# 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 im

### portant 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 beenmade in

### reducing airborne 

### levels ofsulfurdioxide and carbon monoxide in U.S. 

### cities, the 

### standard for ozone was exceeded

### during 1983-1985 in 76 

### urban areas 

### ofthe 

### 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 energyofthe 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 

### rapidlywith 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] predictedbythephotostationary stateat solarnoon inU.S. latitudes is about 20 parts per billion (ppb) by volume. While ozone concentrations in 

### unpolluted tropospheric airvary 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 ofozone circumvents reaction 3 and allows

ozone to accumulate. Such a path is provided by the presence of hydrocarbons (2). Inparticular, peroxyradicals (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 ofozone 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 

### ofozone generation is related closely to the rate of

### R02 production. Generation of the organic peroxy radicals 

### R02 occurs largely

### bythe 

### attackofthe hydroxyl radical (OH) onhydrocarbons. Interestingly, OH attack, which ultimately scavenges hydrocarbons from the 

### atmosphere, is also 

### that which, 

### through oxidation ofNO toNO2, produces ozone. The chemical structureofthehydrocarbon determines the number and form ofperoxy radicals and hence the numberofNO to

### NO2 conversions thatoccuras thehydrocarbon is oxidized. There are three principal sources ofhydroxyl radicals inthe sunlit urban atmosphere: (i) photochemical dissociation of ozone, (ii) 

### photochemical dissociationofcarbonyl compounds, and (iii) photochemical dissociation ofnitrous 

### acid. Photochemical dissociation of

ozone leads to the formation ofexcited 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 acidachieves concentrations in

### pollutedurban airof1 to 10

ARTICLES 745 The author is in the Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125.

IO FEBRUARY I989on May 22, 2014www.sciencemag.orgDownloaded fromppb, with 

### maximum levels occurring just 

### before sunrise. In the presence 

### ofsunlight, 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 ofthe 

### time, result in regeneration

### ofOH.) 

### Nitric acidis also a major sink for nitrogen 

### oxides through the 

### above reaction. One important 

### free radical removalprocess 

### forthe

### organicperoxy 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 

### thermallywith a 

### lifetime at 298 K

### ofabout 45 minutes. The PAN 

### molecule serves as a temporary reservoir for both the 

### peroxyacetyl radical andNO2, with its 

### concentration 

### dependent on 

### temperature andonthe ratioofNOto

### 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 formationofozone andother

### secondaryspecies isnot instantaneous 

### following emission 

### of 

### hydrocarbons and 

### nitrogen oxides, 

### significant transport and mixing occur 

### simultaneously with chemical reaction. The 

### complex interplay between 

### atmospheric chemistry

### andtransportprocesses canbe

### visualized

### byconsidering a 

### column

### ofair

### of

### arbitrarycross 

### sectionthat extends 

### fromthe 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 airhydrocarbons in 39U.S. cities-the25 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 so

