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164 Atmospheric Chemistry

b. The nitrate radical

Because OH is produced primarily by photochemical

reactions and has a very short lifetime, it is present

in the atmosphere at currently measurable con-

centrations only during the day. At night, the nitrate

radical NO

takes over from OH as the major

reactive oxidant in the troposphere. The nitrate radi-

cal is formed by Eq. (5.4). During the day, NO

photolyzed rapidly by solar radiation via two path-

ways

(5.10)

and

(5.11)

The resulting atmospheric residence time of NO

at noon in sunlight is 5s.NO

also reacts with NO

(5.12)

Although NO

is much less reactive than OH, at

night it is present in much higher concentrations than

OH is during the day so it can serve as an effective

oxidant.

c. Odd nitrogen and other “chemical families”

Odd nitrogen (NO

), or total reactive nitrogen as it is

sometimes called, refers to the sum of NO

plus all

compounds that are the products of the atmospheric

oxidation of NO

, including HNO

,NO

, dinitrogen

pentoxide (N

), and peroxyacetyl nitrate (PAN for

short). Grouping the nitrogen-containing species in

this manner is useful in considering their budgets in

the atmosphere.

The main sources of NO

are anthropogenic emis-

sions (see Section 5.6.1) but, away from pollution

sources, soils and lightning can play dominant roles.

Interactions between NO

and NMHC can lead to

photochemical smogs in cities (see Section 5.5.2).

On regional and global scales, interactions of NO

with odd hydrogen have a strong influence on OH

concentrations.

and NO

are examples of “chemical families.”

Other useful chemical families are odd oxygen

(e.g., O, O*, O

and NO

), odd hydrogen (HO

, where

x  0, 1 or 2), and odd chlorine (ClO

,where x  0, 1

or 2). Note that odd hydrogen includes the OH radi-

cal and NO

includes the NO

radical.

 NO : 2NO

 h



(



 0.580 mm) : NO

 O

 h



(



 0.700 mm) : NO  O

d. Ammonia

Ammonia (NH

) originates from soils, animal waste,

fertilizers, and industrial emissions. It is the principal

basic gas in the atmosphere. Ammonia neutralizes

acid species by reactions of the form

(5.13)

The primary removal mechanisms for NH

involve

its conversion to ammonium-containing aerosols

by reactions such as (5.13). The aerosols are then

transported to the ground by wet and dry deposition.

The residence time of NH

in the lower troposphere

is 10 days.

5.3.3 Organic Compounds

Organic compounds contain carbon atoms. The four

electrons in the outer orbital of the carbon atom can

form bonds with up to four other elements: hydrogen,

oxygen, nitrogen, sulfur, halogens, etc. Hydrocarbons

are organics composed of carbon and hydrogen.

There are large natural and anthropogenic sources of

atmospheric hydrocarbons, gases that play key roles

in many aspects of the chemistry of the troposphere.

Methane is the most abundant and ubiquitous

hydrocarbon in the atmosphere. The present concen-

tration of CH

in the northern hemisphere is

1.7 ppmv; it has a residence time in the atmosphere

of 9 years. Sources of CH

include wetlands, land-

fills, domestic animals, termites, biomass burning,

leakages from natural gas lines, and coal mines. The

primary sink for tropospheric CH

is its oxidation by

OH to form formaldehyde (HCHO); HCHO then

photodissociates into CO (see Section 5.3.6) or, in air

with sufficient NO

, OH oxidizes CO to produce O

(see Section 5.3.5).

There are numerous nonmethane hydrocarbons

(NMHC) in the Earth’s atmosphere, and many of

them play important roles in tropospheric chemistry.

Based on their molecular structures, NMHC can be

grouped into several classes. Examples include alkanes

2n  2

), which include ethane (CH

!CH

) and

propane (CH

!CH

); alkenes, which have a

double bond, such as ethene (CH

"CH

) and

propene (CH

CH"CH

); and aromatics, such as ben-

zene (C

) and toluene (C

). Oxygenated hydro-

carbons, which contain one or more oxygen atoms,

such as acetone (CH

COCH

), may provide an

important source of HO

in the upper troposphere,

thereby influencing O

chemistry in this region.

2NH

 H

: (NH

)

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5.3 Some Important Tropospheric Trace Gases 165

5.3.4 Carbon Monoxide

Carbon monoxide is produced by the oxidation of

or NMHC, such as isoprene. Other important

sources of CO are biomass burning and the com-

bustion of fossil fuels. The dominant sink of CO is

oxidation by OH

(5.14)

Because (5.14) is generally the major sink for OH

in nonurban and nonforested locations, the concen-

tration and distribution of OH are often determined

by the ambient concentrations of CO, although CH

O, etc. can also be determining factors. An

important feature of CO in extratropical latitudes is

its seasonal cycle; it accumulates in the atmosphere

during winter when OH concentrations are low, but

in spring CO is depleted rapidly due to reaction (5.14).

