Computer simulation of air pollution chemistry

Computer Simulation of Air Pollution Chemistry Christian Seigneur Bechtel Environmental, Inc., P.O. Box 3965, San Francisco, California, 94119, USA

Abstract The computer simulation of a i r pollution chemistry is a powerful technique for the understanding of the formation of pollutants and the developmont of effective strategies for the control of a i r pollution. The present status computerized mathematical modeling of a i r pollution chemistry is reviewed, typical examples of recent applications presented and present areas of uncertainties are discussed. The four major areas of a i r pollution chemistry that reviewed include photochomical oxidants, acid deposition, atmospheric aerosols and a i r toxics. The effect atmospheric physical processes on a i r pollution chemistry is b r i e f l y discussed. The numarlcal techniques used for simulation of atmospheric chemistry are reviewed. I.

INIROOUCIION

Atmospheric p o l l u t i o n consists in the adverse e f f e c t s of a v a r i e t y of airborne chemical species on human health, vegetation, m a t e r i a l s , ecosystems and a e s t h e t i c values. These species may be d i r e c t l y emitted from p o l l u t i o n sources i n t o the atmosphere o r may be formed from primary p o l l u t a n t s by chemical reactions in the atmosphere. The former are called primary a i r pollutants whereas the l a t t e r are called secondary a i r pollutants. The residence tlme of chemical species in the atmosphere depends on their chemical reactivity and their rate of removal by dry and wet deposition processes. Atmospheric pollution is a multlfaceted problem that includes phenomena involving complex chemistry such as photochemical smog, acid deposition, atmospheric aerosols and a i r toxics. Ihe complexity of a l r pollution chemistry is due to the large number of chemical reactions that involve many interacting chemical species. rhi~ complexity generally results in nonlinear responses of the concentrations of secondary p o l l u t a n t s to changes in primary p o l l u t a n t levels. It is thus necessary t o use computerized mathematical models of a i r p o l l u t i o n chemistry to understand and determine the complex nonlinear r e l a t i o n s h i p s between these secondary a i r p o l l u t a n t s and their precursors. The computer simulation of air p o l l u t i o n chemistry has, t h e r e f o r e , become a powerful technique f o r the understanding of the formation of a i r p o l l u t a n t s and the development of e f f e c t i v e s t r a t e g i e s f o r the control of a i r p o l l u t i o n . We present here a review of the present status of computerized mathematical modeling of a i r pollution chemistry with some typical examples of recent applications and discussion of present areas of uncertainties. Our review addresses in Sections 2 through 5 four major forms of air pollution: photochemical oxidant formation, acid deposition, atmospheric aerosols and a i r toxics. Because the chemistry of photochemical oxidants a f f e c t s the formation of acid species, atmospheric aerosols and c e r t a i n a i r t o x i c s , Section 2 is s l i g h t l y more d e t a i l e d than Sections 3 to 5. This review o u t l i n e s the major atmospheric chemical pathways of a i r p o l l u t i o n chemical systems and focuses on the computer simulation of these systems. A more d e t a i l e d d e s c r i p t i o n of the chemical mechanisms is given in textbooks such as those by Ftnlayson-Pitts and Pitts [1] and Setnfeld [2]. Interactions between

atmospheric physics and chemistry are b r i e f l y discussed in Section 6. Numerical techniques t h a t are u t i l i z e d t o simulate a i r p o l l u t i o n chemistry are reviewed in Section 7. Concluding remarks are presented tn Section 8.

2.

PHOIOCHENICALSROG

Photochemical smog is a form of p o l l u t i o n t h a t is most prominent in sunny urban areas such as Los Angeles, Mexico City and Madrid. Photochemical smog results from the reactions of nitrogen oxides (NOx) and r e a c t i v e hydrocarbons (RHC) in the presence of s u n l i g h t . Major p o l l u t a n t products include ozone (03), nitrogen dioxide (NO2), p e r o x y a c e t y l n t t r a t e (PAN), nitric acid (HN03) and condensible organic products. Formation of secondary aerosols such as s u l f a t e , n i t r a t e and organic aerosols g e n e r a l l y also occurs during photochemical smog episodes and leads to v i s i b i l i t y degradation. The chemistry of atmospheric aerosols is discussed in d e t a i l in Section 4 and t h i s section pertains to the gas-phase chemistry of photochemical smog. In ] a b l e I , the major reactions of nitrogen oxides (NO and NO2) and formaldehyde (HCHO) are presented. Clear|y, m a n y other hydrocarbons (other aldehydes, ketones, p a r a f f i n s , o l e f t n s , and aromatics) are involved in the atmospheric chemistry of photochemical smog. However, most of the major features of photochemical smog chemistry can be exampltfted by means of t h i s NOx/HCHO mechanism. Formation of 03 three-step process.

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during

(2) The HO2 r a d i c a l converts r a d i c a l s (Reaction 3). (3) NO2 is then (Reaction l ) .

daytime

through

a

photolyzed

NO t o to

NO2 t o

produce

produce OH NO and

03

l h e r e f o r e , f o r each HO2 radtcal produced, one molecule of 0 3 can be formed. (The y i e l d is s l i g h t l y less since HO2 r a d i c a l s may undergo other reactions such as Reaction 13). Since the conversion of NO to NO2 produces an OH radical (Reaction 3), the oxidation of

©ComputationalMechanicsPublications

ENVIRONMENTAL

occurs

(1) Formaldehyde is photolyzed (Reaction 4) or oxidized (Reactions 6 through 8) to lead to the formation of HO2 r a d i c a l s .

Paper received on 15 October 1986, and in final form on 2 June 1987. Referee: Prof. Gregory R. Carmichael 116

air of are are of the

Air Pollution Chemistry: i"

formaldehyde continues through Reaction 6 to produce more HO2 radicals. This chain reaction process leads to the formation of 03 through a continuous oxidation of the hydrocarbon, conversion of NO to NO2 and photolysis of NO2 • I t may be noted that NO can also be converted to NO2 via Reaction 2. However, this pathway involves the consumption of 03 and the net yield of Reactions l and 2 is a stoichiometric balance between NO, NO2 and 03 . Therefore, in the absence of hydrocarbons, 03 formation is limited to a steady-state concentration that results from Reactions l and 2. Another aspect of the i n t r i c a t e relationship between NOx and hydrocarbons in photochemical smog chemistry is the competition between NO2 and hydrocarbons (here, formaldehyde) for OH radicals in Reactions 9 and 6, respectively. A sink for nitrogen dioxide is n i t r i c acid (HN03) (Reactions 7, 9 and 12). Organic nitrates are also formed in the presence of other hydrocarbons. For example, peroxyacetylnltrate (PAN) is f o r m e d when acetaldehyde reacts with NOx. The HO2 radicals can recombine according to Reaction 13 to form hydrogen peroxide (H202), a key oxidant in acld deposition chemistry. Although photochemical smog formation requires sunlight, the nighttime chemistry of smog is of p a r t i c u l a r interest because i t leads to the formation of HNO3 and modifies the composition of the pollutant mixture.