### called mixing height. Turbulent convection in the boundary layer

ensures 

### rapidvertical mixing in 

### the 

### layer. The air parcel can be 

### thought to follow a 

### trajectory through the region as determined

### bythe 

### horizontal windcomponents. Ifthe 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-richareas. Overatime

### scaleofabout 1 

### hour, hydroxyl radical attackonthehydrocarbons commences. As the airparcelmoves out 

### ofthe

### urbancore, the emissiondensitydecreases. 

### Concentrations of precursor

### hydrocarbon and

### nitrogenoxides 

### (NO,) peakand

### startto declinewithdownwinddistance as aresult 

### ofthe

### effects ofchemical reaction andmixing. Ozone intheparcel continues toincrease asthe array 

### of

### hydrocarbons is 

### photooxidized. The increase in 

### 03 level is 

### sustainedovera 1- to 5-hourperiodas themore 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 airparcel 

### ultimately begins to 

### decrease reflecting the effects 

### of

### dilution, conversionof

### NO2 to

### nitricacid, andremoval atthe 

### surface. At sunset, 

### allphotochemical ozoneproductionceases. At night when the 

### regeneration of 

### ozone by means of 

### NO2 photolysis does not occur, the overall 

### effect 

### ofthe ozone reactions withNO and

### NO2 is toprovide a route 

### forconvertingthe nitrogen oxides to 

### nitric acid. (We 

### will discuss nighttime 

### chemistry shortly.) During nighttime, radiative 

### cooling at the ground usually leads to the

### establishment ofa

### ground-based inversion layerwitha

### height of several tens to 

### hundreds ofmeters 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 ofmaximum emission 

### density, that 

### is, the urban core, to the location ofpeak ozone concentration is the central idea 

### behind the U.S. 

### Environmental ProtectionAgency's 

### Empirical Kinetic Model

### ing 

### Approach (EKMA) used to 

### relate 

### hydrocarbon and 

### NO. emissions 

### changes tochanges inmaximumozone 

### concentrations for

a

### particular urban area (3).



## Hydrocarbon 

## Chemistry

The feature 

### that 

### distinguishes the chemistry ofthe 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

### majorclass 

### 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)

orwith other 

### R02 radicals. At NO concentrations above about 30 parts per 

### trillion (ppt), reaction with NO is the dominant 

### reaction

SCIENCE, VOL. 243pathway for 

### R02 radicals. The 

### yield ofthe 

### 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 

### byunimolecular

### 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

### decompositionorreactionwith 2. The reactions

### occurringafter

### alkoxyradical 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 

### likelythat 

### these intermediate 

### products continue to react 

### with other 

### species present. Althoughit is

### predictedthat, foralkaneswith 

### fourormore 

### carbons, B-bifunctional compounds of the 

### general form 

### RCOCH2CH2- CHOR' 

### would be 

### formed, at present no 

### definitive evidence for the 

### formation ofthese 

### compounds exists. 

### Also, ourpresent 

### 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 ofalkanes with 

### five or more 

### carbons. In summary, 

### the 

### chemistry ofC2 to 

### C4 alkanes is 

### reasonablywell understood. For

### C5 and

### largeralkanes, the 

### fateofthe 

### corresponding alkoxy radicals is not 

### well 

### established, and the role of RO + 

### NO2-) RONO2 for such

### species as an

### NO. sinkis not

### fullyknown. 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 ofa

### hydrocarbon mixture into the 

### atmosphere. All ofthe 

### OH-radical-alkene reactions 

### proceed by OH addition to the 

### double bond followed

### by rapidadditionof

### 02 to

### yieldthe

### corresponding

### hydroxyalkyl peroxy radical. Subsequent reaction ofthe

### hydroxyalkyl peroxyradicalwith NO 

### yields the 

### ,B-hydroxyalkoxy radical. 

### (The hydroxynitrate 

### path accounts for 

### only about 1% and can be 

### neglected.) While the 

### I

### hydroxyalkoxy radicals can react 

### with 

### 02 or 

### undergo unimolecular decompositionor

### 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 ofthe 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 

### ofthe 

### reported high toxicity ofthe 

### dinitrates, further research is 

### needed to 

### better define their

### yields. Furthermore, the reactionsequences 

### occurring in the 

### absence of

### NO, as 

### typically occurs at 

### night when 

### NO3 radical reactions are ofimportance, 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 ofthe 

### initially energy-rich biradical

(23)



## coR

R2 j

which can be 

### collisionally stabilized or undergo unimolecular de

### composition (9). The

### thermalizedbiradicals 

### canreactwithanumber 

### 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 rapidlyadds 

### 02 toyieldaperoxyacyl radical 

### [RC(O)OO]. Asnoted 

### 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% 

### ofthe formaldehyde inthe

### urbanatmosphere 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 process

### es, although, withtheexceptionofacetone, 

### photolysis is

### probablyof minor 

### importance. The major reaction products from the atmo

### spheric reactions 

### ofketones are 

### aldehydes and PANprecursors, 

### and bifunctional 

### oxygen-containingcompounds 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 reactionofthe 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 