Figure 5.5 shows satellite measurements of CO.

The high concentration of CO emanating from South

America and Africa are due to biomass burning. The

plumes travel slowly across the southern hemisphere

and can be detected in Australia during the dry

season. A strong source of CO, due to industrial

emissions, is also apparent in southeast Asia; this

plume can sometimes reach North America.

5.3.5 Ozone

Because about 90% of the O

in the Earth’s atmos-

phere is in the stratosphere (see Section 5.7.1), it was

suggested in the middle of the 20th century that the

CO  OH : H  CO

stratosphere was a primary source for tropospheric

and that a balance existed between this source

and surface sinks. It has subsequently been recog-

nized that trace gases, such as NO, CO, and organic

compounds, which are emitted by human activities,

lead to the formation of O

through photochemical

reactions. In addition, various natural sources, such

as hydrocarbons from vegetation and NO from light-

ning, produce O

precursors.

Ozone plays a controlling role in the oxidizing

capacity of the troposphere. Much of the O

in the

troposphere is produced by in situ homogeneous

gas-phase reactions involving the oxidation of CO,

, and NMHC by OH in the presence of NO

,as

outlined later.

Fig. 5.5 Concentrations of CO at an altitude of 4.5 km

measured from a satellite. Concentrations range from back-

ground values 50 pptv in regions shaded blue to as high

as 450 ppbv in the regions shaded red. The CO can be

transported upward and also be carried over large horizontal

distances. [Courtesy of NASA.]

Christian Friedrich Schönbein (1799–1868) German chemist. Discovered nitrocellulose (an explosive) when he used his wife’s cotton

apron to wipe up some spilled nitric and sulfuric acid and the apron disintegrated. Taught in England before joining the faculty at the

University of Basel.

Arie Jan Haagen-Smit (1900–1977) Dutch biologist and chemist. Joined the Biology Department, California Institute of Technology,

in 1937, where he worked on terpenes and plant hormones. Carried out pioneering work on analysis of Los Angeles’ smogs and the

emissions responsible for them.

Schönbein

discovered ozone by its odor follow-

ing an electrical discharge (it can sometimes be

smelled after a thunderstorm). He called it ozone,

after the Greek word ozein, meaning to smell.

Ozone is an irritating, pale blue gas that is toxic

and explosive. Because of its high reactivity, ozone

is an extremely powerful oxidizing agent that

damages rubber and plastics and is harmful to

humans and plants even at low concentrations

(several tens of ppbv). Haagen-Smit

suggested

that the formation of O

in cities is due to photo-

chemical reactions involving nitrogen oxides and

5.2 “Bad” and “Good” Ozone

Continued on next page

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166 Atmospheric Chemistry

At wavelengths 0.430



m, NO

is photolyzed

(5.15)

which is followed quickly by

(5.16)

where j and k

are rate coefficients.

However, much

of the O

produced by (5.16) is removed rapidly by

(5.17)

In combination, (5.15)–(5.17) constitute a null

cycle that neither creates nor destroys ozone.

Exercise 5.5 Derive an expression for the steady-

state concentration of O

given by Eqs. (5.15)–(5.17) in

terms of the concentrations of NO and NO

, j and k

Solution: All three reactions are fast: NO is formed

by (5.15) and NO

is formed by (5.17) as rapidly as it

is depleted by (5.15). Therefore, from the definition

of a rate coefficient,

where the square brackets indicate species concen-

tration. Therefore,

(5.18)[O

] 

[NO

]

[NO]

j [NO

]  k

] [NO]

 NO :

 O

O  O

 M :

 M

 h



NO  O

Expression (5.18) is called the Leighton

relationship.

It is an example of a photostationary state relation. ■

If typical concentrations of NO

and NO are

substituted into (5.18), together with values for j and

, the concentrations of O

obtained are far below

observed concentrations even in the free troposphere.

