Urban Air Pollution: State of the Science

JOHN H. SEINFELD

Urban air pollution is comprised of a highly complex

mixture of gaseous and particulate components. Much

progress has been made in our understanding of the

detailed chemistry and physics of air pollution, but important areas of uncertainty still remain.

T HE URBAN ATMOSPHERE IS A GIANT CHEMICAL REACTOR IN

which pollutant gases such as hydrocarbons and oxides of

nitrogen and sulfur react under the influence of sunlight to

create a variety of products, including ozone and submicrometer

aerosols. The National Ambient Air Quality Standard for ozone

allows no more than one exceedance per year (on average over 3

years) of a daily maximum 1-hour average ozone concentration of

0.12 parts per million (ppm) by volume. Although substantial

progress has been made in reducing airborne levels of sulfur dioxide

and carbon monoxide in U.S. cities, the standard for ozone was

exceeded during 1983-1985 in 76 urban areas of the United States

(1). Of the six "criteria" pollutants for which air quality standards

have been established, ozone remains the most resistant to efforts at

abatement.

Ozone

The only significant chemical reaction producing ozone in the

atmosphere is that between atomic and molecular oxygen:

0+02+M -03+M (1)

where M is any third body, such as N2 or 02, that removes the

energy of the reaction and stabilizes 03. At high altitudes (above 20

km) the oxygen atoms are produced by photodissociation of

molecular oxygen by radiation in the deep ultraviolet. At lower

altitudes, where only radiation with wavelengths greater than 280

nm is present, the only significant oxygen atom production is from

photodissociation of nitrogen dioxide:

N02 + hv- NO+O (2)

where the photon hv has a wavelength between 280 and 430 nm.

The nitric oxide produced in this reaction reacts rapidly with ozone

to regenerate NO2:

NO+ 03- N02 + 02 (3)

The above three reactions occur rapidly, establishing a steadystate ozone concentration given by the so-called photostationary

state relation,

[0] - J[N02] k[NO] (4)

where k is the rate constant for reaction 3 and J[NO2] is the

photolysis rate of NO2. At a ratio of [NO2]/[NO] = 1, [03]

predicted by the photostationary state at solar noon in U.S. latitudes

is about 20 parts per billion (ppb) by volume. While ozone

concentrations in unpolluted tropospheric air vary between 20 and

50 ppb, levels as high as 400 ppb are obtained in polluted urban

areas.

In reaction 3, one ozone molecule is required to regenerate NO2

from NO. A reaction path that converts NO to NO2 without

consuming a molecule of ozone circumvents reaction 3 and allows

ozone to accumulate. Such a path is provided by the presence of

hydrocarbons (2). In particular, peroxy radicals (RO2 where R is an

alkyl group) produced in the oxidation of hydrocarbon molecules

react with NO to form NO2, thereby shifting the photostationary

state in favor of ozone production. The net process is:

R02 + NO-- NO2 + RO

NO2 + hv--* NO + 0

0 + 02 + M -*03 + M

(5)

(6)

(7)

Net: R02 + 02+ hv-RO+03 (8)

The rate of ozone generation is related closely to the rate of R02

production. Generation of the organic peroxy radicals R02 occurs

largely by the attack of the hydroxyl radical (OH) on hydrocarbons.

Interestingly, OH attack, which ultimately scavenges hydrocarbons

from the atmosphere, is also that which, through oxidation of NO

to NO2, produces ozone. The chemical structure ofthe hydrocarbon

determines the number and form of peroxy radicals and hence the

number of NO to NO2 conversions that occur as the hydrocarbon is

oxidized.

There are three principal sources of hydroxyl radicals in the sunlit

urban atmosphere: (i) photochemical dissociation of ozone, (ii)

photochemical dissociation ofcarbonyl compounds, and (iii) photochemical dissociation of nitrous acid. Photochemical dissociation of

ozone leads to the formation of excited oxygen atoms, O('D), that

react with water vapor to form hydroxyl radicals:

03 + hv- O('D) + 02

O('D) + H20 -* 20H

(9)

(10)

where the photon has a wavelength between 280 and 310 nm.

Photochemical dissociation of aldehydes (RCHO) leads to the

formation of radicals that through a rapid series of reactions lead

ultimately to the formation of OH:

RCHO + hv -* R + HCO

HCO + 02-* HO2 + CO

HO2 + NO -* NO2 + OH

(11)

(12)

(13)

where the photon has a wavelength between 280 and 360 nm. An

important source of OH radicals is also nitrous acid (HONO).

Nitrous acid achieves concentrations in polluted urban air of 1 to 10

ARTICLES 745

The author is in the Department of Chemical Engineering, California Institute of

Technology, Pasadena, CA 91125.

IO FEBRUARY I989

 on May 22, 2014 www.sciencemag.org Downloaded from 
ppb, with maximum levels occurring just before sunrise. In the

presence of sunlight, HONO has a photolysis lifetime of about 15

minutes, and its photolysis

HONO+ hv-OH+ NO (14)

provides an early morning source of OH. Its nighttime formation

route is thought to involve a heterogeneous hydrolysis of NO2.

The primary OH removal process is the reaction with NO2 to

form gaseous nitric acid:

OH + NO2 + M-> HNO3 + M (15)

(Reaction with hydrocarbons is not actually a sink for OH since

subsequent reactions of the OH-hydrocarbon reaction products,

most of the time, result in regeneration of OH.) Nitric acid is also a

major sink for nitrogen oxides through the above reaction.

One important free radical removal process for the organic peroxy

radicals involves the peroxyacetyl radical reaction with NO2 to

produce peroxyacetylnitrate, denoted PAN:

CH3C(0)02 + NO2 Z± CH3C(O)02N02 (16)

The formation reaction is reversible as the PAN molecule can

decompose thermally with a lifetime at 298 K of about 45 minutes.

