Pergamon - Paris-Est Créteil Val-de-Marne Universityaumont/contenu/recherche/paper/1996... · 2006. 3. 28. · representation of the organic peroxy chemistry for NO concentration

Pergamon Atmospheric Environment Vol. 30, No. 12, pp. 2061-2077, 1996 Copyright © 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 1352-2310/96 $15.00 + 0.00

1352-2310(95) 00279-0

TESTS OF SOME R E D U C T I O N HYPOTHESES MADE IN P H O T O C H E M I C A L MECHANISMS

B. AUMONT,* A. JAECKER-VOIROL,i" B. MARTINt and G. TOUPANCE* * LISA-Universit6 Paris VII-Paris XII, URA CNRS 1404, Centre multidisciplinaire de Cr~tefl, 61 Avenue du G6n6ral De Gaulle, 94010 Cr~teil Cedex, France; i" Institut Fran~ais du P6trole, Techniques d'applica-

tion 6nerg6tiques, 1&4 avenue de Bois Pr6au, BP311, 92506 Rueil Maimaison Cedex, France

(First received 20 November 1993 and in final form 29 May 1995)

Abstraet--The purpose of this paper is to evaluate the errors induced by different hypotheses used to elaborate reduced kinetic mechanisms of tropospheric chemistry. To do that, a reference chemical kinetic scheme was developed: it includes a limited number of representative primary species (6 alkanes, 4 alkenes, 2 aromatics) for which up to date kinetic constants and mechanisms were used. This chemical scheme constitutes a reference against which the various reduction methods were tested. The tests were performed for three different scenarios characterised by various VOC/NO x ratios (5,10, 20). These scenarios are assumed to correspond to the chemical state of the atmosphere in urban areas and in rural situations. In a first step, the high NO x approximation (i.e. a chemical scheme without treatment of peroxy + HO= and peroxy + peroxy reactions) was tested. The results show that this scheme does not produce significant error on the simulated concentrations for NO concentrations above 2 ppb. In a second step, three successive reduction methods were applied to the reference mechanism: (1) use of the chemical operator concept to treat the organic peroxy chemistry, (2) loss of information on the organic peroxy class, (3) lumping of secondary organic species into surrogate species. The use of chemical operators provides a satisfactory representation of the organic peroxy chemistry for NO concentration down to 100 ppt. The scheme obtained after the loss of information on the organic peroxy class increases only slightly the errors compared to the reference scheme. The lumping of secondary VOCs into surrogate species does not generate significant errors on most of the key species (Oa, NOx, OH, etc.) but induces a significant overestimation of PAN. At this step, the final reduced scheme involves 71 species and 150 reactions. These reduction methods seem to be relevant for most of the diurnal situations encountered in the lower continental troposphere. Copyright © 1996 Elsevier Science Ltd

Key word index: Air quality simulation models, reduced chemical mechanisms, tropospheric chemistry, air pollution, photochemical mechanisms.

INTRODUCTION

It is now well known that the continental atmosphere is concerned at various spatial scales by the photochemical pollution. The problem is especially harmful in urban and surrounding areas where most of the anthropic emission occur. A rapid increase of the ozone concentration in the background troposphere has also been observed from the beginning of the 19th century (~ 10 ppb) to the end of the 20th (~40 ppb) (Voltz and Kley, 1988; Marenco et al., 1994).

Chemical kinetic schemes involving 100 to 150 reactions, assumed to be representative of the thou- sands of reactions involved in atmospheric chemistry, have been developed to approximate the chemistry of the continental troposphere (Carter et al., 1986; Lurmann et al., 1986; Gery et ai., 1988; Stockwell et al., 1990; Carter, 1990). However, these kinetic mechanisms, because they are based on approximations, introduce deviations from actual evolutions. These reduction procedures have to take into account

three key factors: the large number of organic compounds (from biogenic hydrocarbons to anthropic hydrocarbons); the large range of concentrations encountered within the model domain (several orders of magnitude); the large range of VOCfNOx ratios (from values where the ozone chemistry is limited by volatile organic compounds (VOC) to values where it is limited by NOA.