Hence, reactions other than (5.15)–(5.17) must be

involved in the control of tropospheric O

.This con-

clusion led to the suggestion that HO

, and related

radicals derived from organic species, are involved in

determining atmospheric concentrations of O

.The

hydroxyl radical can be produced from small amounts

of highly reactive atomic oxygen by (5.6b). Then, in

unpolluted air, OH is rapidly transformed to HO

(5.19a)

(5.19b)

and, in the absence of NO

,HO

is mostly con-

verted back to OH through reactions with NO or

,e.g.,

(5.19c)

In sunlight, a photostationary steady state is

established quickly between OH and HO

.HO

([OH]  [HO

]) is lost by

(5.20)2HO

: H

 O

: O

 OH

H  O

 M : HO

 M

OH  CO : H  CO

hydrocarbons released by cars and oil refineries.

Ozone reacts with hydrocarbons from automobile

exhausts and evaporated gasoline to form second-

ary organic pollutants such as aldehydes and

ketones. Ozone produced in urban areas can be

transported into rural areas far removed from

industrial regions. For example, during a summer

heat wave in 1988 the Acadia National Park in

Maine had dangerously high concentrations of

5.2 Continued

, which probably originated in New York City.

Ozone alone, or in combination with SO

and

, accounts for 90% of the annual loss of

crops due to air pollution in the United States.

In contrast to the bad effects of O

in the

troposphere, the much greater concentrations

of O

in the stratosphere reduce the intensity

of dangerous UV radiation from the sun (see

Section 5.7.1).

It is usual to represent chemical rate coefficients for photolytic reactions, such as (5.15), by j and the rate coefficients for nonphotolytic

reactions, such as (5.16), by k.

Philip Albert Leighton (1897–1983) American chemist. Faculty member at Stanford University. Main research interest was photochemistry.

His monograph The Photochemistry of Air Pollution (1961) was particularly influential in promoting research on urban air pollution.

P732951-Ch05.qxd 12/09/2005 09:05 PM Page 166

5.3 Some Important Tropospheric Trace Gases 167

and

(5.21)

Because H

and HNO

are highly soluble, they are

removed quickly from the troposphere by wet depo-

sition. The aforementioned reactions, together with

values for the appropriate rate coefficients, suggest

that OH concentrations near the Earth’s surface in

sunlight should be 3  10

molecules m

3

, which

is close to the measured concentration of OH during

daylight hours.

In the presence of a sufficiently high mixing

ratio of NO (10 pptv), HO

formed by the oxida-

tion of VOCs converts NO to NO

, which then

forms O

(5.22a)

(5.22b)

(5.22c)

(5.22d)

Note that OH and NO are regenerated in cycle

(5.22), and that NO is converted to NO

without

consuming O

, providing a pathway for net ozone

production. Thus, increases in NO might be expected

to produce increases in O

The OH radical also oxidizes methane

(5.23)

Then, if the concentration of NO

is low,

(5.24)

where CH

is the methylperoxy radical and

OOH is methyl hydroperoxide. CH

OOH is

removed by wet deposition, resulting in a net loss of

. If NO

is present, NO

from the reaction

(5.25)

leads to O

formation by (5.15)–(5.16). These reac-

tions demonstrate the importance of photochemistry

and NO

in determining the concentration of O

the global troposphere, as well as the complexity of

tropospheric ozone chemistry.

Figure 5.6 shows the global distribution of O

in the troposphere obtained by subtracting satellite

 NO : CH

O O

 CH

: CH

OOH  O

OH  CH

 O

: H

O  CH

O  O

 M : O



 O

 h



: NO  O

 NO : OH  NO

OH  CO  O

: HO

 CO

OH  NO

 M : HNO

 M

measurements of O

in the stratosphere from the

total column ozone concentration. It is evident that

is generally low throughout the year over the

tropical oceans. Over middle latitudes O

exhibits

pronounced seasonal variations, with a pronounced

spring maximum. Over industrialized regions of the

northern hemisphere ozone concentrations also tend

to be high during summer.

Dobson Units (DU)

December–February

40N

30N

20N

10N

20S

10S

30S

40S

180 120W 60W 0 60E 120E 180

March–May

40N

30N

20N

10N

20S

10S

30S

40S

180 120W 60W 0 60E 120E 180

June–August

40N

30N

20N

10N

20S

10S

30S

40S

180 120W 60W 0 60E 120E 180

September–November

40N

30N

20N

10N

20S

10S

30S

40S

180 120W 60W 0 60E 120E 180

15 20 25 30 35 40 45 50

Fig. 5.6 Composite seasonal distribution of the tropospheric

ozone column (in Dobson units) determined from satellite

measurements from 1979 to 2000. [From Atmos. Chem. Phys. 3,

895 (2003).]