Table i .

Reaction N.umber

. . . . . . .

"

dl/

'

C. Seigneur

"

At night, photolysls of NO2 d o e s not take place. Therefore, there is no pathway for 03 formation. Moreover, 03 is depleted rapidly by fresh NO emissions (Reaction 2) and by reaction with NO2 (Reaction lO). The radical NO3 that is formed in the l a t t e r reaction plays a p i v o t a l role in nighttime smog chemistry. HNO3 is formed at night when NO3 reacts with NO2 (Reactions I I and 12) and with HCHO (Reaction 7). The l a t t e r reaction also leads to the formation of HO2 radicals. Therefore, although the photochemical pathways are not available to generate the HO2 and associated OH radicals, some nonnegligible HO2 and OH levels may be present at night due to reactions such as Reaction 7. Another nighttime source of HO2 radicals is the thermal decomposition of PAN in presence of NO. This brief description of photochemical smog chemistry shows t h a t during daytime the chemistry is driven by the HO2 and OH radicals that p a r t i c i p a t e in the oxidation of hydrocarbons, conversion of NO and NO2 and formation of 03 . At night, the NO3 radical plays the major role and 03 is depleted. Gautier et a l . [3] have presented a comprehensive analysis that clearly illustrates the daytlme/nighttlme aspects of photochemical smog chemistry in d e t a i l . The chemistry of photochemical smog becomes rather complicated when other hydrocarbons that are present in the atmosphere need to be considered. Such hydrocarbons include, f o r example, in addition to formaldehyde, other aldehydes, ketones, paraffins ( i . e . , alkanes), olefins ( i . e . , alkenes), and aromatics. Table I I presents a summary of the typical lifetimes of these hydrocarbons in a polluted atmosphere. Aldehydes and ketones are the

A Simple Chemical Mechanism of Photochemical Smog

Major Role of the Reaction

ChemicalReaction

Oaytime/NightLime Reaction*

1

NO2+ hv ( + 0 2 )

--

NO + 03

Formation of 03

O

2

NO + 03

~

NO2 + 02

NO to NO 2 conversion,

D&N

03 consumption 3

NO + HO2

--

NO 2 + OH

0

NO to NO 2 conversion without 03 consumption, OH formation

4

HCHO + hv (+ 0 2 )

~

2 HO2 ÷ CO

O

HO 2 formation via HCHO

photolysis

5

HCHO + hv

~

H 2 + CO

No radical formation

O

6

HCHO + OH (+02)

~

HO 2 + CO + H20

HO 2 formation via

O

HCHO oxidation

7

HCHO + NO 3 (+02)

~

HO 2 + CO + HNO 3

N

HO 2 formation via HCHO oxidation

O

B

HCHO + HO2(+NO+02) ~

HCOOH + HO2 + NO2

HCOOH formation,

g

NO2 + OH

--

HNO3

NO 2 sink

O

lO

NO2 + 03

--

NO3 + 02

NO 3 formation

N

II

NO2 + NO3

~

N205

NO 2 sink

N

12

N205 + H20

~

2 HNO3

NO2 sink

N

H202 ~ 02

HO2 s i n k ,

O

NO to NO 2 conversion

13

HO2 + HO2

~

H202 f o r m a t i o n

*

0 and N characterize reactions that are n~Jor during daytime and nighttime, respectively.

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1 9 8 7 , Vol. 2, N o . 3.

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Air Pollution C h e m i s t w :

C. Seigneur

only hydrocarbons t h a t are photolyzed at s i g n i f i c a n t rates. The OH and NO3 r a d i c a l s o x i d i z e most of the hydrocarbons r e l a t i v e l y r a p i d l y . Oleftns are the only hydrocarbons t h a t are o x i d i z e d by 03 and 0 at a significant rate. Methane ts a fairly unreacttve hydrocarbon.

are r e f e r r e d [e.g., 9].

to

E x p l i c t t mechanisms are the most comprehensive models of photochemical smog chemistry and provide t h e r e f o r e the most accurate representation of the e v o l u t i o n of chemical species concentrations. However, t h e i r hlgh level of d e t a i l involves large computational requirements t h a t prevent t h e i r widespread a p p l i c a t i o n 6 . Mechanisms based on surrogate molecule representations o f f e r the advantage of an e x p l i c i t chemical mechanism f o r the surrogate molecules selected. However, the performance of such mechanisms becomes poor when the hydrocarbon mixture that ts simulated involves hydrocarbons w i t h r e a c t t v t t t e s t h a t d i f f e r s i g n i f i c a n t l y from the r e a c t t v l t t e s of the surrogate molecules.

develop

(1) A few hydrocarbons are selected as r e p r e s e n t a t i v e of t h e i r respective hydrocarbon class (e.g. propylene fs taken as r e p r e s e n t a t i v e of o l e f t n s ) . Such mechanisms

Table I I .

Condensed

mechanisms

based

on

lumped

Photolysls and Oxidation Lifetime of

Hydrocarbons in a Typical Polluted Atmosphere (a)

OH

0

Reaction

Reaction

HO2

Hydrocarbon

Photolysis

O3 Reaction

Formaldehyde

2 h

--

6h

--

Other Aldehydes

6 h

--

2 to 5 h

.

.

.

.

--

2 h to 10 d

.

.

.

.

Ketones Methane

.

.

.

.

g mo

.

Other Paraffins

.

.

.

.

10 mln to

.

NO3

Reaction

Reaction

4h

. .

. .

8d 2 to 4 d (b)

.

.

.

.

20 d to

9d Anthropogenic

--

01eflns

5 mln to

50 rain to 7h

2 d

Natural Oleflns

--

1 to 7 h

30 rain to

8too 3 d to

--

8 s to

1 y

3me

(b)

--

40 s to

1 h

20 min

Alkynes

--

7hto3d

.

Aromatics

.

.

.

--

2 h to 2 d

.

.

.

.

18 d to

--

1 to 2 h

.

.

.

.

10 to 30 s

(nonoxygenated) Aromatics

.

. 8 mo

(oxygenated)

(a)

(b)

Conditions are f o r summertime, concentrations of 120 ppb of 03 , 0.2 ppt of OH, 0.002 ppt of O, 40 ppt of HO2 and 100 ppt of NO3. Photolysls and reactions wtth 03 , OH, 0 and HO2 are important during daytime, reactions wtth N% are important at night. Lifetimes which exceed 1 year are not l i s t e d . The l i f e t m e is defined as the inverse of the f i r s t - o r d e r or p s e u d o - f i r s t - o r d e r reaction rate parameter. Therefore, i t corresponds to the time necessary f o r the hydrocarbon concentration to decrease to 37 percent of l t s t n t t l a l value. Not avat 1able.

i

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mechanisms

The r e l a t i v e advantages and disadvantages of the three major types of chemical k i n e t i c mechanisms may be summarized as f o l l o w s .