Two extreme cases can first be considered. The first case corresponds to areas of intense anthropic emissions: the ratio VOC/NOx is low and the peroxy + peroxy radical reactions can be neglected, leading

to simple chemical schemes. Nevertheless, the number of organic compounds is so large that most of them must be lumped into model or surrogate species. Models developed for these situations are adapted to urban areas but are not suitable for rural areas. The second case corresponds to the general troposphere: the NOx concentration is low and the peroxy + peroxy radical reactions must be taken into account. However, in these environments, the number of

2061

2062 B. A U M O N T et al.

organic compounds is small and quasi-explicit chemical schemes can be used. Obviously, there is a large gap between these two extreme cases corresponding to the lower continental atmosphere. This is the most difficult situation to simulate because no easy chemical simplification can be made: many organics have significant concentrations, and the high VOC/NOx ratios imply a complete treatment of the peroxy chemistry. In this case, an intense lumping is neces- sary to make the model size reasonable. A large effort has been made in the past ten years to develop models matching these mesoscale or regional scale situations. The aim of these models is to represent with the same accuracy the chemistry of metropolitan areas and the chemistry of remote rural areas, and also the oceanic atmosphere downwind of the continent.

The important question to assess is the limits of validity of such condensed schemes with respect to these drastic chemical conditions. The aim of this paper is not to elaborate a new chemical scheme but to identify and justify the errors introduced by the different reduction steps in order to help the interpretation of mesoscale and regional scales model simulations.

Some specific constraints have been considered when choosing the reduction approaches to test:

• The evaluation of fuel reformulation and the impact of biogenic emissions, in fact the definition of reactivity scales requires the differentiation of the effect due to each hydrocarbon (HC) on the evolution of the photooxidant properties of the atmosphere. Thus, procedures involving the lumping of primary HCs or the carbon bond approach have not been considered here.

• As mentioned above, the chemical conditions evolve rapidly at mesoscale or regional scales. Thus, the scheme must allow to simulate correctly the chemical processes for a large range of NO~ concentrations. This is why special attention was given to methods allowing a good representation of the RO2 radical chemistry.

METHODOLOGY

A reference mechanism for a limited number of representative primary species, with recent kinetic and mechanistic data has first been developed. This mechanism is not an operational mechanism but only a reference scheme to test the impact of different reduction methods. The first tested reduction method was the hioh NOx approximation. Then three successive reduction levels were applied to the reference mechanism: peroxy radicals lumping using the chemical operator concept, loss of information on the alkyl peroxy radical class, lumping of secondary VOC with some surrogate species. The relevance of each reduction step is checked against the reference mechanism.

ELABORATION OF THE REFERENCE MECHANISM

The reference chemical scheme describes the degradation of 12 primary HCs selected to represent the main chemical processes encountered in the lower continental troposphere. The whole reference mechanism treats 174 species and 657 reactions. The detailed description of this mechanism is beyond the scope of the present paper and has been detailed by Aumont (1994). Only the main characteristics of this reference mechanism will be described here. This mechanism will be referenced as MOCA-ref in the remainder of this paper. The primary degradation mechanism used in MOCA-ref for n-butane is given in Table 1 for illustrative purposes.

Choice of the representative primary species

Alkanes. The various reactive pathways of alkanes can be represented by treating only C < 6 alkanes. Using longer alkanes does not provide supplementary tests of new reaction pathways and strongly increases the size of the kinetic scheme. Thus methane, ethane, propane, n-butane, /-butane and n-pentane were selected as the representative species describing the al- kane chemistry.

Alkenes. The same approach has been used to select representative alkenes. Ethene, propene, 2-butene and 2-methyl-2-butene have been retained. These species allow a representation of various types of products as a function of the primary alkene carbon number and present different substitution levels at the double bond carbons. Moreover, ethene, propene, and 2-butene are also the most emitted compounds of this family.

Aromatics. For these compounds, the problem is quite different since the degradation mechanisms are not really well known. The most documented species are toluene and xylenes which are also among the most emitted by anthropic sources (Middleton et al., 1990; Derwent and Jenkin, 1991). Thus, only toluene and m-xylene were considered in this mechanism.

Inorganic chemistry

As commonly done, an explicit formulation of the inorganic chemistry, involving the NOffOJHOx system, has been used, because of the relatively small number of species and reactions. The inorganic scheme involves 42 reactions and 11 species. The photolysis rates are those derived from Hough (1986). The rate constants used in MOCA-ref result from the IUPAC digest (Atkinson et al., 1992).

