Structure and Composition of the Present-Day Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 55

N 2 (g), O 2 (g), Ar(g), and CO 2 (g). The standard value of
m d is 28.966 g mol − 1 .The equation of state for water
vapor is analogous to that for dry air.

Table 3.2. Volume mixing ratios of well-mixed gases
in the lowest 100 km of the Earth's atmosphere

Volume mixing ratio

Chemical

Gas name formula (Percent) (ppmv)

Example 3.5
When p d =

1,013 hPa and T = 288 K, what is the

Molecular nitrogen N 2 (g) 78.08 780,000

density of dry air?

Molecular oxygen O 2 (g) 20.95 209,500

Argon Ar(g) 0.93 9,300

Solution
From Equation 3.12,

Neon Ne(g) 0.0015 15

1.23 kg m −3 . d =

Helium He(g) 0.0005 5

Krypton Kr(g) 0.0001 1

The number concentration of a gas (molecules per
unit volume of air) is an absolute quantity. The abun-
dance of a gas may also be expressed in terms of a
relative quantity, volume mixing ratio , defined as the
number of gas molecules per molecule of dry air, and
expressed for gas q as

Xenon Xe(g) 0.000005 0.05

molecules in this region. The mixing ratios of well-
mixed gases do not vary much in time or space over
short time scales (hundreds to thousands of years). Nev-
ertheless, it is the spatially and temporally varying
gases ,whose mixing ratios are small but vary in time
and space, that are the most relevant to air pollution.

N q
N d =

p q
p d

q = (3.14)

where N q and p q are the number concentration and
partial pressure, respectively, of gas q .Volume mix-
ing ratios may be multiplied by 100 and expressed as a
percentage of dry air volume ,multiplied by 10 6 and
expressed in parts per million volume (ppmv), multi-
plied by 10 9 and expressed in parts per billion volume
(ppbv), or multiplied by 10 12

3.5.1. Well-Mixed Gases
Gases can become well mixed in the bottom 100 km
of the atmosphere only if they (1) are long lived (have
low chemical, physical, and biological loss rates) and
(2) are emitted uniformly over time. Table 3.2 lists most
well-mixed gases in the homosphere. Although carbon
dioxide is a long-lived gas, it is not well mixed because
its emission rate changes over short time scales due
to human activity and because human emissions are
concentrated in urban areas, giving rise to higher mixing
ratios in urban areas than in surrounding rural areas.
These higher mixing ratios in urban areas are referred
to as carbon dioxide domes .
Molecular nitrogen [N 2 (g)] and molecular oxygen
[O 2 (g)] are the most abundant well-mixed gases. At
any altitude, N 2 (g) comprises about 78.08 percent, and
O 2 (g) comprises about 20.95 percent and of all non-
water gas molecules by volume. Although these gases
have mixing ratios that are constant with increasing
altitudes, they have partial pressures that decrease with
increasing altitude because air pressure decreases with
increasing altitude (Figure 3.2a), and O 2 (g) and N 2 (g)
partial pressures are constant fractions of air pressure.
Together, N 2 (g) and O 2 (g) make up 99.03 percent
of all gases in the atmosphere by volume. Argon (Ar)
makes up most of the remaining 0.97 percent. Argon,
the “lazy gas,” is colorless and odorless. Like other
noble gases, it is inert and does not react chemically.

and expressed in parts

per trillion volume (pptv).

Example 3.6
Find the number concentration and partial pres-
sure of ozone if its volume mixing ratio is

q = 0.10 ppmv. Assume T = 288 K and p d =

1,013 hPa.

Solution
From Example 3.4, N d = 10 19 molec cm −3 .

Thus, from Equation 3.14, the number concen-
tration of ozone is N q

2.55 ×

= 0.10 ppmv × 10 −6 ×

2.55 10 12 molec

cm −3 .FromEquation 3.9, the partial pressure
exerted by ozone is p q =

× 10 19 molec cm −3 = 2.55 ×

0.000101 hPa.

3.5. Composition of the Present-Day
Atmosphere
The present-day atmosphere below 100 km (the homo-
sphere) contains only a few well-mixed gases that,
together, comprise more than 99 percent of all gas