The PAN molecule serves as a temporary reservoir for both the

peroxyacetyl radical and NO2, with its concentration dependent on

temperature and on the ratio ofNO to NO2 concentrations (because

the peroxyacetyl radical also reacts with NO to form NO2). PAN

concentrations peak in late afternoon and early evening; levels of 5

to 10 ppb are measured in Los Angeles.

Because the formation ofozone and other secondary species is not

instantaneous following emission of hydrocarbons and nitrogen

oxides, significant transport and mixing occur simultaneously with

chemical reaction. The complex interplay between atmospheric

chemistry and transport processes can be visualized by considering a

column of air of arbitrary cross section that extends from the surface

to the top of the boundary layer. The height of the daytime

boundary layer is usually constrained by an elevated temperature

Table 1. Ambient air hydrocarbons in 39 U.S. cities-the 25 most abundant

compounds based on median concentration (66, 67).

Compound

Concentration (ppbC)

Median Maximum

Isopentane 45.3 3393

n-Butane 40.3 5448

Toluene 33.8 1299

Propane 23.5 393

Ethane 23.3 475

n-Pentane 22.0 1450

Ethylene 21.4 1001

m-Xylene, p-xylene 18.1 338

2-Methylpentane 14.9 647

Isobutane 14.8 1433

Acetylene 12.9 114

Benzene 12.6 273

n-Hexane, 2-ethyl-1-butene 11.0 601

3-Methylpentane 10.7 351

1,2,4-Trimethylbenzene 10.6 81

Propylene 7.7 455

2-Methylhexane 7.3 173

o-Xylene 7.2 79

2,2,4-Trimethylpentane 6.8 106

Methylcyclopentane 6.4 293

3-Methylhexane 5.9 168

2-Methylpropene, 1-butene 5.9 365

Ethylbenzene 5.9 159

m-Ethyltoluene 5.3 83

n-Heptane 4.7 233

746

inversion (that is, warm air over cold) at about 1 to 2 km, the socalled mixing height. Turbulent convection in the boundary layer

ensures rapid vertical mixing in the layer.

The air parcel can be thought to follow a trajectory through the

region as determined by the horizontal wind components. If the air

parcel starts upwind of the urban core, the relatively clean parcel

picks up hydrocarbon and NO, emissions as it is carried over the

source-rich areas. Over a time scale ofabout 1 hour, hydroxyl radical

attack on the hydrocarbons commences. As the air parcel moves out

of the urban core, the emission density decreases. Concentrations of

precursor hydrocarbon and nitrogen oxides (NO,) peak and start to

decline with downwind distance as a result of the effects of chemical

reaction and mixing. Ozone in the parcel continues to increase as the

array of hydrocarbons is photooxidized. The increase in 03 level is

sustained over a 1- to 5-hour period as the more reactive olefins and

aromatics are consumed. After 5 to 10 hours of travel, medium

reactivity hydrocarbons contribute to net ozone production. The

ozone level in the air parcel ultimately begins to decrease reflecting

the effects ofdilution, conversion ofNO2 to nitric acid, and removal

at the surface. At sunset, all photochemical ozone production ceases.

At night when the regeneration of ozone by means of NO2

photolysis does not occur, the overall effect of the ozone reactions

with NO and NO2 is to provide a route for converting the nitrogen

oxides to nitric acid. (We will discuss nighttime chemistry shortly.)

During nighttime, radiative cooling at the ground usually leads to

the establishment of a ground-based inversion layer with a height of

several tens to hundreds of meters inhibiting ground-based emissions from reaching any ozone stored aloft. The following sunrise,

the inversion is rapidly eroded, bringing ozone and other products

from the previous day stored aloft down to mix with the fresh

emissions and to feed the second day's photochemistry.

This concept of an "air parcel" traveling downwind from the

point of maximum emission density, that is, the urban core, to the

location of peak ozone concentration is the central idea behind the

U.S. Environmental Protection Agency's Empirical Kinetic Modeling Approach (EKMA) used to relate hydrocarbon and NO.

emissions changes to changes in maximum ozone concentrations for

a particular urban area (3).

Hydrocarbon Chemistry

The feature that distinguishes the chemistry of the urban atmosphere from that of the natural troposphere is the relatively high

concentrations of a great number of anthropogenic hydrocarbons

(Table 1).

Alkanes. Alkanes constitute a major class of anthropogenic hydrocarbon emissions (see Table 1). Under urban conditions alkanes

react with OH radicals during daylight hours and with the NO3

radicals at night, with the latter process being of minor (<10%)

importance to total alkane consumption (4-6). Both reactions

proceed by H-atom abstraction from C-H bonds. The resulting

alkyl (R) radical reacts rapidly, and exclusively, with 02 to yield an

alkyl peroxy radical (RO2). Under tropospheric conditions, these

alkyl peroxy radicals react with NO by two pathways

R02 + N) NO2 + RO

)- RONO2

with HO2 radicals

R02 + HO2 -> R02H + 02

(17)

(18)

(19)

or with other R02 radicals. At NO concentrations above about 30

parts per trillion (ppt), reaction with NO is the dominant reaction

SCIENCE, VOL. 243
pathway for R02 radicals. The yield of the alkyl nitrate (RONO2)

increases with the number of carbon atoms in R02, for example,

from <0.014 for a C2 alkane up to about 0.33 for a C8 alkane. The

alkoxy (RO) radicals react by unimolecular decomposition, unimolecular isomerization, or with 2. For the 2-pentoxy radical, for

example,

0.

decompoition CH3CHCH2CH2CH3 3 CH3CHO + CH3CH2CH2 (20)

CH3CHOHCH2CH2CH2 (21)

02 CH3COCH2CH2CH3 + HO2

(22)

The isomerization reactions occur mainly by a 1,5 shift of

hydrogen, so they require a four-carbon chain; where this is

available, these reactions are expected to dominate over decomposition or reaction with 2. The reactions occurring after alkoxy radical

isomerization have not been experimentally established, although

many routes have been postulated.