Orffanic chemistry

In order to ease the description, various mechanisms used to represent the VOC degradation are considered from a general point of view (initiation of the VOC degradation, peroxy reactions, alkoxy reactions). The more particular reactive pathways (aromatic chemistry and alkene + 03 reactions) will be treated later. At least, the validity limits of the reference mechanism will be briefly stressed.

Tab

le

1. P

rim

ary

degr

adat

ion

mee

bani

sm

used

in

M

OC

A-r

ef

(see

tex

t) f

or

n-bu

tane

. R

ate

cons

tant

s k

are

in t

he

form

: k

= A

(T)”

ex

p(

- E

/T)

(uni

t sy

stem

: c

m, m

olec

ule,

s)

Rea

ctio

n A

n

E

Not

e

OH

+

NC

‘H,,

-0.1

48

CsH

,CH

,Ot

+ 0.

852C

H3C

H (0

,) C

2Hs

( -

03

CH

3CH

(02)

C3H

5 + N

Oqa

N02

+

dJH

sCH

(0)

CIH

, +

WH

sCH

(O

NO

J C

zHs+

. C

&O

, +

CH

sCH

(0

s)

CIH

, *0

.75H

CH

O

+ 0.

2SC

HsO

H

+ 0.

5H02

+

0.5

CH

,CH

(0

) &

HI

+ O

XC

HsC

(0

) C

zHS

+ 0.

25C

HJC

H(O

H)C

zH,’

( +

HO

* +

CH

,CH

(O,)

CsH

s 3

CH

sCH

(O

OH

) C

sHs’

(

+ 01

) C

H,C

H

(0,)

C

,H,

+ R

,Ol

~O.@

ZH

&H

(O)

C2H

S +

0.2

CH

,C(O

) C

2H,

+ O

.ZC

H,C

H

(OH

) C

,Hs#

(

+ 0,

) C

HsC

H(O

&H

s +

CH

sCH

(O&

Hs

=, L

ZC

HsC

H

(0)C

2H,

+ O

.CH

sC(O

) C

,H,

+ 0.

4CH

sCH

(OH

)C2H

S’

( +

02)

CH

sCH

(O&

,Hs

+ R

sO,

=-0.

6CH

&H

(0)C

2Hs

+ 0.

2CH

sqO

)GH

s +

0.2C

H,C

H(O

H)

&H

I+

( $0

2)

CH

sCH

(OJC

2HS

+ R

sOZ

*0.

8CH

sCH

(0)C

zHs

+ O

.ZC

Hsq

OG

H~

( +

0,)

CH

sCH

(02)

C2H

, +

RC

Os

*O.~

CH

~CH

(O)C

,HI

+ O

.ZC

H,q

O)C

>HS

( +

02)

CH

sCH

(O&

H,

+ O

2 *

HO

2 +

CH

,qO

) C

IH,

CH

,CH

(0)C

2H,

=-‘X

&H

O

+ C

2HsO

2 (

- 0,

) C

3H7C

H20

2 +

NO

*

aNO

+

aCsH

,CH

20

+ bC

sH&

H20

N02

’ C

Hs0

2 +

C3H

7CH

202

*0.7

5HC

HO

+

0.25

CH

SOH

+

0.5H

02

+ O

SCsH

,CH

,O

+ 0.

25 C

SH,C

HO

+

0.25

C,H

,CH

,OH

+ (

+ 0,

) H

O2

+ C

sH,C

Hs0

3 ==

- CsH

,CH

zOO

HL

(

+ 0,

) C

,H,C

H20

2 +

RIO

l*0.

6CsH

,CH

20

+ O

JC,H

,CH

O

+ 0.

2CSH

,CH

20H

+ (

+ 0,

) C

sH&

HZ

OI

+ C

sH,C

H20

1 =

l.2C

sH,C

H20

+

OA

CaH

,CH

O +

0.2

CsH

,CH

sOH

+ (

+ 03

C

sH,C

H20

t +

R20

2 eO

.6

C,H

,CH

,O

+0.2

C

sH,C

HO

+

0.2C

sH,C

HIO

H’

( +

02)

C,H

,CH

20,

+ R

sO,

-0.8

C

sH,C

H20

+

0.2

CsH

,CH

O

( +

0,)

CsH

,CH

202

+ R

CO

s =

0.8

C,H

,CH

20

+ 0.