An e x p l t c i t d e s c r i p t i o n of these reactions may tnvolve several hundreds of steps. For example, the mochanlsm of Carter et a l . [ 7 ] f o r propene and n-butane comprises 130 reactions and the mechanism of Leone and Seinfeld [8] f o r 2 aromatics, 3 o i e f t n s , 3 p a r a f f i n s , 4 aldehydes and 2 ketones comprises 217 reactions. Recently, Kerr and Calvert [ 5 ] developed a chemtcal k i n e t i c mechanism t h a t comprises over 1000 chemtcal r e a c t i o n s . C l e a r l y , the simulation of photochemlcal smog by an e x P l i c i t mechanism ts computattonally expensive and t t has been necessary t o develop condensed k i n e t i c mechanisms t h a t can be used to stmulate smog chemistry tn a c o s t - e f f e c t i v e manner. taken

species

(3) A representation based on chemtcal bonds of hydrocarbons (e.g. single bonds, double bonds, aromatic r i n g s , carbonyl groups) is used to decompose the hydrocarbon s t r u c t u r e i n t o these carbon groups and to develop a k i n e t i c mechanism based on the r e a c t i v i t y of the carbon groups. Thts approach is referred to as the Carbon-Bond Mechanism [11, 12].

The mathematlcal modeling of photochemical smog chemistry requires the simulation of a large number of chemtcal reactions t h a t take place between NOx and a large number of p a r a f f i n s , o l e f i n s , aldehydes, ketones, and aromatic compounds and the products of t h e i r reactions.

been

surrogate

(2) Hydrocarbons from a same class (e.g. o l e f t n s ) are lumped I n t o one category and a lumped mechanism ts developed f o r one h y p o t h e t i c a l species t h a t ts r e p r e s e n t a t i v e of t h l s class of hydrocarbons. These mochanisms are referred to as lumped molecule mechanisms [ e . g . , 10].

More d e t a t l e d discussions of photochemical smog chemistry are given by F t n l a y s o n - P t t t s and P l t t s [ 1 ] , Setnfeld [ 2 ] , and Nhttten [ 4 ] . Reviews of chemical reaction mechanisms and rate parameters are given by Kerr and Calvert [5] and Atkinson and Lloyd [ 6 ] .

Three major approaches have condensed k i n e t i c mechanisms.

as

to

molecule

Air P o l l u t i o n C h e m i s t r y :

representations require some degree of parameterization in the development of the kinetic and mechanistic models of the hydrocarbon chemistry. By selecting these parameters appropriately, i t is possible to obtain good representation of the chemistry of mixtures of hydrocarbons. However, several d i f f i c u l t i e s arise in the parameterization scheme. As the mixture of hydrocarbons evolves with time, the composition of the hydrocarbon mixture changes since some hydrocarbons are more reactive than others. Since the parameterlzatlon is defined for a single composition, it will become approximate as the composition changes. Also, such mechanisms generally do not verify carbon mass balance. Condensed mechanisms based on carbon-bond representations involve some degree of parameterization since the kinetics of the carbon-bond e n t i t i e s must be determined based on the kinetics of molecules. However, carbon-bond mechanisms offer several advantages such as the conservation of carbon mass, the representation of the kinetic properties of RHC mixtures with a better accuracy than condensed mechanisms using molecular representations, and a mechanistic representation t h a t is little affected by the composition of the hydrocarbon mixture. A disadvantage is that some of the mechanism parameters may vary with time. A comparative study of the predictions of several chemical kinetic mechanisms was performed by Leone and Seinfeld [ 8 ] . Their study suggests that d i f f e r e n t photochemical smog models can lead to different predictions of 03 and other secondary pollutant formation because of differences in the parameterlzatlon of these mechanisms. However, good agreement can g e n e r a l l y be obtained between d i f f e r e n t models once the differences in the choice of rate constants, photolysis rates and stolchlometrlc parameters used in lumped mechanisms have been identified and resolved. I t should be noted that the computational implementation of these chemical kinetic mechanisms can be optimized by means of repro-modellng techniques [13]. However, the r e l a t i v e importance of individual chemical reactions varies with

C. Seigneur

time [3] and this variation must be taken into account in the implementation of a mechanism. The simulation of photochemical smog chemistry by means of c h e m i c a l kinetic mechanisms is presently satisfactory. An example of the simulation of ozone concentrations in the Los Angeles basin at the Pasadena monitoring station is shown in Figure I . T h i s simulation was conducted for the two-day episode of 26-27 June 1974 with an a i r quality model that includes emissions, transport, dispersion and dry deposition of pollutants and treats photochemical smog chemistry with a carbon-bond mechanism consisting of 68 reactions among 31 species [14]. The use of a two-day simulation period is of particular importance to minimize the influence of the model i n i t i a l conditions on the simulation results. S i m i l a r i l y , the simulation domain must be selected to encompass a s u f f i c i e n t l y large area to minimize the effect of boundary conditions. The complexity of photochemical smog chemistry is reflected in the effect of changes in precursor (NOx and RHC) levels on secondary pollutants such as 03 and NO2. Reductions in NOx and RHC levels may lead to either increases or decreases in 03 levels depending on the r e l a t i v e levels of NOx and RHC. Figure 2 depicts the response of ambient 03 concentrations to the emission levels of NOx and RHC based on a i r quality model simulations [15]. The model used to develop this diagram simulates the chemistry of an a i r parcel along a wind t r a j e c t o r y using a carbon-bond mechanism. This diagram corresponds to conditions typical of the Los Angeles basin. I t appears that the 03 levels are very sensitive to the value of the r a t i o of NOx/RHC emission levels and that for a constant value of this r a t i o , the ambient 03 concentration w i l l be l i t t l e affected by the absolute value of the NOx and RHC emission levels. no obtain more precise estimates of the effect of specific emission control strategies, air quality simulation models with a three-dlmenslonal representation of the emissions, transport, dispersion and chemical

50

*,,.o S ~

40

~

Ol~e~ed

r-

30

A //

,o E 0

o

20~

10 m

0

r

0

6

12 June 26, 1974

18

24 T~ne (Houm)

30

36

42

48

June 27, 1974

Fig. 1. Simulation of the ozone concentration at the Pasadena monitoring s t a t i o n in the Los Angeles metropolitan area.

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119

Air P o l l u t i o n C h e m i s t r y : C.

