IV. Measurement Systems 47

the primary pollutant -• that react are termed the precursors of the secondary
pollutants. With the ki owledge that each secondary pollutant arises from
specific chemical reactions involving specific primary reactants, we must
control secondary pollute its by controlling how much of each primary
pollutant is allowed to be e.Tiitted.

IV. MEASUREMENT SYSTEMS

Many methods of air quality measurement have inherent averaging
times. In selecting methods for measuring air quality or assessing air pollu-
tion effects, this fact must be borne in mind (Table 4-1). Thus, an appropriate
way to assess the influence of air pollution on metals is to expose identical
specimens at different locations and compare their annual rates of corrosion
among the several locations. Since soiling is mainly due to the sedimenta-
tion of particulate matter from the air, experience has shown that this can
be conveniently measured by exposing open-topped receptacles to the
atmosphere for a month and weighing the settled solids. Human health
seems to be related to day-to-day variation in pollutant level. It is accepted
practice the world over to assess suspended particulate matter levels in the
air by a 24-hour filter sample, which in the United States is acquired by a
high-volume sampler, known to workers in the field as a hi-vol.
Because a filter sample includes particles both larger and smaller than
those retained in the human respiratory system (see Chapter 7, Section
III), other types of samplers are used which allow measurement of the size
ranges of particles retained in the respiratory system. Some of these are
called dichotomous samplers because they allow separate measurement of the
respirable and nonrespirable fractions of the total. Size-selective samplers rely
on impactors, miniature cyclones, and other means. The United States has
selected the size fraction below an aerodynamic diameter of 10 /-tm (PM 10)
for compliance with the air quality standard for airborne particulate matter.

TABLE 4-1

Air Quality Measurement
Measure of Cyclic factor Measurement method with Effect with same
averaging time measured same averaging time averaging time
Year Annual trend Metal specimen Corrosion
Month Seasonal cycle Dustfall Soiling
Day Weekly cycle Hi-vol Human health
Hour Diurnal cycle Sequential sampler Vegetation damage
Minute Turbulence Continuous instrument Irritation (odor)