2 C

sH,C

HO

(

+ 0,

) C

,HX

H>O

+

0,

*CIH

,CH

O

+ H

O,

021

C;H

;CH

;O

=bi2

(O

$CH

2CH

2CH

;02

( -

0,)

CH

2(O

H)C

H2C

HsC

HI0

1 +

NO

*C

Hs(

OH

)CH

2CH

2CH

0 +

NO

2 +

HO

I (

- 0,

) C

H,0

2 +

CH

,(OH

)CH

,CH

,CH

,O,

=z-0

.75H

CH

O

+ 0.

25C

HsO

H

+ H

O1

+ 0.

75C

H2(

OH

)CH

2CH

,CH

0 +

0.25

CH

~(O

H)C

H2C

HzC

HZ

OH

+ (

+ 0,

) H

Os

+ C

Hs(

OH

)CH

2CH

2CH

202

= C

H2(

OH

)CH

&H

2CH

200H

+ (

+ 0,

) C

Hs(

OH

jCH

2CH

2CH

202

+ R

,Ol

=90.

6H02

+

0.8C

H2(

0H)C

H2C

H2C

H0

f 0.

2CH

z(O

H)C

H2C

HtC

HZ

OH

+ (

+ 0,

) C

Hs(

OH

)CH

2CH

2CH

202

+ C

H2(

0H)C

HIC

H2C

H20

2 q

1.2

HO

2 +

l.6C

HZ

(OH

)CH

2CH

aCH

0 +

0.4C

HI(

OH

)CH

2CH

ZC

H20

H1

( +

0,)

CH

s(O

H)C

H2C

H,C

H20

, +

R,O

, *

0.6H

02

+ 0.

8CH

2(0H

)CH

2CH

2CH

0 +

0.2

CH

2(0H

)CH

sCH

2CH

ZO

H#

( +

0,)

CH

s(O

H)c

HsC

HsC

H,O

2 +

RsO

s *

CH

2(O

H)c

H2C

H#H

O

+ O

.8H

O2

( +

02)

CH

s(O

H)C

H2C

H2C

H20

2 +

RC

Os

=CH

t(O

H)C

H2C

H2C

H0

+ 0.

8HO

z (

+ 0,

) C

2HsO

s +

NO

qr

CH

,CH

O

+ H

O2

+ N

O2

( -

01)

CIH

S02

+ C

H,O

s 12

0.75

H

CH

O

+ O

.ZSC

H,O

H

+ 0.

75C

H,C

HO

+

HO

2 +

0.25

C

2HsO

H

( +

0,)

CsH

sO2

+ H

O2

=CH

&H

&O

H

( +

0,)

C2H

T02

+

C,H

,02

* 1.

2 H

O2

+ 0.

4 C

2HrO

H

+ 1.

6CH

&H

O

( +

02)

C2H

b02

+ R

IO2

*0.6

HO

z +

O.Z

C,H

,OH

+

0.8

CH

,CH

O

( +

0,)

C2H

502

+ R

202

s0.6

H02

+

0.2C

zH50

H +

0.8

CH

aCH

O (

+ 0

,)

C2H

s02

+ R

sO,

*0.8

H02

+

CH

$HO

(

+ 0,

) C

2H10

z +

RC

OJ

=-0.

8H02

+ C

H&

HO

( +

0,)

1SlE

-

17

4.2E

-

12

3.8E

-

14

4.1E

-

13

2.O

E -

14

5.

OE

-

16

l.OE

-

15

2.9E

-

16

2.5E

-

13

l.SE

-

14

2.7E

l4

4.2E

-

12

3.8E

-

13

4.1E

-

13

l.OE

-

13

5.O

E -

14

2,

OE

-

14

2.9E

-

15

2.5E

-

12

3.7E

-

14

1.3E

11

4.2E

-

12

3.8E

-

13

4,lE

-

13

l.OE

-

13

5.O

E -

14

2.

OE

-

14

29E

-

15

2.5E

-

12

4.2E

-

12

3.8E

-

13

4.1E

-

13

5,O

E -

14

l.O

E -

13

2.

