120 ATMOSPHERIC CHEMISTRY
Hence, there is considerable room for continued research.
Atmospheric chemistry is involved in several steps through
the air-pollution system. First is chemically characterizing
and quantifying the emissions of primary pollutants. Second
is understanding the chemical and physical transformations
that these primary pollutants undergo. Third is measuring the
quantities of the various pollutants in the ambient air. Fourth
is quantifying the deposition processes for the various pol-
lutants. Finally, a mathematical formulation of the sources,
chemical and physical transformations, and removal pro-
cesses must be incorporated into the atmospheric model.
The chemistry of the formation of secondary pollutants
is extremely complex. It requires the identification of all of
the important reactions contributing to the chemical system.
There must be a thorough investigation of each specific reac-
tion, which can be achieved only when the reaction-rate
constant has been carefully determined for each elementary
reaction involved in the properly specified reaction mecha-
nism. Because of the large number of important reactions
that take place in the atmosphere, the rapid rates of many of
them, and the low concentrations of most of the reactants, the
experimental investigations of these atmospheric chemical
kinetics is an enormously large and complex task.
In the United States, a set of National Ambient Air Quality
Standards (NAAQS) have been established, as shown in Table 2.The primary standards are designed to protect the public health
of the most susceptible groups in the population. Secondary
NAAQS have also been set to protect the public welfare,
including damage to plants and materials and aesthetic effects,
such as visibility reduction. The only secondary standard that
currently exists that is different from the primary standard is for
SO 2 , as shown in the table. For comparison purposes, Table 3
shows recommended limits for air pollutants set by the World
Health Organization and various individual countries.
To illustrate the importance and complexity of atmospheric
chemistry, a few examples will be presented and discussed:
(1) urban photochemical-oxidant problems, (2) secondary
organic aerosols, (3) chemistry of acid formation, and
(4) stratospheric ozone changes in polar regions. These
examples also illustrate the differences in the spatial scales
that may be important for different types of air-pollution
problems. Considering urban problems involves dealing with
spatial distances of 50 to 100 km and heights up to a few kilo-
meters, an urban scale or mesoscale. The chemistry related
to acid formation occurs over a much larger, regional scale,
extending to distances on the order of 1000 km and altitudes
of up to about 10 km. For the stratospheric ozone-depletion
problem, the chemistry of importance occurs over a global
scale and to altitudes of up to 50 km. Secondary organic aero-
sol formation can be an urban to regional scale issue.TABLE 2
U.S. National Ambient Air Quality Standards
Pollutant Primary Averaging Times SecondaryCarbon monoxide 9 ppm 8-hour^1 None
35 ppm 1-hour^1 None
Lead 1.5 g/m^3 Quarterly average Same as primary
Nitrogen dioxide 0.053 ppm Annual (arith. mean) Same as primary
Particulate matter (PM 10 ) 50 g/m^3 Annual^2 (arith. mean) Same as primary
150 g/m^3 24-hour^1
Particulate matter (PM2.5) 15 g/m^3 Annual^3 (arith. mean) Same as primary
65 g/m^3 24-hour^4
Ozone 0.08 ppm 8-hour^5 Same as primary
0.12 ppm 1-hour^6 Same as primary
Sulfur oxides 0.03 ppm Annual (arith. mean) —
0.14 ppm 24-hour^1 —
— 3-hour^1 0.5 ppm
1 Not to be exceeded more than once per year.
2 To attain this standard, the expected annual arithmetic mean PM 10 concentration at each
monitor within an area must not exceed 50 μ g/m^3.
3 To attain this standard, the 3-year average of the annual arithmetic mean PM 2.5 concentrations
from single or multiple community-oriented monitors must not exceed 15 μ g/m^3.
4 To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at
each population-oriented monitor within an area must not exceed 65 μ g/m^3.
5 To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour
average ozone concentrations measured at each monitor within an area over each year must
not exceed 0.08 ppm.
6 (a) The standard is attained when the expected number of days per calendar year with
maximum hourly average concentrations above 0.12 ppm is 1.
(b) The 1-hour NAAQS will no longer apply to an area one year after the effective data of the
designation of that area for the 8-hour ozone NAAQS.
Source: Data is from the U.S. EPA Web site: http://www.epa.gov/air/criteria.html.C001_008_r03.indd 120C001_008_r03.indd 120 11/18/2005 10:15:19 AM11/18/2005 10:15:19 AM