For alkanes, the expected products of photooxidation include

aldehydes, ketones, alkyl nitrates, and bifunctional compounds

including keto-alcohols and aldehyde-alcohols. It is likely that these

intermediate products continue to react with other species present.

Although it is predicted that, for alkanes with four or more carbons,

B-bifunctional compounds of the general form RCOCH2CH2-

CHOR' would be formed, at present no definitive evidence for the

formation of these compounds exists. Also, our present understanding suggests the possibility of formation of 8-hydroxynitrate alkanes, for example, RCHOHCH2CH2CH(ON02)R'. Investigations are needed to determine whether such compounds are formed

in the photooxidation of alkanes with five or more carbons.

In summary, the chemistry of C2 to C4 alkanes is reasonably well

understood. For C5 and larger alkanes, the fate of the corresponding

alkoxy radicals is not well established, and the role of RO +

NO2-) RONO2 for such species as an NO. sink is not fully known.

The chemistry of cycloalkanes is essentially unknown.

Alkenes. The rates of reaction of alkenes with OH radicals are

significantly higher than those of the corresponding alkanes. Alkenes also react with NO3 and 03. Alkene reactions tend to

dominate product generation shortly after the emission of a hydrocarbon mixture into the atmosphere. All of the OH-radical-alkene

reactions proceed by OH addition to the double bond followed by

rapid addition of 02 to yield the corresponding hydroxyalkyl peroxy

radical. Subsequent reaction of the hydroxyalkyl peroxy radical with

NO yields the ,B-hydroxyalkoxy radical. (The hydroxynitrate path

accounts for only about 1% and can be neglected.) While the Ihydroxyalkoxy radicals can react with 02 or undergo unimolecular

decomposition or isomerization, available data indicate that at room

temperature isomerization does not compete with decomposition

(6). Furthermore, only for ethene does reaction with 02 compete

with decomposition. Thus, for alkenes with more than three

carbons, OH radical reaction leads, by cleavage of the C=C double

bond, to the formation of aldehydes or ketones or both.

NO3 reacts with alkenes by addition to the alkene double bond.

The adduct is expected to react first with 02 and then NO, leading

to aldehydes, ketones, or dinitrates (7, 8). Because of the reported

high toxicity of the dinitrates, further research is needed to better

define their yields. Furthermore, the reaction sequences occurring in

the absence of NO, as typically occurs at night when NO3 radical

reactions are of importance, are not presently known.

The 03-alkene reaction proceeds by addition of 03 across the

olefinic double bond followed by rapid decomposition of the

resulting molozonide. The major uncertainty associated with this

reaction concems the fate of the initially energy-rich biradical

coR (23)

R2 j

which can be collisionally stabilized or undergo unimolecular decomposition (9). The thermalized biradicals can react with a number

of species, including H20, NO, NO2, SO2, CO, and aldehydes,

while the decomposition products have not been fully established.

Carbonyls. Aldehydes undergo photolysis and reaction with OH

and NO3 radicals, the first two being the major loss processes. The

formaldehyde-OH reaction yields the formyl (HCO) radical which

subsequently reacts with 02 to form HO2 and CO:

OH + HCHO -- H20 + HCO

1 02

HO2 + CO (24)

For the higher aldehydes, the acyl (RCO) radical initially formed

rapidly adds 02 to yield a peroxyacyl radical [RC(O)OO]. As noted

earlier, the peroxyacyl radical can react with NO or NO2, the latter

reaction leading to the peroxyacylnitrates (PANs). It is estimated

that 80 to 90% of the formaldehyde in the urban atmosphere results

from the photooxidation of other organics and only about 10 to

20% from direct emission.

Ketones are exemplified by acetone and its higher homologues.

Photolysis and OH reaction are the major atmospheric loss processes, although, with the exception ofacetone, photolysis is probably of

minor importance. The major reaction products from the atmospheric reactions of ketones are aldehydes and PAN precursors, and

bifunctional oxygen-containing compounds may be formed in small

yield.

The most important uncertainties in carbonyl chemistry involve

photolysis rates and quantum yields (for example, formaldehyde is

known only to ±+30%), the fate of hydroxycarbonyls (OH-olefin

adducts and RO isomerization products), and the existence of

nitrate carbonyls resulting from higher alkanes. Because carbonyl

photolysis is such an important source offree radicals, uncertainties

in photolysis rates translate into uncertainties in ozone formation

rates.

Alcohols. The atmospheric reaction of the simple aliphatic alcohols

involves OH attack, for example, for methanol:

OH + CH30H )L CH20H + H20

b- CH30 + H20

(25)

(26)

with path (a) accounting for about 85% of the overall reaction

pathway at 298 K. Both the CH20H and CH30 radicals react with

02 to give formaldehyde and HO2. Thus, the overall methanol-OH

reaction can be written as:

CH30H + OH 0 H20 + HO2 + HCHO (27)

Carboxylic acids. Carboxylic acids, such as formic and acetic acid,

react with the OH radical through the formation of an addition

complex, for example, for formic acid

OH + HCOOH -- H20 + COOH

1 02

CO2 + HO2 (28)

IO FEBRUARY 1989 ARTICLES 74-7
Products formed from the reactions of OH with higher carboxylic

acids have not been established.