OE

-

14

2.9E

-

15

2.5E

-

12

2 0 0 0 0 0 0 0 0 0 : 0 0 0 0 0 0 0 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

-190

-1

80 0

-790

0 0 0 0 0 26

0 74

OQ

-1

80 0

-790

0 0 0 0 0 46

0 41

30

-180

0 -7

90 0 : 0 0

- 18

0 0 -

790 0 0 0 0 0

(4 (

0 0)

(4

0 0

) (4

0 0

1 (4

0 0

) id

) (0

0)

+ ne

deg

rada

tion

mec

hani

sm

of t

his

spec

ies

is n

ot

trea

ted

in M

OC

A-r

ef.

(a)

Rat

e co

nsta

nt

from

A

tkin

son

(199

0).

(b)

Stoi

chio

met

ric

coet

licie

at

estim

ated

fr

om

Akt

inso

n (1

987)

. (c

) St

oich

iom

etri

c co

effi

cien

t II

and

b es

timat

ed

from

Car

ter

and

Atk

inso

n (1

989)

. T

empe

ratu

re

and

pres

sure

de

pend

ant.

(d)

Rat

e co

nsta

nt

and

stoi

chio

met

ric

coe&

ient

s es

timat

ed

usin

g M

adro

nich

an

d C

alve

rt

(199

0) m

etho

dolo

gy.

See

also

te

xt a

nd

Tab

le

2. (

e) S

ee t

ext

and

Tab

le

2. (

f)

RIO

Z,

R20

2,

RaO

Z

and

RC

O,

repr

esen

t, re

spec

tivel

y,

the

tota

l co

ncen

trat

ion

OS p

rim

ary,

se

cond

ary

and

tert

iary

pe

roxy

alky

l ra

dica

l an

d ac

ylpe

roxy

ra

dica

ls.

(g)

Nitr

ate

form

atio

n ne

&ct

ed

(see

tex

t).

(h)

The

al

koxy

ra

dica

l fo

rmed

fo

llow

ing

this

re

actio

n is

sup

pose

d to

und

ergo

on

ly

1,5-

H

shif

t is

omer

izat

ion,

yi

eldi

ng

fina

lly

HO

2 an

d a

carb

onyl

sp

ecie

s.

(i)

CIH

IO

form

ed

follo

win

g th

is r

eact

ion

is s

uppo

sed

to u

nder

go

only

re

actio

n w

ith 0

, to

giv

e H

O2

and

CH

XH

O.

(j)

CH

30

form

ed

follo

win

g th

is r

eact

ion

is s

uppo

sed

to u

nder

go

only

re

actio

n w

ith O

z to

giv

e H

O1

and

HC

HO

. (k

) D

= 0

.916

, b

= 0

.084

at

T =

300

K a

nd

atm

osph

eric

pr

essu

re.

(I)

n =

0.96

7, b

= 0

.033

at

T =

300

K a

nd

atm

osph

eric

pr

essu

re.

(m)

Bas

ed

on t

he

stru

ctur

e ac

tivity

re

latio

nshi

ps

give

n by

Atk

inso

n

2064 B. AUMONT et al.

Table 2. Rate constants k associated to the peroxy radicals reaction used in the reference mechanism

Reaction k (cm 3 molec- 1 S 1) References (see also text)

RO2 + NO 4.2 x 10-12 exp(180/T) b RO2 + HO2 4.1 x 10-13 exp(790/T) c CH302 + CH302 3.7x 10 -13 C R102 + R102 a 1.0× 10 -13 b R202 + R202 a 1,0X 10 -t5 b R302 + R302 a 2.0x 10 -1~ b RCO3 + NO 2.0 x 10- I I C RCO3 + HOE 4.3 × 10-1~ exp(1040/T) c RCO3 + NO2 See text RCO3 + RCO3 1.6 × 10-1 l b

Note. b: Atkinson (1990). c: Lightfoot et al. (1992). aRiO2, R202, R303 are respectively primary, secondary and

oxyalkyl radicals. tertiary per-

Initiation of the VOC degradation VOC + OH reactions. When available, rate

constants provided by Atkinson (1989) for the VOC + OH reactions have been selected. For missing data, the rate constants were estimated using structure activity relationships provided by Atkinson (1987). The same relationships were also used to com- pute the probabilities for OH attack on the various carbon sites of aikanes and secondary VOC. For alkenes, only OH addition on the C = C bond was considered. When two distinct OH additions are pos- sible, a 65% addition yield on the terminal carbon for propene (Atkinson, 1990) was used, and for 2-methyl- 2-butene this reaction was assumed to form only the most stable alkyl (100% addition on the terminal carbon).