### pathwayat 298 K. Boththe CH20H

### andCH30 radicals reactwith 

### 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 74-7Products 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 solelywith the OH radical, a reaction that can proceed by two pathways: (i) H-atom abstraction ofalkyl 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 ofaromatic aldehydes, such as benzaldehyde, with subsequent reactions ofthese 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 thirdbody. As notedearlier, 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 ofwater:


### N205 + 

### H20-- 

### 2HN03

+ other isomers CH3 $1JOH

CH30H

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 reactionpathway 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 totalyieldofproducts as a percentage ofthe parent aromatic is onlyabout 30% forthe 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 ofreactive 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 ofphotolytic reactions, is the accumulation at night of the nitrate free radical, NO3, which is rapidlyphotolyzed during the day so that its daytime concentration isnegligible. Duringthenight, NO3 is produced by

NO2 + 03-> NO3 + 02 (30)

748 

### (35)

Themain importance ofthenighttime chemistryis the conversion of

## NO, into nitric acid by a route not 

### involving OH radicals. The relative importance ofdifferentpathways leadingtothe formationof 

### HNO3 atnight, namelythe 

### 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 ofthe above mechanism

### plus a number ofother, less important reactions) andobserved

### NO3 concentrations provides a test 

### ofthe degree 

### of

### understandingofthis 

### 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 UrbanAir 

## Pollution

Air 

### pollution 

### chemistry is studied 

### experimentally at two levels: 

### laboratory studies of the kinetics and mechanisms 

### of individual reactions, and smog chamber experiments, inwhich 

### 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 manyofthe 

### chemical details fromthe 

### 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% ofthe 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. 243fraction ofthese data and

### generally simulate 

### peak ozone concentrations to within 

### ±+30%. It is 

### generally agreed that an 

### acceptable chemicalmechanismmust simulate this large

### bodyofsmogchamber data at least as 

### accurately as mechanisms it is 

### proposed to 

### replace. Because 

### ofa lack

### ofmeasurements ofmany intermediate species and 

### products, fitting of the 

### chamber data for 03, 

### NON, PAN, and the starting organics 

### provides only a 

### global evaluation 

### ofa mechanism. It is possible that compensating errors in the mechanism mayyield reasonable 03 predictions. Because it is not 

### practical to use a 

### mechanism that includes 

### the 

### detailed, specific chemistryofeachorganic compoundpresent 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 

### similarorganics, and 

### (ii) the 

### "lumped structure" approach inwhich 

### 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 

### ofthose areas where uncer

### tainties 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 threemajor

### differences in the 

### representation of the organic chemistry in the above three mechanisms. As notedearlier, themechanismofring 

### fragmentation in the aromatic-OH reaction has not yet 

### been identified. The RADMmechanismuses

### dicarbonyls as surrogates foralltheuniden

### tified 

### products, and, 

### consequently, dicarbonyl yields in 

### the RADM mechanismare 

### approximatelyafactorof2 higherthanthose 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 secondmajor 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 energyused inthe 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 oforganic peroxyradicals withotherradicals, 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 nearlyidentical levels ofozoneundermost 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 oforganic chemistrywhere onlythe 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 ofthe 

### 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 ofsizes below about 2-,umdiameter are largely responsible 

### for the severe visibility deterioration observed in areas like Los Angeles. Such particles are also easily respirable and may contain toxicor

### mutagenic components. Control ofthe fineparticle component ofthe urban aerosol is a

### particularly important problem facing air

### pollution control agencies. The 

### atmospheric aerosol is ahighlycomplexentity, 

### representinga mixture of primary emissions, such as soot and fly ash, and secondary species that are the 

### "sink" ofmany 

### 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 ofurban

### aerosols are 

### sulfates, nitrates, 

### ammonium, trace 

### metals, carbonaceous 

### material, andwater (Fig. 1). Our 

### detailed knowledge of the size-composition distribution of the

### urbanaerosol is limited (29-32). Primary

### particles, once emitted, undergo chemical and

### physical processes that result in a 

### continually changingchemical composition andparticle size distribution. Particlesofasingle substanceemittedfromaspecific source, forexample, soot or sea salt, may react with the products ofgas-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 conversionprocess includes (i) 