P732951-Ch05.qxd 12/09/2005 09:05 PM Page 167

168 Atmospheric Chemistry

An increase in tropospheric O

has occurred glob-

ally over the past century, from 10–15 ppbv in the

preindustrial era to 30–40 ppbv in 2000 in remote

regions of the world. The increase is attributable to

the increase in NO

emissions associated with the

rapid increase in the use of fossil fuels since the

industrial revolution.

5.3.6 Hydrogen Compounds

Hydrogen compounds are the most important oxi-

dants for many chemicals in the atmosphere and are

involved in the cycles of many chemical families.

Hydrogen compounds include atomic hydrogen (H),

which is very short-lived because it combines quickly

with O

to form the hydroperoxyl radical (HO

);

molecular hydrogen (H

), which, next to CH

,is

the most abundant reactive trace gas in the tropo-

sphere; the hydroxyl radical OH (see Section 5.3.1);

the hydroperoxyl radical HO

; hydrogen peroxide

), which is formed by the reaction of HO

radicals and is an important oxidant for SO

cloud droplets; and H

O, which plays multivarious

roles in atmospheric chemistry, including its reac-

tion with excited atomic oxygen to form OH (reac-

tion 5.6b).

The primary source of odd hydrogen species

(HO

), in the form of OH, in the lower troposphere

where water vapor is abundant, is (5.7). Another

source of HO

is the photolysis of formaldehyde

(5.26)

followed by (5.19b) and

(5.27)

both of which produce HO

.The simplest loss mech-

anisms for HO

are of the form (5.20), and

(5.28)

and NO

can react to produce O

(see

Section 5.3.5). Consequently, there is considerable

interest in HO

chemistry in the upper troposphere

where NO

is emitted in significant quantities by jet

aircraft. Cycling between OH and HO

occurs on

a timescale of a few seconds and is controlled by

OO

: H

O  O

CO  O

: HO

 CO

CHO  h



: H  HCO

reactions (5.22). These reactions account for 80%

of the measured concentrations of HO

in the upper

troposphere. NO

controls the cycling within HO

that leads to O

production. NO

also regulates the

loss of HO

through reactions of OH with HO

,NO

and HO

5.3.7 Sulfur Gases

Sulfur is assimilated by living organisms and is then

released as an end product of metabolism. The most

important sulfur gases in the atmosphere are SO

S, dimethyl sulfide (CH

SCH

or DMS for short),

COS, and carbon disulfide (CS

Sulfur compounds exist in both reduced and oxi-

dized states, with oxidation numbers from 2 to 6.

After being emitted into the Earth’s oxidizing atmos-

phere, the reduced sulfur compounds (most of which

are biogenic in origin) are generally oxidized to the

4 oxidation state of SO

[i.e., S(IV)] and eventually

to the 6 oxidation state of H

[i.e., S(VI)]. The

6 oxidation state is the stable form of sulfur in

the presence of oxygen. The oxidation of sulfur

compounds illustrates an effect that often applies to

other compounds, namely the more oxidized species

generally have a high affinity for water (e.g., sulfuric

acid). Consequently, the more oxidized species are

removed more readily from the atmosphere by wet

processes.

The principal natural sources of SO

are the oxida-

tion of DMS and H

S. For example,

(5.29a)

The HS then reacts with O

or NO

to form HSO

(5.29b)

(5.29c)

The HSO is then converted rapidly to SO

(5.29d)

(5.29e)

Volcanoes and biomass burning are also sources of

atmospheric SO

. However, the largest source of SO

is fossil fuel combustion (see Fig. 5.15 for sources and

sinks of sulfur compounds).

HSO

 O

: HO

 SO

HSO  O

: HSO

 O

HS  NO

: HSO  NO

HS  O

: HSO  O

H  H

S : H

O  HS

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5.4 Tropospheric Aerosols 169

In the gas phase, SO

is oxidized by

(5.30a)

(5.30b)

(5.30c)

is also oxidized to H

in cloud water (see

Section 6.8.5).

The main sources of H

S are emissions from soils,

marshlands, oceans, and volcanoes. The only signifi-

cant sink for H

S is oxidation to SO

by (5.29).

Biological reactions in the oceans, involving

phytoplankton, emit several sulfur gases, of which

DMS has the largest emission rate. DMS is

removed from the atmosphere primarily by its

reaction with OH to produce SO

.The sulfur gas

with the largest concentration in the unpolluted

atmosphere is COS (0.5 ppbv). The major sources

of COS are biogenic and the oxidation of CS

OH; the source of CS

is also biogenic. Because

COS is very stable in the troposphere, it is eventu-

ally transported into the stratosphere where it is

the dominant source of sulfate particles during the

 H

O : H

SO

 O

: HO

 SO

 H  SO

 M : HOSO

 M

Aitken

nuclei

Large

particles

Giant

particles

Cloud and precipitation

physics

Atmospheric electricity

Large

ions

Small

ions

Atmospheric

radiation

and optics

Air chemistry

(including

air pollution)

Particle diameter (

µ m)

–4

–3

–2

–1

Fig. 5.7 Size range of particles in the atmosphere and their

importance.

quiescent periods between large volcanic eruptions

(see Section 5.7.3).