VOC + hv reaction. Most of the photolysis rates are those derived from Hough (1986). The photolysis frequencies not studied in his paper were approxim- ated by considering those of the chemical compounds which have the closest molecular structure.

Peroxy reactions. Peroxy radicals are the main species which are formed after the initiation of the VOC degradation. Five removal processes were considered:

(a) X O 2 + NO --, XO + NO2, (a') XO2 + NO ~ XONO2, (b) XO2 + NO2 "-~ XO2NO2, (C) XO2 + HO2 --~ XOOH + 02, (d) XO2 + RO2 --~ products.

A distinction is performed between peroxyalkyl radicals (RO2) and peroxyacyl radicals (RCO3) due to their difference in rate constants. The selected rate constants of all these reactions are given in Table 2.

Peroxy + NO reactions. For RO2 radicals, the rate constant recommended by Atkinson (1990) was used. The branching ratio between (a) and (a') depends on the substitution level of the alkyl chain. For the non- substituted chains, the branching ratio was estimated from structure activity relationships (Carter and At- kinson, 1989b). According to Carter (1990), the (a') pathway can be neglected for those peroxy radicals

having (i) a ~C(OH)-C(O2)~ structure mainly resulting from alkene + OH reactions, (ii) a ~C(OH) C2-C(O2)~ structure mainly resulting from the 1,5-

H shift isomerization on alkoxy radicals. The rate constant proposed by Lightfoot et al.

(1992) for the CH3C(O)O2 + NO reaction has been systematically used for all RCO3.

Peroxy + NO2 reaction. These reactions were neglected for all the RO2, due to the rapid thermal decomposition of peroxyalkylnitrate at ambient temperature: RO2NOz ~ RO2 + NO2. As concerns the CH3C(O)O 2 4- N O 2~ P A N reactions, the IUPAC recommendations (Atkinson et al., 1992) are retained. These reaction rates were also used for all the double carbon RCO3 reactions. For longer chains, the high pressure kinetic rate of the CH3COO2 4- NO2 reaction was assumed relevant (k = k~ = 1.2x 10 - z l (T/300)-°9). For RC(O) O2NO2 (C 1> 3) thermal decomposition, the kinetic rate proposed by Atkinson (1990) was preferred.

Peroxy + HO2 reactions. The kinetic rate of the CH302 + HO2 reaction reported in the review of Lightfoot et al. (1992) was used for all RO2 + HO2 reactions. When comparing with the other measured rate constants reported in this review, it appears that the kinetic rates of RO2 + HO2 reactions are prob- ably underestimated or at least represent lower limits. The formation of ozone and carboxylic acids following the RCO3 + HO2 reactions were included in this scheme. Yields and kinetic rates for the CH3CO3 + HO2 reaction have been extended to all RCO3 species (Lightfoot et al., 1992), leading to a generic equation: RCO3 + HO2 --, 0.303 + 0.3 RC(O)OH + 0.7 RC(O)OOH + 0.7 02.

Peroxy + peroxy reactions. These reactions are represented in this scheme according to the method proposed by Madronich and Calvert (1990). This method is based on the assumption that the kinetic rates and the branching ratios only depend on the peroxy radical class. Following the Madronich and Calvert methodology, five classes of organic peroxy radicals are distinguished in MOCA-ref: primary,

Tests of some reduction hypotheses 2065

secondary and tertiary peroxyalkyl radicals, peroxyacyl radicals, and an explicit treatment for CH302. The kinetic rates selected for the "self-reactions" (reaction between two peroxy radicals in the same class) are given in Table 2. The branching ratios needed to evaluate the reaction products results from the work of Madronich and Calvert. The rate constants for the "cross reactions" (reaction between two peroxy radicals in different class) are estimated as a geometric average of the self-reaction rate constants. For the estimation of the branching ratios, an arithmetic average is used. Within the assumption stated above, a simplified "counter" scheme can be performed, which is mathematically equivalent to the explicit permutations, both in rates of reaction and in the identity of products (Madronich and Calvert, 1990).