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

### particle from a 

### singlevapor 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 ofvapor molecules to

### preexistingparticles (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 

### supersaturatedwith respect to these 

### species. The degree 

### ofsupersat10 FEBRUARY I989 ARTICLES 74.9urationwill determine the 

### competitionbetween 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

### throughtheN205 

### hydrolysis route at

### night. Unlike 

### sulfuric 

### acid, nitric acid has a 

### high vapor pressure and does not 

### readily condense on 

### aerosols. The formation ofaerosol nitrate requires the presence ofNH3, orotheralkaline 

### species, to 

### formsalts such as NH4NO3. An

### equilibriumis

### presumed toexist amongNH3, 

### HNO3, and NH4NO3:

NH3 + 

### HNO3 a NH4NO3 (36)
This equilibrium is strongly 

### influenced by temperature, relative humidity, andpH (37, 38), andfavors the formationofNH4NO3 at low temperature, 

### high humidity, and lowpH. Stelson et al. (37, 38) and others (34, 35, 39, 40) have 

### postulated that the 

### sulfate-nitrate-ammonium aerosol constituents 

### shouldbe 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 fora 

### 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 oftheurban 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 sevenormore 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 

### theAmes test, and soots have been shown to be carcinogenic inanimal studies (53). Aerosol carbon is not routinely

### monitored, andemissions 

### 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 

### ofmost large urban areas. 

### One-quarter ofthe total came 

### from 

### gasoline-powered vehicle exhaust, one-fourth from diesel engine exhaust, and the remainder from a 

### largevarietyofstationary fuel 

### burning, industrial, and distributed sources. Black elemental carbon emissions were dominated 

### bydiesel exhaust. The ratio oftotal 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 ofthe extent 

### ofenrichment in 

### organic aerosol attributable to 

### secondary aerosol formation. Field measurements carried out in 1982 in Los 

### Angeles showed 

### little seasonal 

### dependence in the ratio oftotal to elemental carbon (43), although some individual days at inlandsites did showlarge 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

### ofthe 

### South CoastAir

### 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 andAcidic Fogs

The 

### importance ofacid 

### fog as a component 

### ofthe 

### 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 ofanions 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 fogencompassesvirtuallythe entire range of

### atmospheric chemical 

### phenomena: gas-phase chemistry to produce 

### HNO3 and H2SO4 

### vapors, aerosol formation, 

### cooling ofmoist air 

### followed by condensation ofwater vapor on the aerosol particles that serve as condensation 

### nuclei, scavenging 

### ofgaseous 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 adense radiationfogleads to rapidremoval 

### fromthe 

### atmosphere ofthe major ionic species. Because 

### ofthe 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 ofsecondary importance in a cloudmaybecome 

### 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 pri

### marily in the gas phase; 

### undermid-continentalwintertime temperatures, the distribution 

### shifts 

### markedly toward the 

### particulate phase. 

### Atmospheric transformations ofPAH

### depend

### dramaticallyon 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 shownto leadto 

### loss ofPAH andtheir derivatives on several 

### substrates, although because 

### ofthe 

### 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. 243Recent 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 ofPAH are needed to assess the atmospheric fate ofthese species.