5.4 Tropospheric Aerosols

Atmospheric aerosols are suspensions of small solid

andor liquid particles (excluding cloud particles) in

air that have negligible terminal fall speeds. Figure 5.7

shows the ranges of particle sizes that play a role in

the atmosphere.

Molecular aggregates that carry an electric charge

are called ions.The number density and type of

ions in the air determine the electrical conductiv-

ity of the air, which, in turn, affects the magnitude

of the fair weather atmospheric electric field. Ions

in the lower atmosphere are produced primarily

by cosmic rays, although very close to the Earth’s

surface ionization due to radioactive materials in

the Earth and atmosphere also plays a role. Ions

are removed by combining with ions of opposite

sign. Small ions, which are not much larger than

molecular size, have electrical mobilities (defined

as their velocity in a unit electric field) between

1 and 2  10

4

m s

1

for an electric field of

1Vm

1

at normal temperature and pressure

(NTP). Large ions have electrical mobilities in

the range from 3  10

8

to 8  10

7

m s

1

in a

field of 1 V m

1

at NTP. Concentrations of small

ions vary from about 40 to 1500 cm

3

at sea level,

whereas concentrations of large ions vary from

about 200 cm

3

in marine air to a maximum value

of about 8  10

3

in some cities. It can be seen

from these numbers that the electrical conductiv-

ity of the air (which is proportional to the product

of the ion mobility and the ion concentration) is

generally dominated by small ions. However, when

the concentrations of large ions and uncharged

aerosols are large, as they are in cities, the concen-

tration of small ions tends to be low due to their

capture by both large ions and uncharged aerosols.

Consequently, the electrical conductivity of air is a

minimum (and the fair weather atmospheric elec-

tric field a maximum) when the concentration of

large ions, and similarly sized uncharged particles,

is a maximum. The observed decrease of at least

20% in the electrical conductivity of the air over

the north Atlantic Ocean during the 20th century

is attributed to a doubling in the concentration of

particles with diameters between 0.02 and 0.2



probably due to pollution.

5.3 Charged Particles

P732951-Ch05.qxd 12/09/2005 09:05 PM Page 169

170 Atmospheric Chemistry

The effects of aerosols on the scattering and absorp-

tion of radiation have been discussed in Section 4.4.

The role that aerosols play in the formation of cloud

particles is discussed in Chapter 6. Here we are con-

cerned with tropospheric aerosols, in particular with

their sources, transport, sinks, properties, and with

their roles in tropospheric chemistry.

5.4.1 Sources

a. Biological

Solid and liquid particles are released into the atmos-

phere from plants and animals. These emissions,

which include seeds, pollen, spores, and fragments of

animals and plants, are usually 1–250



m in diameter.

Bacteria, algae, protozoa, fungi, and viruses are gen-

erally 1



m in diameter. Some characteristic con-

centrations are: maximum values of grassy pollens

200 m

3

; fungal spores (in water) 100–400 m

3

;

bacteria over remote oceans 0.5 m

3

; bacteria in

New York City 80–800 m

3

; and bacteria over

sewage treatment plants 10

3

. Microorganisms

live on skin: when you change your clothes, you can

propel 10

bacteria per minute into the air, with

diameters from 1 to 5



The oceans are one of the most important sources

of atmospheric aerosols [1000–5000 Tg per year,

although this includes giant particles (2–20



diameter) that are not transported very far]. Just

above the ocean surface in the remote marine atmos-

phere, sea salt generally dominates the mass of both

supermicrometer and submicrometer particles.

The major mechanism for ejecting ocean materials

into the air is bubble bursting (some materials enter

the air in drops torn from windblown spray and foam,

but because these drops are relatively large, their

residence times in the air are very short). Aerosols

composed of sea salt originate from droplets ejected

into the air when air bubbles burst at the ocean

surface (Fig. 5.8). Many small droplets are produced

when the upper portion of an air bubble film bursts;

these are called film droplets (Fig. 5.8b). Bubbles

2 mm in diameter each eject 100–200 film

droplets into the air. After evaporation, the film

droplets leave behind sea-salt particles with diame-

ters less than 0.3



m. From one to five larger drops

break away from each jet that forms when a bubble

bursts (Fig. 5.8d), and these jet drops are thrown

about 15 cm up into the air. Some of these drops

subsequently evaporate and leave behind sea-salt

particles with diameters 2



m. The average rate

of production of sea-salt particles over the oceans

is 100 cm

2

1

Hygroscopic salts [NaCl (85%), KCl, CaSO

(NH

)

] account for 3.5% of the mass of sea-

water. These materials are injected into the atmos-

phere by bubble bursting over the oceans. In

addition, organic compounds and bacteria in the sur-

face layers of the ocean are transported to the air by

bubble bursting.