Alkoxy reactions. Three removal processes were taken into account for alkoxy radicals: reaction with 02, unimolecular decomposition and 1,5-H shift isomerization. These reactions are represented in this scheme following the recommendation given by At- kinson (1990) and Atkinson and Carter (1991). For the non-substituted alkyl chain, rate constants were estimated using structure activity relationships (Atkin- son, 1990). For alkoxy radicals having the ~C(OH)- C(O')~ and - C ( =O)--C(O')~ structures, unimolecular decomposition was systematically supposed to be the major pathway (Atkinson and Carter, 1991), except for CH2(OH)CH20" and CH3C(O)CH20" for which the reaction with 02 was also considered. The rate constants used for these two radicals are those of Atkinson (1990).

Aromatic rings fragmentation. The kinetic scheme proposed by Carter (1990), calibrated by matching smog chamber results, has been selected. However, due to the lack of precise knowledge on the aromatic ring fragmentation chemistry, this scheme is already a simplified scheme. Thus, it will not be further reduced and consequently the evaluation of the errors introduced by the various procedures of simplification will not concern this aspect of the atmospheric chemistry.

Alkene + 03 reactions. The rate constants provided by Atkinson (1990) for the alkene + O3 reactions have been selected, as well as for the removal of the activated biradicals [H2C'O~] #, [CH3C'HO~ # formed by these reactions. For the [(CH3)2C'O~ ~ biradicals resulting from 2-methyl-2-butene + O3 reaction, the chemical mechanism proposed by Carter (1990) was used.

Limitations of the reference mechanism. In order to limit the number of reactions and species, the chemistry of some VOC was not treated in MOCA-ref, especially alkyl nitrates (RONO2), organic hydroperoxides (ROOH) and carboxylic acids (RC(O)OH). Some reaction products resulting from RO2 + RO2 reactions are also not represented: alco- hols (C/> 3) and most of the bi-functional products (like bi-alcohol and bi-earbonyl products). This im- plies that simulations with MOCA-ref should not

exceed time scales of half a day, so that the contribution of the chemistry induced by these compounds (especially RONO2 and ROOH compounds) can be neglected without a significant modification of the chemical system. This point was tested and verified for our simulation scenarios by reintroducing these reactions in the final reduced scheme. The organic chemistry driven by the NOa radical has also been neglected except for the phenol + NOa and the cresol + NO3 reactions which are rapid enough to be con-

sidered during the daytime. This limits the use of MOCA-ref to simulations of diurnal situations.

REDUCED MECHANISM

Step 1: the high NOt approximation

In urban or suburban situations, where NOt concentrations are high, the major removal process of peroxy radicals is their reaction with NO and peroxy + peroxy and peroxy + HO2 reactions can be neglected. According to Sillman et al. (1990), this will be the case for NOt concentrations greater than ~4 ppb. This approximation has been commonly used in the past, particularly in the ALW mechanism (Atkinson et ai., 1982). With this approximation and further assuming that all organic radicals are in a quasi steady state, a generic global equation can be written for each primary HC:

(A) HC + OH - , al VOCI + a2 VOC2

+ -.. + btHO2 + b2NO2

- b a N O , k = k o n

where the as coefficients are the final organic product yields, and bl, b2 and ba are, respectively, the total HO2, NO2 production yields and NO consumption yield. As an example, this approximation, when applied to the n-butane degradation mechanism given in Table 1, leads to the final global equation (at 300 K and atmospheric pressure):

(B) n-C4Hlo + OH ~0.688 CH3C(O)C2H5

+ 0.110 CH2 (OH)CH2CH2CHO

+ 0.033 C3HTCHO + 0.186 CHaCHO

+ 0.005 CaHTCH2ONO2

+ 0.071 CH3CH(ONO2)C2H 5

+ 0.924 HO 2 -I- 1.127 NO2 - 1.203 NO

k = 1.51 x 10" 17T2 exp(190/T)

(on 3 melee- 1 s - 1 ).

Since the branching ratios associated with the peroxy and particularly alkoxy removal reactions are a function of temperature, the stoichiometric coefficients are also temperature dependent. Analogous reactions can be written for the 12 hydrocarbons treated in this


study. The RCO3 + RO2 and RCO3-t-HO2 reactions were also removed to be coherent with the general formulation of the high NO~ approximation. The use of this approximation leads to a mechanism with 111 species and 210 reactions.