Seigneur

l.qO

1.20

1.00 oq Z 0 w

ru x

0.80

o

%

0.60 < .-I uJ

o.~ 0.~

0.40

0.20

%

''-

0.00 0.00

0.15

0.30

0.45

0.60

0.75

0.90

1.05

1.20

1.35

!.50

RELATIVE ROG EMISSIONS Fig. 2. Simulation of the maximum l-hour average ozone concentrations (ppm) as function of reactive hydrocarbons (ROG) and nitrogen oxides (NOX) emission levels in the Los Angeles metropolitan area. Emission levels of l refer to 1987 levels. Reprinted with permission from Seigneur and Roth [15].

reactions can be used. For example, Roth et a l . [ 1 6 ] simulated photochemical smog formation in the Los Angeles basin with an urban a i r q u a l l t y model comprising a carbon-bond mechanism to simulate the chemistry. Reductions in NOX by 22% led to increases in peak 03 of 5% for a summer case and 3% f o r an autumn '~ase. Reductions in RHC by 15% led to reductions in peak 03 of 8% f o r the summer case and 9% f o r the autumn case. Although these results are specific to these model simulations, they i l l u s t r a t e the complex nonlinear relationship between photochemical oxidants and t h e i r precursors. Presently, the chemistry of paraffins, olefins and carbonyls is f a i r l y well established. Uncertainties remain, however, in the chemistry of aromatics and long-chain paraffins. Although simulation of the chemistry of mixtures of NOx and aromatics have reproduced measured concentrations successfully, the mechanistic d e t a i l s of the reactions s t i l l involve uncertainties. S i m i l a r l y , the r e l a t i v e yields of the oxidation reactions of long-chaln paraffins require to be established m o r e precisely. The chemistry of photochemical smog at low NOX and RHC concentrations typical of non-urban areas also deserves f u r t h e r investigation.

3.

ACIO RAIN

The chemistry of acid deposition can be p r i m a r i l y defined as the chemistry of sulfate and n i t r a t e formation since other acidic species such as hydrochloric and organic acids generally do not contribute in s i g n i f i c a n t amounts to t o t a l acid deposition. Dry and wet deposition of acidic species may lead to the a c i d i f i c a t i o n of lakes, streams and s o i l s .

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The o x i d a t i o n of s u l f u r d i o x i d e (S02) to s u l f u r i c acid (H2SO4)/sulfate and of NOx to nitric acid (HNO3)/nltrate occurs In both the gas phase and the aqueous phase. A detailed review of the aqueous chemistry of sulfate and n i t r a t e formation has been presented by Hoffmann and Calvert [17]. The gas-phase chemistry of these species was reviewed by Atkinson and Lloyd [6] and Kerr and Calvert [ 7 ] . Although a large number of chemical reactions are involved in the oxidation of SO2 and NO2 t o H2SO4 and HN03, r e s p e c t i v e l y , only a few major o x i d a t i o n pathways appear to contribute to the majority of atmospheric sulfate and n i t r a t e formation. The major chemical pathways leading to the formation of these acids are presented in fable Ill. lhe gas-phase oxidation of SO2 by OH radicals takes place p r i m a r i l y during daytime and is faster during summertime and in polluted atmospheres because of higher OH concentrations under these conditions. The aqueous oxidation of SO2 by H202 generally predominates in summertime when the H202 concentration reaches a few ppb. This reaction is very fast and i t s rate increases as the droplet becomes more acidic. In many cases, this reaction is oxidant limited and is, therefore, a source of n o n l i n e a r i t y between SO2 and sulfate levels. In wintertime, H202 levels are lower (e.g., of the order of O.l p p b ) and this reaction is, therefore, less important, lhe aqueous oxidation of SO2 by 03 takes place at a s i g n i f i c a n t rate only in droplets with pH above 4. Therefore, t h i s reaction occurs in clouds and fogs t h a t are not too much p o l l u t e d . I t is an important pathway f o r s u l f a t e formation in r a i n i n g clouds since the rainout of p o l l u t a n t s leads to higher pfl values that favor t h i s reaction. The reaction of SO2 with 02 catalyzed by t r a c e metals such as iron and manganese occurs at a f a s t e r rate when the ~H is high. Its kinetics is complex and synergism exists between the two

Air P o l l u t i o n C h e m i s t r y :

Table I I I . Major Chemical Pathways of Atmospheric Sulfate and Nitrate Formation

Gas phase SO2 + OH (+ H20 . 02)

H2SO4 ÷ HO2

NO2 + OH

-

HNO3 02

NO2 ÷ 03

NO3 ÷

NO3 + RH*

HNO3 + R

NO3 ÷ NO2

N205

N205 ÷ H20

2HNO3

Aqueous phase+ S(VI) + H20

S([V) ÷ H202 s(zv) + 03 1

S(IV) + - 02 2 NO3

,

S(VI)

•

S(VI)

,

NO3

+

02

Mn2+, Fe3÷

* RH represents hydrocarbons such as aldehydes, o l e f i n s , and aromatics (see Table I I ) + S(IV) represents dissolved SO2, S(VI) represents dissolved H2SO4. Several pathways exist to convert NO3 to HNO3 in the aqueous phase.

trace metals. The importance of t h i s reaction c l e a r l y depends on the concentrations of trace metals. I t is a potential major pathway f o r SO2 oxidation in urban fog. M o r e ambient measurements of trace metals are needed to assess the importance of t h i s reaction in

clouds. The gas-phase reaction of NO2 with OH takes place p r i m a r i l y during daytime while the gas-phase reactions that Involve the NO3 radical predominate at night as discussed in the previous section. I t should be noted, however, t h a t in clouds, where the photolytic a c t i v i t y is considerably reduced, the NO3 concentrations may reach appreciable levels and the NO3 pathway may then become important. I t should be noted that the r e l a t i v e importance of these pathways w i l l vary according to the ambient conditions, i . e . background p o l l u t i o n , l i g h t i n t e n s i t y , l i q u i d water content and temperature. Some other reactions may contribute s i g n i f i c a n t l y to acid formation under special conditions. For example, the aqueous reactions of SO2 with free radicals may be a nonnegligible pathway to sulfate formation in clean background environments. The chemistry of acid formation is closely related to photochemical smog chemistry since species such as 03,H202, OH and NO3 t h a t are key oxidants in the conversion of SO2 and NO2 to H2SO 4 and HN03, respectively, are products of photochemical smog. The simulation of sulfate and n i t r a t e chemistry must, therefore, involve photochemical smog gas-phase chemistry as a necessary component. In addition, i t ~st involve simulations of both gas-phase and aqueous-phase chemistries of SO2 and NO2 oxidation. An important component of such a system is the reversible transport of chemical species from the gas phase to the aqueous phase. Mass transfer consists of diffusion in the gas phase, transport through the gas/liquid interface according to thermodynamic equilibrium (Henry's law) and diffusion in the aqueous