Dry sea-salt particles will not form solution droplets

until the relative humidity exceeds 75%. Ambient

gases (e.g., SO

and CO

) are taken up by these

droplets, which changes the ionic composition of

the droplets. For example, the reaction of OH(g)

with sea-salt particles generates OH



(aq) in the

droplets, which leads to an increase in the production

of (aq) by aqueous-phase reactions and a

reduction in the concentration of Cl



(aq). Conse-

quently, the ratio of Cl to Na in sea-salt particles

collected from the atmosphere is generally much less

than in seawater itself. The excess of (aq) over

that of bulk seawater is referred to as non-sea-salt

sulfate (nss).

The oxidation of Br



(aq) and Cl



(aq) in solutions

of sea-salt particles can produce BrO

and ClO

species. Catalytic reactions involving BrO

and ClO

similar to those that occur in the stratosphere

(see Section 5.7.2), destroy O

.This mechanism has

2

Surface

water

Air

bubble

Film

droplets

(a) (b)

Jet

drops

(d)(c)

Fig. 5.8 Schematics to illustrate the manner in which film

droplets and jet drops are produced when an air bubble

bursts at the surface of water. Over the oceans some of

the droplets and drops evaporate to leave sea-salt particles

and other materials in the air. The time between (a) and (d)

is 2 ms. The film droplets are 5–30



m diameter before

evaporation. The size of the jet drops are 15% of the diam-

eter of the air bubble.

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5.4 Tropospheric Aerosols 171

been postulated to explain the depletion of O

from 40 to 0.5 ppbv, that occurs episodically over

periods of hours to days in the Arctic boundary layer

starting at polar sunrise and continuing through

April.

Smoke from forest fires is a major source of

atmospheric aerosols. Small smoke particles (prima-

rily organic compounds and elemental carbon) and

fly ash are injected directly into the air by forest fires.

Several million grams of particles can be released

by the burning of 1 hectare (10

). It is estimated

that about 54 Tg of particles (containing 6 Tg of

elemental carbon) are released into atmosphere each

year by biomass burning. The number distribution of

particles from forest fires peak at 0.1



m diameter,

which makes them efficient cloud condensation

nuclei. Some biogenic particles (e.g., bacteria from

vegetation) may nucleate ice in clouds.

b. Solid Earth

The transfer of particles to the atmosphere from the

Earth’s surface is caused by winds and atmospheric

turbulence. To initiate the motion of particles on the

Earth’s surface, surface wind speeds must exceed cer-

tain threshold values, which depend on the size of the

particle and the type of surface. The threshold values

are at least 0.2 m s

1

for particles 50–200



m in

diameter (smaller particles adhere better to the

surface) and for soils containing 50% clay or tilled

soils. To achieve a frictional speed of 0.2 m s

1

requires a wind speed of several meters per second

a few meters above ground level. A major source of

smaller (10–100



m diameter) particles is saltation,

in which larger sand grains become airborne, fly a

few meters, and then land on the ground, creating a

burst of dust particles.

On the global scale, semiarid regions and deserts

(which cover about one-third of the land surface) are

the main source of particles from the Earth’s surface.

They provide 2000 Tg per year of mineral particles.

Dust from these sources can be transported over

long distances (see Section 5.4.3).

Volcanoes inject gases and particles into the

atmosphere. The large particles have short residence

times, but the small particles (produced primarily by

gas-to-particle (g-to-p) conversion of SO

) can be

transported globally, particularly if they reach high

altitudes. Volcanic emissions play an important role

in stratospheric chemistry (see Section 5.7.3).

c. Anthropogenic

The global input of particles into the atmosphere

from anthropogenic activities is 20% (by mass) of

that from natural sources. The main anthropogenic

sources of aerosols are dust from roads, wind erosion

of tilled land, biomass burning, fuel combustion, and

industrial processes. For particles with diameters

5



m, direct emissions from anthropogenic sources

dominate over aerosols that form in the atmosphere

by g-to-p conversion (referred to as secondary parti-

cles) of anthropogenic gases. However, the reverse is

the case for most of the smaller particles, for which

g-to-p conversion is the over-whelming source of the

number concentration of anthropogenically derived

aerosols.