Aromatics. Aromatic hydrocarbons are a major class of organics

associated with the urban environment (Table 1). The monocyclic

aromatic hydrocarbons react solely with the OH radical, a reaction

that can proceed by two pathways: (i) H-atom abstraction of alkyl

H-atoms for alkyl-substituted aromatics from the alkyl-substituent

groups and (ii) OH radical addition to the aromatic ring (10). The

H-atom abstraction path leads mainly to the formation of aromatic

aldehydes, such as benzaldehyde, with subsequent reactions of these

aldehydes leading to the production of peroxyacylnitrates and

nitrophenols. This H-atom abstraction path accounts for less than

10% of the overall OH radical reaction for benzene and the alkylsubstituted aromatic hydrocarbons. For toluene, for example, the

OH-addition path is assumed to proceed by two routes:

CH3

+OH

which is followed by

M

NO2 + NO3 2 N205 M

(31)

where M is a third body. As noted earlier, NO and 03 can scavenge

each other by

NO + 03 -* NO2 + 02

and NO3 can be reconverted to NO2 by

NO + NO3--* 2NO2

or thermally decompose:

NO3-> NO + 02

(32)

(33)

(34)

Finally, N205 is irreversibly converted to nitric acid in the presence

of water:

N205 + H20-- 2HN03

+ other isomers

CH3

$1JOH

CH3

0H

02

The H-atom abstraction by 02 from the OH-aromatic adduct to

yield phenolic compounds has been shown to be relatively minor,

accounting for about 16% of the overall OH radical products of

toluene. The major reaction pathway is thought to be that involving

ring cleavage of the OH-aromatic-02 adduct. The ot-dicarbonyls,

glyoxal, methylglyoxal, and biacetyl, which have been identified as

products from benzene and the methyl-substituted benzenes, are

indications of ring cleavage (11-14). In laboratory studies of the

ring-fragmentation process, however, the total yield of products as a

percentage ofthe parent aromatic is only about 30% for the tolueneOH reaction and approximately 50% for the m-xylene reaction (11-

16). The remaining carbon-containing products have not been

identified.

The uncertainties in atmospheric aromatic chemistry are major:

(i) ring-opening versus ring-conserving pathways; (ii) the nature

and yield of all products in ring-opening; and (iii) the phenol

distribution and subsequent reactions of ring-conserving paths.

These uncertainties have been well known to atmospheric chemists

for 10 years; progress in reducing them requires development of

new analytical techniques for large (more than six carbons) molecules at very low concentrations. Because aromatics constitute

probably the most important class of reactive organics in the urban

atmosphere, determining the detailed reaction pathways of these

compounds constitutes one of the greatest needs in air pollution

chemistry.

Nighttime Chemistry

The key difference between day and night chemistry, aside from

the absence of photolytic reactions, is the accumulation at night of

the nitrate free radical, NO3, which is rapidly photolyzed during the

day so that its daytime concentration is negligible. During the night,

NO3 is produced by

NO2 + 03-> NO3 + 02 (30)

748

(35)

The main importance of the nighttime chemistry is the conversion

of NO, into nitric acid by a route not involving OH radicals. The

relative importance of different pathways leading to the formation of

HNO3 at night, namely the N205-H20 reaction and NO3 reactions

with organic species, needs to be elucidated. The origin and fate of

nitrous acid (HONO) at night also must be understood because of

its role as an early moming OH source.

Since the NO3 radical is the key to nighttime chemistry, comparison of predicted (by means of the above mechanism plus a number

of other, less important reactions) and observed NO3 concentrations

provides a test of the degree of understanding of this chemistry (17-

21). Predicted NO3 concentrations have somewhat exceeded those

measured, although the temperature-dependent cross section of

NO3 (22) has yet to be employed to relate its concentration to its

absorption spectrum. Further comparisons of measurements and

predictions for nighttime urban conditions are needed.

Chemical Reaction Mechanisms for

Urban Air Pollution

Air pollution chemistry is studied experimentally at two levels:

laboratory studies of the kinetics and mechanisms of individual

reactions, and smog chamber experiments, in which single organics

or mixtures of organics are photooxidized in the presence of NO,.

The main function of smog chambers is to provide data to test our

degree of understanding of air pollution chemistry. Because the

photodegradation of a hydrocarbon can involve 100 or more

individual reactions, for even one organic molecule one must infer

many of the chemical details from the relatively few species that can

be measured in such chambers. Smog chambers are not a perfect

simulation of atmospheric chemistry. For example, the light flux

may not be the same as the solar flux, and, to facilitate measurement,

organic concentration levels in chambers are usually about ten times

those in the atmosphere. The most important problem relating to

the use of smog chambers, however, is that the walls generally

influence the chemistry occurring. Typically, the release of species

absorbed on the walls may contribute 10% of the total free radicals

in the chamber. Although empirical models of the wall radical

source have been proposed (23), a fundamental understanding of

the processes occurring at chamber walls is lacking. Elucidation of

these heterogeneous processes involving NO, is the most pressing

need associated with evaluating smog chamber experiments.

About 500 smog chamber experiments have been carried out in

both indoor and outdoor chambers over the past 15 years. Current

chemical reaction mechanisms have been evaluated against a large

SCIENCE, VOL. 243
fraction of these data and generally simulate peak ozone concentrations to within ±+30%. It is generally agreed that an acceptable

chemical mechanism must simulate this large body ofsmog chamber

data at least as accurately as mechanisms it is proposed to replace.

Because of a lack of measurements of many intermediate species and

products, fitting of the chamber data for 03, NON, PAN, and the

starting organics provides only a global evaluation of a mechanism.

It is possible that compensating errors in the mechanism may yield

reasonable 03 predictions.

Because it is not practical to use a mechanism that includes the

detailed, specific chemistry ofeach organic compound present in the

atmosphere (for example, for each of the compounds in Table 1),

"lumping" procedures are required to limit both the number of

species and the number of reactions included in chemical reaction

mechanisms. Two general lumping approaches have emerged: (i)

the "lumped molecule" approach in which a particular organic

molecule, or a generalized organic species, is used to represent

similar organics, and (ii) the "lumped structure" approach in which

organics are grouped according to the type and number of carbon

bonds in each compound.

