Page 72 - Fundamentals of Air Pollution

48 4. Air Quality

Because an agricultural crop can be irreparably damaged by an excursion
of the level of several gaseous pollutants lasting just a few hours, recording
such an excursion requires a measuring procedure that will give hourly
data. The least expensive device capable of doing this is the sequential
sampler, which will allows a sequence of 1- or 2-hr samples day after day
for as long as the bubblers in the sampler are routinely serviced and ana-
lyzed. As already discussed, the hourly data from sequential samplers can
be combined to yield daily, monthly, and annual data.
None of the foregoing methods will tell the frequency or duration of
exposure of any receptor to irritant or odorous gases when each such
exposure may exceed the irritation or odor response threshold for only
minutes or seconds. The only way that such an exposure can be measured
instrumentally is by an essentially continuous monitoring instrument, the
record from which will yield not only this kind of information but also all
the information required to assess hourly, daily, monthly, and annual
phenomena. Continuous monitoring techniques may be used at a particular
location or involve remote sensing techniques.

V. AIR QUALITY LEVELS

A. Levels
Air quality levels vary between concentrations so low that they are less
than the minimum detectable values of the instruments we use to measure
them and maximum levels that are the highest concentrations ever mea-
sured. Table 4-2 gives data from monitoring sites reporting concentrations
approaching maximum values for the four principal gaseous pollutants.
Figure 4-7 gives national data (1982-1991) for CO in the United States. The
mean chemical composition and atmospheric concentration of suspended
particulate matter (total, coarse, and fine) measured in the United States
in 1980 are shown in Table 4-3. The percentages do not add up to 100%
because they exclude the oxygen (except for the nitrate and sulfate compo-
nents), nitrogen (except for the nitrate component), hydrogen, and other
components of the compounds of the listed elements in the form in which
they actually exist in the atmosphere; for example, the most common form
of particulate sulfur and sulfate in the atmosphere is (NH 4) 2(SO 4). The table
indicates that about 30% of the mass of particulate matter is in the fine
fraction (<2.5 /-on), 30% is in the coarse fraction (>2.5 < 15 /am), and 40%
is coarser still—between 15 and ca. 50 ^tm.
Because there has been no recent analysis of the relationship between
the concentration of total suspended particulate matter in the air of cities
with populations of different sizes, we are forced to use data for the decade