C. Seigneur

phase. Schwartz [18] presented a comprehensive analysis of mass transfer in cloud systems. In general, the chemistry of cloud droplets is generally not limited by diffusion but solely by Henry's law except for large drops such as rain drops. T h e r e are some cases for which, in small drops ( i . e . , about lO pm) an aqueous reaction is limited by diffusion rather than by kinetics. For example, the oxidation of SO2 by 03 is limited by aqueous diffusion at high pH whereas the oxidation of SO2 by H202 is limited by aqueous diffusion at very low pH [18]. The aqueous chemistry of species that react very fast such as the OH, HO2 and NO3 radicals is limited by t h e i r diffusion from the gas phase to the droplet. Mathematical models of s u l f a t e / n i t r a t e chemistry must necessarily involve treatments of gas-phase chemistry, aqueous-phase chemistry and gas-phase/aqueous-phase mass transfer. Some recent models that include these components with various levels of detail include the models developed by Chameldes [19], Seigneur and Saxena [20, 21], Carmlchael et a i . [ 2 2 ] , Lurmann et a i . [ 2 3 ] , and Jacob [24]. An example of the simulation of acidic species formation is presented in Figure 3. This simulation represents the evolution of species concentrations during a fog event which includes fog condensation, stagnation and evaporation. The increase in sulfate and n i t r a t e concentrations due to the chemical reactions that oxidize SO2 and NO2 is evident. As the fog evaporates, the droplets become more concentrated in sulfate and n i t r a t e and the pH decreases quickly. The nonlinear nature of the chemistry of sulfate and n i t r a t e formation is p a r t i c u l a r l y apparent in the cases where aqueous chemistry predominates. Seigneur et a l . [ 2 5 ] simulated acid formation for conditions typical of the eastern U.S using the model of Seigneur and Saxena [20]. They showed that the gas-phase chemistry of sulfate and n i t r a t e formation is nearly linear whereas the cloud chemistry of sulfate and n i t r a t e formation shows strong nonlinearities. When these factors are combined with a budget of cloud cover; model simulations showed that a 50% reduction in SO2 levels would lead to reductions in sulfate ranging from 20 to 32% [26]. Although the major chemical processes that govern acid deposition and fog chemistry seem to be well established, there are s t i l l areas of uncertainties. Primarily, the role of the free radicals, OH, HO2 and NO3 needs to be determined m o r e accurately. The radicals OH and HO2 are involved in the aqueous oxidation of SO2 and the formation of H202. The NO3 radical plays a key role in the nighttime chemistry of n i t r a t e formation and i t s conversion to n i t r a t e in droplets must be elucidated. F i n a l l y , the kinetics of the aqueous oxidation of SO2 by 0 when catalyzed by trace metals such as Mn2+ and Fe~+ and the concentrations of these trace metals in droplets must be investigated further.

4.

ATMOSPHERIC AEROSOLS

Atmospheric aerosols consist of primary aerosols that are d i r e c t l y emitted into the atmosphere (e.g. from i n d u s t r i a l sources, automobile exhausts, f u g i t i v e dust, sea s a l t and pollen) and secondary aerosols that are formed in the atmosphere through chemical reactlons. Secondary aerosols may constitute a s i g n i f i c a n t fraction of the t o t a l aerosol m a s s . l y p i c a l l y , secondary aerosols range in size from nucleating particles of about O.OOl ~m to aerosols of about 2 pm. Primary aerosols are generally present in larger mass concentrations as coarse aerosols with sizes ranging from about l pm up to about lO0 ~m.

Secondary several

aerosols

ways.

contribute

Small

particles

to

air

can

be

pollution

in

inhaled

and

E N V I R O N M E N T A L SOFTWARE, 1 9 8 7 , Vol. 2, No. 3.

121

Air Pollution Chemistnj: C. Seigneur

I 4

A

==0.5

0.5~

1 0 0500

!

I

0600

0700

0800

0500

0600

Local Standard Time

0700

0 0800

Local Standard Time

30 50

~

20

/

REAC'IO.__ m

F

.202 REACTION

¢

~

~

OH REACTION

~2o INITIAL SULFATE

INITIAL NITRATE 10

0 0500

I 0600

I 0700

0800

Local Standard Time Fig. 3.

I 0700

0800

Simulation of acid fog f o m a t t o n in the Los Angeles m e t r o p o l i t a n area. Reprinted w i t h permission from Seigneur and Saxena. [ 2 0 ] .

The chemistry of aerosols is complex because i t t h r e e phases: condensible

involves

species

(2) The aerosol aqueous phase where chemical can take place.

are

reactions

(3) The aerosol s o l i d phase t h a t may deliquesce i n t o an aqueous phase and v i c e - v e r s a . The chemical composition of atmospheric aerosols is t h e r e f o r e governed by gas-phase chemistry, aqueous aerosol chemlca] k i n e t i c s , and thermodynamic e q u i l i b r i u m and mass t r a n s f e r between the ambient gas phase and the aerosol phase. Mathematical s i m u l a t i o n of the e v o l u t i o n of atmospheric aerosols is e s s e n t i a l t o i n t e g r a t e a l l these i n t e r a c t i n g physical and chemical processes in a consistent ii

122

I 0600

Local Standard Time

adversely a f f e c t the human pulmonary system. Aerosols can i m p a i r atmospheric v i s i b i l i t y because of t h e i r ability t o s c a t t e r l i g h t and, a l s o , in the case of carbon soot aerosols t o absorb l i g h t . Aerosols in the size range of O.l t o 1 pm are the most e f f i c i e n t a t scattering light. F i n a l l y , s u l f a t e and n i t r a t e aerosols c o n t r i b u t e t o acid d e p o s i t i o n . We focus here on secondary aerosols since they are the most c h e m i c a l l y reactive.

(1) The gas phase where c h e m i c a l l y formed.

0 0500

ENVIRONMENTAL SOF33/VARE, 1987, Vol. 2, No. 3.

framework. Secondary aerosols c o n s i s t p r i m a r i l y of sulfate, nitrate, ammonium, w a t e r , and organics. A summary of the chemical and physical processes t h a t govern the behavior of these aerosol specfes is presented in l a b l e IV. S u l f u r i c acid, t h a t can be formed in both the gas phase and the aqueous phase condenses I r r e v e r s i b l y on the aerosol because of i t s low vapor pressure. N i t r i c acid and condensible organic species are formed s o l e l y in the gas phase and may condense on or evaporate from the aerosol depending on t h e t r c o n c e n t r a t i o n , c o n c e n t r a t i o n s of o t h e r species, relative humidity, temperature and aerosol size. Ammonia condenses t o form s a l t s w i t h s u l f a t e and n i t r a t e and is d t s t r l b u t e d between the gas and aerosol phases. Mathematical models t h a t have been developed include: ( l ) Models of the HNO3 f o r m a t i o n reactions).

gas-phase chemistry of H2SO4 and (see Table I I I f o r these chemical

(2) Models of the aerosol chemistry of s u l f a t e formation ( e . g . the model of Saxena and Seigneur [ 2 7 ] ) . (3) Models of the thermodynamics of s u l f a t e / n i t r a t e / ammonium aerosols ( e . g . the models described by Saxena et a l . [ 2 8 ] ) . (4) Models of the gas-phase chemistry of organic aerosol formation that are in an early stage of development.