Three mechanisms that are based on the most recent kinetic and

mechanistic information are: (i) the Carbon Bond MechanismVersion IV (CB4) (24), a lumped structure mechanism; (ii) the

Carter-Atkinson-Lurmann (CAL) mechanism (25); and (iii) the

Regional Acid Deposition Model (RADM) mechanism (26), the

latter two being lumped molecule mechanisms. Although each of

these mechanisms is based on the same body of available kinetic

data, important uncertainties in those data have led to somewhat

different treatments in the mechanisms of those areas where uncertainties exist.

Inorganic chemistry in all mechanisms is very similar, and thus

discrepancies in predictions among them are the result of differing

treatments oforganic chemistry. There are three major differences in

the representation of the organic chemistry in the above three

mechanisms. As noted earlier, the mechanism of ring fragmentation

in the aromatic-OH reaction has not yet been identified. The

RADM mechanism uses dicarbonyls as surrogates for all the unidentified products, and, consequently, dicarbonyl yields in the RADM

mechanism are approximately a factor of 2 higher than those in CB4

GASEOUS ORGANICS

H202, HOP2, OH

03, O° Z

Fig. 1. Processes influencing the composition of a typical urban aerosol

particle.

and CAL. The second major difference in the mechanisms concerns

the treatment of PAN chemistry. The CB4 assumes a different

temperature dependency than the other two mechanisms for the

reaction of peroxyacetyl radicals with NO2. The large negative

activation energy used in the CB4 causes PAN formation to increase

significantly as temperature decreases, while PAN formation in the

CAL and RADM mechanisms is relatively insensitive to temperature changes. The third major difference among the mechanisms

concems the reactions of organic peroxy radicals with other radicals,

which can lead to substantial differences in H202 predictions.

Dodge (27) has reported extensive simulations with the three

mechanisms. While the three mechanisms were found to predict

nearly identical levels of ozone under most conditions, differences in

predictions were noted with (i) high NO, levels, (ii) high levels of

aromatics, and (iii) low temperatures.

Inorganic atmospheric NO, chemistry is well established. There

exist, however, significant areas of organic chemistry where only the

initial reaction and the first-generation products are known. A key

question is whether resolving the uncertainties in the detailed

chemistry will substantially change our understanding of the overall

features of hydrocarbon/NO. chemistry. Our current ability to

simulate reasonably well the global behavior of the large body of

smog chamber data would suggest not, although a more definitive

resolution awaits the simulation of ambient data, particularly for

intermediate species such as formaldehyde, hydrogen peroxide,

PAN, and nitric acid. The 1987 Southem California Air Quality

Study (SCAQS) offers promise in this regard (28).

Urban Aerosol

Particles of sizes below about 2-,um diameter are largely responsible for the severe visibility deterioration observed in areas like Los

Angeles. Such particles are also easily respirable and may contain

toxic or mutagenic components. Control of the fine particle component of the urban aerosol is a particularly important problem facing

air pollution control agencies.

The atmospheric aerosol is a highly complex entity, representing a

mixture of primary emissions, such as soot and fly ash, and

secondary species that are the "sink" of many atmospheric chemical

processes. Because of the role of the aerosol as an end product,

knowledge of its chemical composition is essential to a full understanding of the chemical processes that preceded its formation.

Prevalent compounds of urban aerosols are sulfates, nitrates, ammonium, trace metals, carbonaceous material, and water (Fig. 1).

Our detailed knowledge of the size-composition distribution of

the urban aerosol is limited (29-32). Primary particles, once emitted,

undergo chemical and physical processes that result in a continually

changing chemical composition and particle size distribution. Particles of a single substance emitted from a specific source, for example,

soot or sea salt, may react with the products of gas-phase reactions

such as H2SO4 produced from the oxidation of SO2. Also, the

products of gas-phase reactions may themselves homogeneously

nucleate to form new particles.

The gas-to-particle conversion process includes (i) homogeneous,

homomolecular nucleation, that is, the formation of a new, stable

particle from a single vapor species; (ii) homogeneous, heteromolecular nucleation, that is, the formation of a new particle involving

two or more vapor species-usually one of the vapor species is

water; and (iii) heterogeneous condensation, the transfer of vapor

molecules to preexisting particles (33). Gas-phase chemical reactions

generate products, some of which have a low vapor pressure. As

these low-volatility products are formed, the atmosphere becomes

supersaturated with respect to these species. The degree of supersat10 FEBRUARY I989 ARTICLES 74.9
uration will determine the competition between new particle formation by nucleation and existing particle growth by condensation.

Aerosol sulfate forms from the gas-phase oxidation of SO2 to

H2SO4 followed by H2SO4 nucleation or condensation or from the

oxidation of SO2 in the aqueous phase of aerosols and water

droplets. Sulfuric acid will also react with ammonia to form salts

such as ammonium bisulfate (NH4HSO4), ammonium sulfate

[(NH4)2SO4], letovicite [(NH4)3H(SO4)2], or mixed salts with

ammonium nitrate (34, 35). Catalytic oxidation of SO2 may occur

on soot aerosol surfaces (36).

Aerosol nitrate exists primarily as nitrate salts, principally ammonium nitrate, but also others such as sodium nitrate. The oxidation

of NO, to nitrate occurs primarily through the N02-OH reaction

during the daytime and through the N205 hydrolysis route at night.