## Some 

## Critical Policy Issues 

## in 

## Urban Air 

## Pollution

Effect ofprecursors on ozone. Despite large efforts and expenditures

to control emissions ofhydrocarbons andnitrogenoxides, 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) andNOR, 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 order20 ppmC/ppm, where ppmC is parts permillionbyvolume of carbon atoms), there is a shortage ofNO that can be oxidized by

peroxyradicals. Ozone formation is controlledprimarily 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 onthis diagram. Strategies that followthe 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 ofthe 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 onwhich side ofthe 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 oforganics available since 03 scavenging by the NO-03 reaction is more effective than the peroxy radical oxidation ofNO to NO2. Furthermore, the

### N02-OH reactionformingnitric acideffectivelycompetes with organics for OH, thereby inhibiting the formation 

### ofperoxy radicals. Decreasing

### NOxunderthese 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 andozone levels close to the

### NOx source regionwill 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 ofnatural sources and ofmaterial 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 andbecause 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, inview 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, forexample, bycomprehensive monitoring in

### highwaytunnels. Models 

### forthe

### spatial and

### temporal distribution oftraffic must also be verified in field programs. 

### Reconciliation of inconsistencies 

### between 

### estimatedhistorical 

### hydrocarbon emissions and ozone trends must be 

### attempted. Role ofbiogenic hydrocarbons in urban air pollution. 

### Hydrocarbons emitted 

### byvegetation are primarily 

### photochemically reactive 

### cyclic and other olefins with internal double bonds. In the 

### United States, trees are the 

### principal emitters ofsuch 

### hydrocarbons, with 

### isoprene and 

### o-pinene 

### being the 

### primary species emitted. Isoprene and the other 

### naturally emitted olefins are highly reactive, but the atmo

### spheric 

### degradation pathways of these 

### compounds are not 

### well known. For 

### example, 

### only about 

### 50% ofthe 

### degradation 

### pathways of

### isoprene have 

### been 

### identified, andthose 

### for

### ao-pinene are evenless 

### well known. The 

### ozone-forming potential of biogenic 

### hydrocar

### bons, underurban

### VOC/NOx conditions, is 

### roughlyequivalent, 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 reactwith ozone, become a net destroyer 

### ofozone 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, includ

### ing 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 inthe 

### studyandiftheir

### chemistryoccurs 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 7SIwhere 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 

### unburnedmethanol, which is 90 to 

### 95% ofthe 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 perunit ofemission (on a mass 

### basis) as 

### the current on-road 

### emissions. The benefits in

ozone reduction appear to diminish in successive days ofa 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-fueledvehicles. It is 

### also a suspected

### carcinogen. 

### Formaldehyde andmethanolemissions are

### generally

### correlated; the exhaust 

### formaldehyde is a certainmass 

### fraction ofthe methanol content. The benefits ofmethanol 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-fueledvehicles 

### increase, acrossoverpointisreachedwhere the 

### photochemical 

### ozone-forming capability of the emissions is equal tothatofconventional 

### 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 onevaporative emissions frommethanol-fueledvehicles. Use 

### ofmethanol in 

### light- andheavy-duty diesel vehicles would leadto adramatic reduction in

### fineparticles emitted. Effects ofmethanol 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 TechnologyAssessment, Washington, DC, 1988). 2. Nationwide in 1985, hydrocarbon emissions in urban areas were estimated to originate from: highwayvehicles, 40%; organic solvent use, 30%; small stationary sources, the remainder. Approximately 45% ofNO, 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 andA. C. Lloyd, J. Phys. Chem. Ref: Data 13, 315 (1984). 5. W. P. L. Carter and R. Atkinson, J. Atmos. Chem. 3, 377 (1985). 6. R. Atkinson, Chem. Rev. 86, 69 (1986). 7. H. Bandow, M. Okuda, H. Akimoto, J. Phys. Chem. 84, 3604 (1980). 8. P. B. Shepson et al., Environ. Sci. Technol. 19, 849 (1985). 9. S. Hatakeyama, H. Kobayashi, H. Akimoto, J. Phys. Chem. 88, 4736 (1984). 10. J. A. Leone, R. C. Flagan, D. Grosjean, J. H. Seinfeld, Int.J. Chem. Kinet. 17, 177 (1985). 11. H. Bandow, N. Washida, H. Akimoto, Bull. Chem. Soc. Jpn. 58, 2531 (1985). 12. H. Bandow and N. Washida, ibid., p. 2541. 13.