Air Pollution Chemistry: C. Seigneur ~, rln

F

Inl

1 i1=

nmrnr

i

ii

Table IV. Summaryof the Major Chemical and Physical Processes Affecting Aerosol Chemical Composition

Formation in the gas-phase

F o r m a t i o nin the aerosol

Sulfate

Yes

Yes

Yes

No

Nitrate

Yes

No

Yes

Yes

Ammonium

No

No

Yes

Yes

Organics

Yes

No

Yes

Yes

Species

Condensation Evaporation

These different components of aerosol chemistry can be integrated into a model of atmospheric aerosols and used to simulate the evolution of the aerosol chemical composition. An example of the prediction of an aerosol chemical composition as function of i t s size is presented in Figure 4. T h i s simulation represents the aerosol chemistry in a moderately polluted environment for a r e l a t i v e humidity of 60%. The model of Hudischewskyj and Seigneur [ 2 9 3 was used f o r this simulation. The largest aerosol concentration occurs in the size range between 0.2 and 0.5 ~m. Most of the secondary aerosol mass ( i . e . , sulfate, n i t r a t e and ammonium) is present between O.l and 1 wm.

As in the chemistry of photochemical smog and acid rain, nonlinearities are present in the chemistry of secondary aerosols. For example, Russell and Cass [34] simulated the formation of an~onium n i t r a t e in the Los Angeles basin along an a i r parcel trajectory. A reduction in NOx by 20% led to a reduction in maximum aerosol n i t r a t e by 40% and a reduction in RHC by 30% led to an increase in maximum aerosol n i t r a t e by 16%. These results emphasize the need for comprehensive and rigorous models to understand and analyze atmospheric aerosol chemistry.

Several areas of uncertainties s t i l l remain in the modeling of atmospheric aerosols. Although inorganic aerosols c o m p r i s e primarily salts of the sulfate/nitrate/ammonium system, other inorganic aerosol species are present such as NAN03, Ca(N03)2, and NH4CI. These species should be incorporated into aerosol models. The chemistry of organic aerosol formation is s t i l l only p a r t i a l l y understood, since i t has been investigated only for a few species [e.g. 30, 3l].

5.

The concern over the t o x i c i t y of airborne chemical species has greatly increased over the past few years. Several chemical species h a v e b e e n classified as suspected carcinogens or mutagens. The assessment of the risk to the human population exposed to these a i r toxics requires the simulation of the atmospheric behavior of these pollutants. Because of the large number of chemical species that are known or suspected to be toxic a i r pollutants, i t is not possible to provide an extensive review of t h e i r chemistry. I t may be noted that the chemical r e a c t i v i t y of toxic a i r pollutants varies greatly and that the requirement f o r computer simulation of t h e i r chemistry depends on the complexity of the chemistry of the specific a i r toxic compound of interest.

The thermodynamics of organic aerosol species is also of primary importance since i t determines the p a r t i t i o n of these condensible organics between the gas and aerosol phases [32]. Although some work is being conducted in this area [33], there is presently a lack of thermodynamic data f o r the development of rigorous organic aerosol models.

6

I

l

I

I

AIR TOXICS

I

I

I

I

0 lq

I o

I

H,O Ammonium

Ed/

W

Nitrate Sulfate

.,o

£'91

n

°f 0

Primary

-

0.01

0.1

Diameter

(~rn)

1.

10.

Fig. 4. Simulation of the size-distributed chemical composition of aerosols in a moderately polluted continental background.

ENVIRONMENTAL SOFTWARE, 1987, Vol. 2, No. 3.

123

Air Pollution Chemistry: C. Seigneur To i l l u s t r a t e the range of chemical behavior of a i r toxics, the chemistry of four a i r toxics is b r i e f l y discussed, lhese a i r toxics are chromium (Cr), benzo(a)pyrcne (naP), benzene (C6H6) and formaldehyde (HCHO). Chromium is emitted into the atmosphere as p a r t i c u l a t e matter. It exists in two stable oxidation states:Cr(lll) or Cr(VI). Whereas Cr(Vl) is a suspected carcinogenic compound, there is no clear evidence of carcinogenicity of Cr(lll) at this time . The chemical transformations of Cr(VI) to Cr(lll) and vice-versa are therefore central to the potential toxicity of chromium emissions. Simulations of chromium chemistry suggested that for t y p i c a l atmospheric conditions, Cr(VI) may be reduced to C r ( I I l ) at a s i g n i f i c a n t rate by vanadium (V2÷, V3+ and V02÷), iron ( F e 2 ÷ ) ~ HSO~ and arsenic, whereas the oxidation of C r ( I I I ) to Cr(VI) may occur at a s i g n i f i c a n t rate by reaction with MnO2 only i f C r ( l l l ) is emitted as a soluble s a l t and not as an insoluble oxide [35].

Benzo(a)pyrene is a p o l y c y c l i c aromatic hydrocarbon (PAN) which i s formed during incomplete combustion processes. Because of i t s low vapor pressure, naP is primarily present in the aerosol phase. Its carclnogenlcity is well recognized. Possible atmospheric chemical reactions of nap include n i t r a t i o n (by NO2 and HN03), ozonolysis (oxidation by 03) and photooxidation. For example under laboratory conditions nap exposed to 0.25 ppm of NO2 and traces of HNO3 was converted to 6-NU2-BaP with a y i e l d of 20% over an 8-hour period; half l i f e of naP exposed to 0.2 ppm of 03 was 35 min., half l i f e of naP exposed to simulated sunlight was 5.3 hours I l l . nap may therefore be r e l a t i v e l y reactive in the atmosphere although i t s atmospheric chemistry has not been simulated yet. Benzene is present as a gas in the ambient atmosphere. Its chemical r e a c t i v i t y consists primarily in i t s reaction with OH radicals. I t s reaction rate constant of 1.3 x lO 12 cm3 molecules - l s -l suggests a l i f e t i m e in the range of 15 (OH concentration of lO7 cm-3) to 150 hours (OH concentration of lO6 cm-3) depending on the level of oxidant p o l l u t i o n [36]. Benzene is therefore an example of a toxic chemical that offers a f a i r l y simple chemistry; however, i t is related to the complex photochemical smog chemistry through the OH radical.