Unlike sulfuric acid, nitric acid has a high vapor pressure and does

not readily condense on aerosols. The formation of aerosol nitrate

requires the presence of NH3, or other alkaline species, to form salts

such as NH4NO3. An equilibrium is presumed to exist among NH3,

HNO3, and NH4NO3:

NH3 + HNO3 a NH4NO3 (36)

This equilibrium is strongly influenced by temperature, relative

humidity, and pH (37, 38), and favors the formation of NH4NO3 at

low temperature, high humidity, and low pH.

Stelson et al. (37, 38) and others (34, 35, 39, 40) have postulated

that the sulfate-nitrate-ammonium aerosol constituents should be in

thermodynamic equilibrium with the local gas phase. Ambient

measurements have generally confirmed the equilibrium relationship among these species (41, 42).

Carbonaceous aerosol generally accounts for a significant fraction

of the fine urban particulate matter; in Los Angeles, for example,

aerosol carbon alone accounts for about 40% of the total fine

particle mass (43). Black graphitic (elemental) carbon particles are

the predominant light-absorbing particulate matter in the atmosphere (44). The carbonaceous fraction of the urban aerosol consists

of both primary (elemental and organic) and secondary components. The elemental carbon, nonvolatile and inert, is from sources

of primary emissions. Organic carbon may be emitted as primary

aerosol from sources or formed in the atmosphere from the low

vapor pressure products of reactions involving hydrocarbons containing approximately seven or more carbon atoms (45-50). Organic

aerosol components present in motor vehicle exhaust (51) and in

ambient samples (52) have been found to be mutagenic in the Ames

test, and soots have been shown to be carcinogenic in animal studies

(53).

Aerosol carbon is not routinely monitored, and emissions data on

carbonaceous particles are scarce. Cass et al. (54) carried out an

inventory of organic and elemental carbon particle emissions in the

Los Angeles area for 1980, which should be typical of most large

urban areas. One-quarter of the total came from gasoline-powered

vehicle exhaust, one-fourth from diesel engine exhaust, and the

remainder from a large variety of stationary fuel burning, industrial,

and distributed sources. Black elemental carbon emissions were

dominated by diesel exhaust. The ratio of total carbon to elemental

carbon in primary aerosol emissions was estimated to be about

3.2: 1. Enhancement of this ratio in ambient samples relative to

source emissions gives an indication of the extent of enrichment in

organic aerosol attributable to secondary aerosol formation. Field

measurements carried out in 1982 in Los Angeles showed little

seasonal dependence in the ratio of total to elemental carbon (43),

although some individual days at inland sites did show large levels of

organic carbon enrichment. An enrichment of up to 0.54 parts

organic carbon added to the 2.43:1 ratio observed at Lennox, a

coastal site, represented a 22% increase in total carbon relative to

750

elemental carbon from one end of the South Coast Air Basin to the

other. On average over long periods of time, it was estimated that

approximately 75% of Los Angeles fine aerosol carbon concentrations are attributable to primary source emissions, with the remainder caused by secondary aerosol formation.

Acid Deposition and Acidic Fogs

The importance of acid fog as a component of the general acidic

deposition phenomenon has recently been recognized. Hoffmann

and co-workers have reported pH values of fogs in southern

California as low as 1.69 (55). These fogs were much more acidic,

and concentrations of anions and cations such as SO'-, NO-, and

NH'7 were higher by one to two orders of magnitude than in

precipitation in the same areas. The physicochemical processes

leading to acid fog have yet to be fully elucidated.

An acid fog encompasses virtually the entire range of atmospheric

chemical phenomena: gas-phase chemistry to produce HNO3 and

H2SO4 vapors, aerosol formation, cooling of moist air followed by

condensation of water vapor on the aerosol particles that serve as

condensation nuclei, scavenging of gaseous species by the droplets,

followed by ionization and aqueous-phase reactions. Gravitational

settling delivers the acidic fog contents to the ground.

Several differences exist between a fog and a cloud environment.

Urban radiation fogs typically develop under stable meteorological

conditions resulting in weak mixing and significant vertical gradients of species like H202, NH3, and HNO3 (56). The deposition

process during a dense radiation fog leads to rapid removal from the

atmosphere of the major ionic species. Because of the proximity of

the fog to sources of pollutants like SO2 and NO,, gas-phase

pollutant concentrations in the fog can reach much higher levels

than in a cloud. In such a case, pathways for aqueous sulfate

formation that are of secondary importance in a cloud may become

significant in a fog.

Polycycic Aromatic Hydrocarbons

A large number of polycyclic aromatic hydrocarbons (PAH),

ranging from naphthalene (C10H8) to 5- and 6-ring and higher

PAH, their alkyl substituted analogues, and their oxygen- and

nitrogen-containing derivatives, are emitted from motor vehicle

sources. In addition, heterocyclic analogues containing ring 0-, S-,

and N-atoms are possible combustion source emissions. The PAH,

their analogues and derivatives, demonstrated in many cases to be

mutagenic or carcinogenic or both, have relatively low vapor

pressures and are distributed between gas and particulate phases

(57). In temperate climates, the 2- to 4-ring PAH are found primarily in the gas phase; under mid-continental wintertime temperatures, the distribution shifts markedly toward the particulate phase.

Atmospheric transformations of PAH depend dramatically on the

phase with which they are associated. For PAH present in the gas

phase, reaction with the OH radical and, to a lesser extent, N205

leads to atmospheric PAH lifetimes of a few hours or less. Many

studies of the chemistry of PAH adsorbed on different substrates

have been carried out. Data obtained indicate that reactions occurring, including photolysis, are strongly dependent on the nature of

the adsorbent species (58). Photolysis and reaction with 03, NO2,

HNO3, and N205 have been shown to lead to loss of PAH and their

derivatives on several substrates, although because of the substrate

dependence of these adsorbed-phase reactions, complete conclusions concerning the full spectrum of possible consumption reactions and the products of those reactions are not yet available.