In 1997 the worldwide direct emission into the

atmosphere of particles 10



m diameter from

anthropogenic sources was estimated to be 350 Tg

per year (excluding g-to-p conversion). About 35%

of the number concentration of aerosols in the

atmosphere was sulfate, produced by the oxidation

of SO

emissions. Particle emissions worldwide

were dominated by fossil fuel combustion (prima-

rily coal) and biomass burning. These emissions are

projected to double by the year 2040, due largely

to anticipated increases in fossil fuel combustion,

with the greatest growth in emissions from China

and India.

During the 20th century, the emission of particles

into the atmosphere from anthropogenic sources was

a small fraction of the mass of particles from natural

sources. However, it is projected that by 2040 anthro-

pogenic sources of particles could be comparable to

those from natural processes.

d. In situ formation

In situ condensation of gases (i.e., g-to-p conversion)

is important in the atmosphere. Gases may condense

onto existing particles, thereby increasing the mass

(but not the number) of particles, or gases may

condense to form new particles. The former path is

favored when the surface area of existing particles

is high and the supersaturation of the gases is low.

If new particles are formed, they are generally

0.01



m diameter. The quantities of aerosols pro-

duced by g-to-p conversion exceed those from direct

emissions for anthropogenically derived aerosols and

are comparable to direct emission in the case of

naturally derived aerosols.

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172 Atmospheric Chemistry

Three major families of chemical species are

involved in g-to-p conversion: sulfur, nitrogen, and

organic and carbonaceous materials. Various sulfur

gases (e.g., H

S, CS

, COS, DMS) can be oxidized to

.The SO

is then oxidized to sulfate ( ), the

dominant gas phase routes being reactions (5.30).

However, on a global scale, heterogeneous reactions

of SO

in cloud water dominate the conversion of

to (see Section 6.8.9).

Over the oceans, sulfates derived from DMS con-

tribute to the growth of existing particles. Sulfates

are also produced in and around clouds, and nitric

acid can form from N

in cloud water.

Subsequent evaporation of cloud water releases

these sulfate and nitrate particles into the air (see

Section 6.8).

Organic and carbonaceous aerosols are produced

by g-to-p conversion from gases released from the

biosphere and from volatile compounds such as crude

oil that leak to the Earth’s surface. Carbonaceous

particles emitted directly into the atmosphere derive

mainly from biomass fires.

e. Summary

Table 5.3 summarizes estimates of the magnitudes of

the principal sources of direct emission of particles

into the atmosphere and in situ sources.

Anthropogenic activities emit large numbers of

particles into the atmosphere, both directly and

through g-to-p conversion (see Sections 5.4.1c and

5.4.1d). For particles 5



m diameter, human acti-

vities worldwide are estimated to produce 15%

of natural emissions, with industrial processes, fuel

combustion, and g-to-p conversion accounting for

80% of the anthropogenic emissions. However, in

urban areas, anthropogenic sources are much more

important. For particles 5



m diameter, human

activities produce 20% of natural emissions, with

g-to-p conversion accounting for 90% of the human

emissions.

5.4.2 Chemical Composition

Except for marine aerosols, the masses of which are

dominated by sodium chloride, sulfate is one of the

prime contributors to the mass of atmospheric

aerosols. The mass fractions of range from

22–45% for continental aerosols to 75% for

aerosols in the Arctic and Antarctic. Because the

sulfate content of the Earth’s crust is too low to

2

explain the large percentages of sulfate in atmos-

pheric aerosols, most of the must derive from

g-to-p conversion of SO

.The sulfate is contained

mainly in submicrometer particles.

Ammonium is the main cation associated

with in continental aerosol. It is produced by

gaseous ammonia neutralizing sulfuric acid to pro-

duce ammonium sulfate [(NH

)

]—see reaction

2

(NH



)

2

Table 5.3 Estimates (in Tg per year) for the year 2000 of

(a) direct particle emissions into the atmosphere and (b) in situ

production

(a) Direct emissions

Northern Southern

hemisphere hemisphere

Carbonaceous aerosols

Organic matter (0–2



Biomass burning 28 26

Fossil fuel 28 0.4

Biogenic (1



m) — —

Black carbon (0–2



Biomass burning 2.9 2.7

Fossil fuel 6.5 0.1

Aircraft 0.005 0.0004

Industrial dust, etc. (1



Sea salt

1



m2331

1–16



m1,420 1,870

Total1,440 1,900

Mineral (soil) dust

1



m9017

1–2



m 240 50

2–20



m1,470 282

Total1,800 349

(b) In situ

Northern Southern

hemisphere hemisphere

Sulfates (as NH

HSO

) 145 55

Anthropogenic 106 15

Biogenic 25 32

Volcanic 14 7

Nitrate (as )