Formaldehyde is considered to be a probable carcinogenic compound. I t is p r i m a r i l y considered as an indoor a i r pollutant. It is nevertheless a key component of photochemical smog chemistry where i t is produced as a r e s u l t of hydrocarbon o x i d a t i o n reactions and destroyed by photolysis and reactions with OH and HO2 (daytime) and NO3 ( n i g h t t i m e ) r a d i c a l s (see l a b l e 1). This compound is an example of a possibly t o x i c chemical t h a t is chemically very a c t i v e in the atmosphere.

b.

THE EFFECI OF AIMOSPHERIC PHYSICAL PROCESSES ON CHEMICAL REACIIONS

lhe simulation of a i r p o l l u t i o n chemistry generally considers the solution of chemical kinetic and thermodynamic equations. However, i t is eventually necessary to incorporate some treatment of physical processes that a f f e c t the evolution of the chemistry of the system. In the simulation of acid precipitation and fog chemistry, the microphyslcs of the droplet may be critical. So far, detailed models of chemistry have neglected cloud microphyslcs, i.e. the formation, interconversion, and evaporation/sublimation of cloud droplets, rain drops, ice crystals, snow and water vapor. Tremblay and Leighton [37] have presented an attempt at coupling a simple chemical k i n e t i c mechanism to a parameterized model of cloud microphysics.

124

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In the simulation of aerosol chemistry, the size of the aerosols can be a c r i t i c a l parameter f o r determining the aerosol chemical composition which depends f o r v o l a t i l e compounds on the aerosol size. P t l t n l s et a1.[38] and Hudtschewskyj and Seigneur [29] have developed mathematical models t h a t couple aerosol chemistry and physics. F i n a l l y , gas-phase reactions might be affected by the microscale turbulence of the atmosphere that creates f l u c t u a t i o n s in species concentrations. Georgopoulos and Seinfeld [39] have reviewed the theory of the i n t e r a c t i o n s between atmospheric turbulence and chemical reactions and developed a mathematical model that takes these i n t e r a c t i o n s i n t o account.

7.

NUMERICAL TECHNIQUES

The s o l u t i o n of mathematical models of chemistry requires to t r e a t

air

pollution

(1) chemical k i n e t i c equations expressed as ordinary d i f f e r e n t i a l equations (OOE)

nonlinear

(2) thermodynamic e q u i l i b r i u m equations nonlinear algebraic equations.

expressed

as

We present in t h t s section an overview of standard numerical techniques t h a t are used in a i r p o l l u t i o n chemistry models. We address successively chemical kinetic equations and thermodynamic equilibrium equations. Chemical k i n e t i c equctlon~ The system of ODE t h a t represents a chemical k i n e t i c mechanism may be expressed mathematically as f o l l o w s .

~t(t) = f t ( t ,

Yl . . . . .

YN)

Y t ( t o ) = Yto

t = 1.....

N (1)

where Yt is the concentration of species i , ~i is the rate of change of Yi with time t , f i is the rate expression t h a t describes the rate of production and consumption of species t through chemical reactions, t o is the i n i t i a l simulation time, Yio is the initial concentration of species t and N is the t o t a l number of species. Let the Jacobian matrix of t h t s ODE system be defined as [aft/ayj]. The ODE system is said to be s t i f f i f the r a t i o of the l a r g e s t elgenvalue of the Jacobian matrix to i t s smallest value is large. 6ear [40] has presented a d e t a i l e d analysis of s t i f f ODE systems. A b r i e f discussion is presented here since a i r p o l l u t i o n chemist£y systems are g e n e r a l l y s t i f f . The existence of a large value of the r a t i o of the largest to smallest eigenvalues of the Jacoblan matrix implies that some terms of the ODE system require small time steps f o r i n t e g r a t i o n whereas others could be integrated with l a r g e r time steps. Clearly, the use of small time steps is computationally expensive and e f f i c i e n t numerical procedures must be developed to i n t e g r a t e s t i f f OOE systems. lhe simplest approach to circumvent the s t i f f n e s s problem consists in assuming that the f a s t - r e a c t i n g species ( i . e . , those t h a t require small time steps) are at pseudo-steady s t a t e , l he ODE system may then be r e w r i t t e n as f o l l o w s . }i(t) = fl (t' Yl ..... YN ) 0 = fl(t,Yl ..... yN) Yi(to )= Yio

i=I ..... M l=M+l ..... N l=l ..... N

(2)

Air Pollution Chemistry:

where species of index greater than M are assumed to be

at steady state. The s t i f f ODE system has therefore been converted to a system of n o n s t i f f ODE and nonlinear algebraic equations. The ODE can then be integrated with standard numerical methods. It should be noted, however, that the exact solution of the nonlinear algebraic equations is not always trivial because of the intricate relationships between chemical species. In typical air pollution systems, the set of algebraic equations may require the solution of up to fifth order equations. If no steady-state approximation is involved, one must resort to numerical techniques that are suitable for s t i f f ODE systems. Among these techniques, two main categories have been applied to a i r p o l l u t i o n chemistry systems. category is based on the numerical integration scheme o r i g i n a l l y developed by Gear [41]. This scheme uses a predictor-corrector technique that u t i l i z e s the solution of the system at previous time steps. This type of technique is therefore referred to as multistep.Thls scheme is computationally e f f i c i e n t and improvements have been made to f u r t h e r improve i t s efficiency when applied to a i r p o l l u t i o n systems [42].

are generally computationally multistep techniques.

C. Seigneur

more efficient

than

Thermodynamic equilibrium equations The nonlinear algebraic equations of thermodynamic equilibrium reactions arise in both acid deposition chemistry and aerosol chemistry. In many cases, the solution of the algebraic equations representing thermodynamic equilibrium can be obtained a n a l y t i c a l l y except for the value of the H+ concentration ( i . e . , the pH). Then, a standard i t e r a t i o n technique such as the Newton-Raphson method [46] can be used to determine the pH value that leads to the e l e c t r o n e u t r a l i t y of the solution and, therefore, is the solution of the equilibrium equations. Such a technique is available from Saxena et an. [28].