SCIENCE, VOL. 243
Recent ambient data from Norway, Denmark, and the United

States have shown that 2-nitrofluoranthene and 2-nitropyrene,

nitro-PAH not observed directly from combustion sources, are the

major nitro-PAH of ambient particulate matter (59). Since no

formation pathway for these two nitro-PAH can be identified

during sampling, it appears that they must be formed in the

atmosphere by gas-phase reactions involving other PAH.

Comprehensive, systematic investigations of the gas- and adsorbed-phase reactions of PAH are needed to assess the atmospheric

fate of these species.

Some Critical Policy Issues in Urban

Air Pollution

Effect ofprecursors on ozone. Despite large efforts and expenditures

to control emissions of hydrocarbons and nitrogen oxides, decreases

in ozone concentrations nationwide have been modest, approximately 10% between 1979 and 1985 (60). The effect of precursor

emission controls on ozone concentrations is complex, depending

on meteorological conditions, the absolute and relative amounts of

volatile organic compounds (VOCs) and NOR, and the background

concentrations of ozone and its precursors (Fig. 2).

There is general agreement that reductions in hydrocarbon emissions will lead to ozone decreases (except at very high VOC/NO,

ratios where ozone levels will remain unchanged). The role of NO,

in ozone formation, however, depends on the level of organics.

When organics are abundant relative to NO, (VOC/NO, ratios of

order 20 ppmC/ppm, where ppmC is parts per million by volume of

carbon atoms), there is a shortage of NO that can be oxidized by

peroxy radicals. Ozone formation is controlled primarily in that case

0.28

0.24-

0.20

E

4

0.X cl- 0.16-

0

or z 0.12 0

0.04

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

VOC, ppm carbon

Fig. 2. Peak ozone concentrations in parts per million (labeled on the

contour lines) as a function of initial organic (VOC) and NO, levels. This

plot was generated using EPA's EKMA method under standard default

conditions. It is useful to envision long-term control strategies for ozone

reduction on this diagram. Strategies that follow the ridge toward the origin

will be less effective in reducing ozone than those favoring steeper descents

from the ridge. Strategies favoring reductions of the limiting precursor

descend more steeply; those favoring reductions of the nonlimiting precursor ascend, cross the ridge, and then descend. To judge whether to favor a

VOC or an NO, strategy for reducing ozone, one must know on which side

of the ridge and how far from the ridge line the region is located. Ozone, of

course, is not the only secondary pollutant of concern. The benefits of

reducing the atmospheric concentrations of nitrogen-bearing species

through NO, emissions reductions and of secondary organic aerosol

through VOC reductions must be factored into the analysis. Also, the

positions of the ozone isopleths in a plot such as that above will vary with

location in a large urban area, so the use of a single plot to define control

strategies for a large urban area is an oversimplification.

IOFEBRUARY I989

by the amount of NO, available and decreasing NOX decreases

ozone formation. At low VOC/NOx ratios, the order of 5

ppmC/ppm, ozone formation is limited by the amount of organics

available since 03 scavenging by the NO-03 reaction is more

effective than the peroxy radical oxidation of NO to NO2. Furthermore, the N02-OH reaction forming nitric acid effectively competes

with organics for OH, thereby inhibiting the formation of peroxy

radicals. Decreasing NOx under these conditions leads to an increase

in ozone formation since it favors the peroxy radical-NO reaction

channel. As NOx emissions are reduced, the rate of formation of

ozone increases and ozone levels close to the NOx source region will

increase. Ozone levels downwind can then be expected to decrease,

although the actual amount must be assessed by computer simulation (61). As overall emissions levels in a region are decreased, the

importance of natural sources and of material flowing in across the

region's boundaries increases. Uncertainties in concentration levels

aloft and at region boundaries and in emissions inventories themselves must be explicitly accounted for and propagated through

computer simulations when determining emissions control strategies.

The U.S. Environmental Protection Agency (EPA) has historically encouraged exclusive reliance on VOC controls for ozone abatement because VOC controls have been assumed to be cheaper and

more readily available than those for NOx and because simultaneous

VOC and NOx reductions frequently are not predicted to be as

effective for ozone control as VOC reductions alone. The critical

policy issue at this point is, to what extent should NOx controls be

invoked for further ozone abatement, in view of the fact that VOC

controls alone do not seem to have been as effective in ozone

reduction as predicted? In this regard, there is mounting evidence

that hydrocarbon emissions inventories in urban areas may be

seriously underestimated (62) and that, therefore, ozone reductions

are less than originally anticipated. Emission factors for mobile

sources must be verified, for example, by comprehensive monitoring

in highway tunnels. Models for the spatial and temporal distribution

of traffic must also be verified in field programs. Reconciliation of

inconsistencies between estimated historical hydrocarbon emissions

and ozone trends must be attempted.

Role of biogenic hydrocarbons in urban air pollution. Hydrocarbons

emitted by vegetation are primarily photochemically reactive cyclic

and other olefins with internal double bonds. In the United States,

trees are the principal emitters of such hydrocarbons, with isoprene

and o-pinene being the primary species emitted. Isoprene and the

other naturally emitted olefins are highly reactive, but the atmospheric degradation pathways of these compounds are not well

known. For example, only about 50% of the degradation pathways

of isoprene have been identified, and those for ao-pinene are even less

well known. The ozone-forming potential of biogenic hydrocarbons, under urban VOC/NOx conditions, is roughly equivalent, on

a molecule-for-molecule basis, to that of the anthropogenic hydrocarbon mix. At high VOC/NOx ratios, however, characteristic of

rural conditions, the natural hydrocarbons, which react with ozone,

become a net destroyer of ozone and form aerosol.

The extent of involvement of biogenic hydrocarbons in urban

photochemical smog has generally been assumed to be small.