___ ibid., p. 2549. 14. E. C. Tuazon, H. MacLeod, R. Atkinson, W. P. L. Carter, Environ. Sci. Technol. 20, 383 (1986). 15. M. Gery, D. L. Fox, H. E. Jeifries, L. Stockburger, W. S. Weathers, Int.J. Chem. Kinet. 17, 931 (1985). 16. P. B. Shepson, E. 0. Edney, E. W. Corse, J. Phys. Chem. 88, 4122 (1984). 17. A. G. Russell, G. R. Cass, J. H. Seinfeld, Environ. Sci. Technol. 20, 1167 (1986). 18. H. S. Johnston, C. A. Cantrell, J. G. Calvert, J. Geophys. Res. 91, 5159 (1986). 19. J. F. Noxon, R. B. Norton, E. Marovich, Geophys. Res. Lett. 7, 125 (1980). 20. J. F. Noxon, J. Geophys. Res. 88, 11017 (1983). 21. D. Perner et al., ibid. 90, 3807 (1985). 22. S. P. Sander, J. Phys. Chem. 90, 4135 (1986). 23. W. P. L. Carter, R. Atkinson, A. M. Winer, J. N. Pitts, Jr., Int.J. Chem. Kinet. 14, 1071 (1982). 24. M. W. Gery, G. Z. Whitten, J. P. Killus, Development and Testing ofthe CMB-IVfor Urban and Regional Modeling, report EPA/600/3-88/012 (U.S. Environmental ProtectionAgency, ResearchTriangle Park, NC, 1988). 25. F. W. Lurmann, W. P. L. Carter, L. A. Coyner, A Surrogate Species ChemicalReacion Mechanismfor Urban-Scale Air Quality Simulation Models, report EPA/600/3-87/014 (U.S. Environmental ProtectionAgency, ResearchTriangle Park, NC, 1987). 26. W. R. Stockwell, Atmos. Environ. 20, 1615 (1986). 27. M. C. Dodge, J. Geophys. Res., in press. 28. Southern California AirQuality Study (SCAQS) Program Plan (Air Resources Board, State ofCalifornia, Sacramento, CA, 1987). 29. G. W. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel, J. J. Wesolowski, Eds., The Characterand Origins ofSmog Aerosols (Wiley, New York, 1980). 30. R. J. Countess, G. T. Wolff, S. H. Cadle, J. Air Pollut. Control Assoc. 30, 1194 (1980). 31. P. J. Groblicki, G. T. Wolff, R. J. Countess, Atmos. Environ. 15, 2473 (1981). 32. P. J. Lioy and J. M. Daisey, J. Air Pollut. Control Assoc. 33, 649 (1983). 33. J. H. Seinfeld, Atmospheric Chemistry and Physics ofAir Pollution (Wiley, New York, 1986). 34. M. E. Bassett andJ. H. Seinfeld, Atmos. Environ. 17, 2237 (1983); ibid. 18, 1163 (1984). 35. C. Pilinis and J. H. Seinfeld, ibid. 21, 2453 (1987). 36. W. H. Benner, R. Brodzinsky, T. Novakov, ibid. 16, 1333 (1982). 37. A. W. Stelson, S. K. Friediander, J. H. Seinfeld, ibid. 13, 369 (1979). 38. A. W. StelsonandJ. H. Seinfeld, ibid. 16, 983 (1982); ibid., p. 993; ibid., p. 2507. 39. P. Saxena, C. Seigneur, T. W. Peterson, ibid. 17, 1315 (1983). 40. P. Saxena, A. B. Hudischewskyj, C. Seigneur, J. H. Seinfeld, ibid. 20, 1471 (1986). 41. L. M. Hildemann, A. G. Russell, G. R. Cass, ibid. 18, 1737 (1984). 42. A. G. Russell and G. R. Cass, ibid., p. 1815. 43. H. A. Gray, "Control ofatmospheric fineprimarycarbonparticle concentrations," thesis, California Institute ofTechnology (1986). 44. H. Rosen, A. D. A. Hansen, L. Gundel, T. Novakov, Appl. 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