The f i r s t

A drawback of Gear's scheme results from i t s multistep structure that makes i t d i f f i c u l t to handle systems with sharp gradients in parameters. Such sharp gradients lead to the selection of small time steps and, therefore, decrease the computational efficiency. The second category is based on numerical techniques that do not use the solution of the system at previous time steps. These techniques are consequently referred to as s e l f - s t a r t i n g . We provide here three examples of such standard techniques used in a i r p o l l u t i o n chemistry modeling. The scheme of Young and Boris [43] is based on a predictor-corrector scheme. This numerical scheme

breaks down the ODE system into two sets of nonstiff and s t i f f ODE. The same predtctor-corrector method is used for both sets of equations. However, an asymptoti F formula is used for the s t i f f equations in the corrector step whereas a standard second-order corrector equation is used f o r the nonstiff equations. The numerical scheme of Lamb [44J may be described as follows. Over a given time step, the solution of the k i n e t i c equations is obtained by assuming that the concentrations of other species are constant. The solution is, then, obtained a n a l y t i c a l l y and is expressed as a series of exponential terms. The size of the time step is then determined so that the assumption of constant concentrations is v e r i f i e d within a prescribed error c r i t e r i o n . Clearly, i f concentrations vary rapidly, small time steps are selected whereas small changes in concentrations lead to the choice of large time steps. The method is optimized by selecting d i f f e r e n t e r r o r c r i t e r i a f o r d i f f e r e n t chemical species. Hesstvedt et an. [45] developed a numerical scheme that is a combination of the pseudo-steady state approximation and of the exponential solution of Lamb [44]. Over a given time step, species that have lifetimes less than no percent of the time step are assumed to be at steady-state whereas species that have lifetimes larger than no0 times the time step have t h e i r concentrations calculated with a f i n i t e difference technique. The concentrations of other species are obtained by using the exponential solution of t h e i r k i n e t i c equations over the time step. All these numerical schemes have been evaluated against the reference scheme of Gear [41] and, assuming the proper selection of error c r i t e r i a and time steps, have been shown to provide solutions t h a t can agree with the reference solution within about l percent. Clearly, numerical schemes that involve pseudo-steady state approximations and do not involve multi-step procedures

In the c a s e where a large number of nonlinear thermodynamic e q u i l i b r i a are included in the model, the i t e r a t i o n procedure becomes more complex since i t involves more than one chemical species. Then, the numerical method of Morel and Morgan [47] is recommended. This method also uses a Newton-Raphson technique for the i t e r a t i o n steps. Other numerical techniques can be used to solve complex nonlinear algebraic systems. The family of techniques referred to as steepest~descent methods can be e f f i c i e n t to reach the neighborhood of the solution point rapidly. Then, a standard Newton-Raphson technique can be used to determine the exact solution. For example, Bassett and Seinfeld [48] used such an hybrid approach to calculate the chemical composition of size distributed secondary aerosols.

8.

CONCLUSION

The computer simulation of a i r pollution chemistry is now a well-established area of environmental and atmospheric sciences. Numerical techniques exist that can provide accurate and cost-effectlve solutions of the chemical k i n e t i c , thermodynamic and mass transfer equations that govern the dynamics of atmospheric chemical systems. The chemistry of major forms of a i r p o l l u t i o n is presently f a i r l y well understood and the evolution of chemical species concentrations in polluted atmospheres can be predicted by computerized mathematical models reasonably w e l l . Some uncertainties nevertheless remain.

In the simulation of photochemical smog, the chemistry of aromatics, long-chain paraffins and natural hydrocarbons s t i l l presents some uncertainties and the chemistry of photochemical smog formation at low concentrations of NOx and RHC typical of regional-scale ( i . e . non-urban) conditions require further testing. In the simulation of acid rain and fog, the aqueous oxidation of SO2 by 02 catalyzed by trace metals, the scavenging of radicals by droplets, the aqueous chemistry of OH and HO2 radicals, and the role of the NO3 radical in the formation of HNO3 deserve further investigation. Interactions of cloud and fog microphysics with chemical processes also require to be investigated. lhe chemistry of secondary sulfate and n i t r a t e aerosols is f a i r l y well simulated. However, a large e f f o r t is required to elucidate the d e t a i l s of organic aerosol formation, lhe amount of l i q u i d water available on aerosols predicted by models needs to be evaluated with ambient data. Simulation of a i r toxic chemistry is specific to the toxic chemical species considered; those range from trace metals in the aerosol phase such as chromium to organic aerosols such as benzo(a)pyrene and organic

E N V I R O N M E N T A L SOFTWARE, 1987, Vol. 2, No. 3.

125

Air Pollution Chemistry: C. Seigneur gases such as benzene or formaldehyde. The chemistry of a i r toxics may be closely related to other aspects of a i r pollution chemistry such as photochemical smog or aerosol chemistry.

[10] Lurmann, F.W., Lloyd A.C. and Atktnson, R. A Chemical Mechanism for Use in Long-Range Transport/Acld Deposition Computer Modeling, 3. Geophys. Res., 1986, 91, 10,905-10,936.

The different aspects of a i r pollution chemistry that have been presented here demonstrate the complexity of these chemical systems. The complex nonlinear relationships that exist between the secondary atmospheric pollutants and t h e i r emitted precursors require the use of reliable computerized mathematical models. Such models are presently available and some of these models are routinely applied to assess a i r pollution impacts and develop strategies for the control of a i r pollutant emissions. Nevertheless, continuous development and refinement of models of a i r pollution chemistry must take place to further improve t h e i r predictive accuracy.

[11] Whitten, G.Z., Hogo, H. and Killus, 3.P. The Carbon Bond Mechanism: A Condensed Kinetic Mechanism for Photochemical Smog, Environ., Sct. Technol., 1980, 14, 690-700.

ACKNOWLEDGEMENTS The author is grateful to F. W. Lurmann, Environmental Research and Technology, Inc. and P. Saxena, Pacific Gas and Electric Company, for providing valuable comments on a draft of this manuscript.

REFERENCES [I]

Finlayson-Pitts, B. and Pltts, J.N., Jr. Atmospheric Chemistry, John Wiley and Sons, New York, 1986.

[2]

Setnfeld, O . H . Atmospheric Chemistry and Physics of Air Pollution, John Wiley and Sons, New York, 1986.

[3]

Gautier, 0., Carr, R. W. and Seigneur, C. Variational Sensitivity Analysis of a Photochemical Smog Mechanism, Int. J. Chem. Kinet., 1985, 17, 1397-1369.

[4]

Whitten, G.Z. The Chemistry of Smog Formation: A Review of Current Knowledge, Environ. I n t . , 1983, 9, 447-463.

[5]

Kerr, J.A. and Calvert, J.G. (1984) Chemical Transformations Modules f o r Eulerian Acid Deposition Models I: The Gas-Phase Chemistry, U.S. Environmental Protection Agency, Research Traiange Park, North Carolina.

[6]

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Atktnson, R. and Lloyd, A . C . Evaluation of Kinetic and Mechanistic Data for Modeling of Photochemical Smog, J. Phys. Chem. Ref. Data, 1984, 13, 315-444. Carter, W.P.L., Lloyd, A.C., Sprung, J.L. and Pitts, J.N., Jr. Computer Modeling of Smog Chamber Data: Progress in Validation of a Detailed Mechanism for the Photooxtdatton of Propene and n-Butane in Photochemical Smog., Int. J. Chem. Klnet., 1979, 11, 45-102.

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