Computer simulations of ozone levels in Atlanta, however, including estimates of biogenic hydrocarbon emissions and their chemistry, indicate that anthropogenic hydrocarbon emissions reductions

needed to reduce ozone concentrations could become much more

stringent if biogenic hydrocarbon emissions occur at the level

assumed in the study and if their chemistry occurs as presumed (63).

These results point to the need to better define biogenic hydrocarbon emissions inventories and to clearly establish the role of these

hydrocarbons in photochemical smog chemistry in urban areas

ARTICLES 7SI
where such emissions are likely to be important.

Methanol-fiueled vehicles. Methanol has been proposed as a replacement for gasoline and diesel fuels. Unlike gasoline-fueled vehicles,

methanol-fueled vehicles emit only a few compounds, primarily

unbumed methanol and formaldehyde (a product of incomplete

combustion). The principal benefit of replacing gasoline with

methanol is that the unburned methanol, which is 90 to 95% of the

organic emissions, is less photochemically reactive, as an ozone

former, than the organics in gasoline-fueled vehicle exhaust. For

conditions favorable to ozone formation over a 1-day period,

computer simulations of incremental reactivity that incorporate

chemical reaction mechanisms indicate that methanol alone will

form approximately 20 to 33% as much ozone per unit of emission

(on a mass basis) as the current on-road emissions. The benefits in

ozone reduction appear to diminish in successive days of a typical,

severe multiday episode. Formaldehyde, on the other hand, is very

reactive and is emitted by methanol-fueled vehicles at rates greater

than those by gasoline-fueled vehicles. It is also a suspected carcinogen. Formaldehyde and methanol emissions are generally correlated;

the exhaust formaldehyde is a certain mass fraction of the methanol

content.

The benefits of methanol as a fuel are greatly dependent on the

level of formaldehyde emissions that can be achieved. Computer

simulations have shown that, as formaldehyde emissions from

methanol-fueled vehicles increase, a crossover point is reached where

the photochemical ozone-forming capability of the emissions is

equal to that ofconventional gasoline-fueled exhaust (64, 65). While

significant progress has been made in developing methanol vehicle

technology, only a relatively small number of vehides have been

emissions tested, and therefore emission levels are still uncertain.

Also, little information exists on evaporative emissions from methanol-fueled vehicles. Use of methanol in light- and heavy-duty diesel

vehicles would lead to a dramatic reduction in fine particles emitted.

Effects of methanol emissions on atmospheric cloud chemistry and

aerosol formation are at this time unknown.

REFERENCES AND NOTES

1. Urban Ozone and the Clean Air Act: Problems and Proposals for Change (Office of

Technology Assessment, Washington, DC, 1988).

2. Nationwide in 1985, hydrocarbon emissions in urban areas were estimated to

originate from: highway vehicles, 40%; organic solvent use, 30%; small stationary

sources, the remainder. Approximately 45% of NO, emissions are from motor

vehicles, 30% from utilities, and most of the remainder from industrial fuel

combustion.

3. Guidelines for Use of City-Specific EKMA in Preparing Ozone SIP's, report EPA450/4-80-027 (U.S. Environmental Protection Agency, Research Triangle Park,

NC, 1981).

4. R. Atkinson and A. C. Lloyd, J. Phys. Chem. Ref: Data 13, 315 (1984).

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13. ___ ibid., p. 2549.

14. E. C. Tuazon, H. MacLeod, R. Atkinson, W. P. L. Carter, Environ. Sci. Technol.

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25. F. W. Lurmann, W. P. L. Carter, L. A. Coyner, A Surrogate Species Chemical Reacion

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G. T. Wolff and R. L. Klimisch, Eds. (Plenum, New York, 1982).

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88, 5109 (1983); D. J. Jacob et al., Environ. Sci. Technol. 19, 730 (1985).

56. S. N. Pandis and J. H. Seinfeld, in preparation.

57. J. F. Pankow, Atmos. Environ. 21, 2275 (1987); T. F. Bidleman, Environ. Sci.

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58. T. D. Behymer and R. A. Hites, Environ. Sci. Technol. 19, 1004 (1985).

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60. N. E. Gallopoulos et al., paper presented at the Air Pollution Control Association

Specialty Conference, Hartford, CT, 1987.

61. J. H. Seinfeld, J. Air Pollut. Control Assoc. 38, 616 (1988).

62. F. M. Black, paper presented at Third U.S.-Dutch International Symposium on

Atmospheric Ozone Research and Its Policy Implications, U.S. EPA/Netherlands

Environment Ministry, Nijmegen, The Netherlands, 1988.

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65. T. Y. Chang, G. Kunstasal, R. A. Gorse, Jr., paper presented at the American

Chemical Society meeting, Division of Environmental Chemistry, Los Angeles, 25

to 30 September 1988.

66. During the period 1984-1986, over 800 ambient air samples from 39 urban areas

in the United States were analyzed by capillary gas chromatography with sample

cryogenic preconcentration (67). Table 1 lists compounds in descending order of

abundance with the median and maximum concentrations in ppbC. Only 25 ofthe

48 compounds listed by Seila and Lonneman (67) are given here. The 48

compounds consisted of 25 paraffins, 15 aromatics, 7 olefins, and acetylene. Three

of the aromatics were not specifically identified. The sum ofmedian concentrations

for the paraffins, 266 ppbC, represented 60% of the total for the 48 compounds.

Aromatics, 116 ppbC, were 26%; olefins, 46.8 ppbC, were 11%; acetylene, 12.9

ppbC, was 2.7%.

67. R. L. Seila and W. A. Lonneman, paper 88-150.8, presented at the 81st Annual

Meeting of Air Pollution Control Association, Dallas, TX, 20 to 24 June, 1988.

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