Anthropogenic 12.4 1.8

Natural 2.2 1.7

Organic compounds

Anthropogenic 0.15 0.45

Biogenic 8.2 7.4

Sizes refer to diameters. [Adapted from Intergovernmental Panel on Climate

Change, Climate Change 2001, Cambridge University Press, pp. 297 and 301, 2001.]



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5.4 Tropospheric Aerosols 173

(5.13). The ratio of the molar concentrations of

to ranges from 1 to 2, corresponding to an

aerosol composition intermediate between that for

HSO

and (NH

)

.The average mass frac-

tions of submicrometer non sea-salt sulfates plus

associated range from 16 to 54% over large

regions of the world.

In marine air the main contributors to the mass

of inorganic aerosols are the ions Na



,Cl



,Mg

2



, and Ca

2

. Apart from , these com-

pounds are contained primarily in particles a few

micrometers in diameter because they originate from

sea salt derived from bubble bursting (see Fig. 5.8).

Sulfate mass concentrations peak for particles with

diameters 0.1–1



m. Particles in this size range are

effective in scattering light (and therefore reducing

visibility) and as cloud condensation nuclei.

Nitrate occurs in larger sized particles

than sulfate in marine air. Because seawater con-

tains negligible nitrate, the nitrate in these particles

must derive from the condensation of gaseous

HNO

, possibly by g-to-p conversion in the liquid

phase.

Nitrate is also common in continental aerosols,

where it extends over the diameter range 0.2–20



It derives, in part, from the condensation of HNO

onto larger and more alkaline mineral particles.

Organic compounds form an appreciable fraction

of the mass of atmospheric aerosols. The most abun-

dant organics in urban aerosols are higher molecular

weight alkanes (C

2x  2

), 1000–4000 ng m

3

, and

alkenes (C

), 2000 ng m

3

. Many of the parti-

cles in urban smog are by-products of photochemical

reactions involving hydrocarbons and nitrogen oxides,

which derive from combustion. In the United States,

carbonaceous materials can account for 50% or

more of the total dry aerosol mass.

Elemental carbon (commonly referred to as

“soot”), a common component of organic aerosols

in the atmosphere, is a strong absorber of solar

radiation. For example, in polluted air masses from

India, elemental carbon accounts for 10% of the

mass of submicrometer sized particles.

5.4.3 Transport

Aerosols are transported by the airflows they

encounter during the time they spend in the atmos-

phere. The transport can be over intercontinental,

even global, scales. Thus, Saharan dust is transported

to the Americas, and dust from the Gobi Desert

(NO



)

2



2



can reach the west coast of North America. If the

aerosols are produced by g-to-p conversion, long-

range transport is likely because the time required

for g-to-p conversion and the relatively small sizes

of the particles produced by this process lead to long

residence times in the atmosphere. This is the case

for sulfates that derive from SO

blasted into the

stratosphere by large volcanic eruptions. It is also

the case for acidic aerosols such as sulfates and

nitrates, which contribute to acid rain.Thus, SO

emitted from power plants in the United Kingdom

can be deposited as sulfate far inland in continental

Europe.

5.4.4 Sinks

On average, particles are removed from the atmos-

phere at about the same rate as they enter it.

Small particles can be converted into larger parti-

cles by coagulation. Because the mobility of a parti-

cle decreases rapidly as it increases in size,

coagulation is essentially confined to particles less

than 0.2



m in diameter. Although coagulation

does not remove particles from the atmosphere, it

modifies their size spectra and shifts small particles

into size ranges where they can be removed by

other mechanisms.

Exercise 5.6 If the rate of decrease in the number

concentration N of a monodispersed (i.e., all parti-

cles of the same size) aerosol due to coagulation

is given by dNdt  KN

,where K is a constant

(assume K  1.40  10

15

1

for 0.10-



diameter particles at 20 °C and 1 atm), determine

the time required at 20 °C and 1 atm for coagulation

to decrease the concentration of a monodispersed

atmospheric aerosol with particles of a diameter of

0.100



m to one-half of its initial concentration of

1.00  10

3

Solution: Because dNdt  KN

where N

and N are the number concentrations of

particles at time t  0 and time t, respectively.

Therefore,







Kt



K



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