Step 2: reduction using the Carter methodology (1990)

Step 2.1: use of the chemical operator concept. In rural areas, where NOx concentrations are lower than a few ppb, peroxy + peroxy and peroxy + HO2 reactions become important and should be introduced in the chemical mechanism. Recently, Carter (1990) proposed a method allowing a condensed treatment of the peroxy radical chemistry. This approach consists of decomposing the reactivity of each peroxyalkyl radical into a limited number of surrogate species with similar reactivity. Carter used the terminology "chemical operator" to identify these species. They are noted (operator) in the remainder of this paper. The 2-butylperoxy radical (CH3CH(Oz)C2Hs) formed in the n-butane degradation will be taken as an example to discuss the methodology briefly. More general de- tails of this approach can be found in Carter's publica- tions (Carter, 1990; Carter et al., 1986).

Under NOx rich situations, CH3CH(O2)C2H5 re- acts with NO as follows at T = 300 K and atmospheric pressure (see Table 1):

(1) CH3CH(O2)C2H5

+ NO --, 0.084 CHaCH(ONO2)C2H5

+ 0.916 CH3CH(O)C2H5 + 0.916 NO2,

(2) CH3CH(O)C2Hs( + 02) - , 0.881 CH3C(O)C2H5

+ 0.881 HO2 + 0.119 CHaCHO

+ 0.119 CzHsO2,

(3) C2H502 + NO - , C2H50 + NO2,

(4) C2H50 + 02 --* CHaCHO + HO2.

Three distinct contributions associated with the 2- butylperoxy removal can be underlined, each of them being represented by a chemical operator: (i) NOx loss and organic nitrate formation ((RONO2) operator), (ii) NO to NO2 conversion with HO2 production ((RO2> operator), (iii) NO to NO2 conversion with the formation of a second generation of peroxy radicals ((R202) operator).

The 2-butylperoxy degradation can then be re- placed by

0.807 CH3C(O)C2H5 + 0.218 CHaCHO

+ 0.084(RONO2) + 0.807(RO2)

+ 0.109(R202)

with the following three reactions for the chemical operators:

(5) ( R O 2 ) "t- NO -* NO2 q- HO2,

(6) (R202) + NO --, NO2 + (RO2),

(7) (RONO2) + NO --, RONO2.

The contribution of the peroxy + peroxy and peroxy + HO2 reactions can then be treated by including the reactions of each operator with HO2 and with the other peroxy radicals. For example, for the (ROE) and (R202) operators, the following reactions are introduced:

(8) (ROE) + n o 2 --*-OOH,

(8') (R202) + n o 2 -*-OOH,

(9) (RO2) + {RO2} - , ~ HO2,

(9') (R202) + {RO2} --*~t(RO2),

(10) (RO2) + {RCO3} ~ t HO2,

(10') (R202) q- {RCO3} --*~(RO2),

where

• {RO2} and {RCO3} represent, respectively, the total concentration of peroxyalkyl and peroxyacyl radical.

• - O O H represents organic hydroperoxide formation. This species has no mass (0 carbon species), organic matter being represented by the organic species formed following reaction with NO (see discussion below).

• ct represents the branching ratio of the peroxy + peroxy reaction which gives alkoxy radicals, and

consequently HO2 in the case of the (RO2) reactions and (RO2) in the case of the (R202) reactions.

As underlined by Carter (1990), this representation incorporates two major approximations: (i) the organic products formed by the peroxy + peroxy reactions can be represented by those formed by the ROE + NO reactions, (ii) the organic hydroperoxides formed by the peroxy + HO2 reactions can be represented by the organic products formed by the RO2 + NO reactions plus the - O O H species.

No chemical mechanism was associated with RONO2 and -OOH, since the chemistry of organic hydroperoxides and alkyl nitrates are not treated in MOCA-ref (see above).

This chemical operator approach allows a sequential treatment of the different generations of peroxy radicals. This is the only major modification with the chemical operators described by Carter (1990) where the different generations of peroxy radicals are treated simultaneously. When successive generations of organic peroxy occur, this simultaneous treatment leads to an overestimation of NO to NO2 conversion, an underestimation of organic hydroperoxide formation and to an over or underestimation of riO2 production depending on the branching ratio between peroxy + NO, peroxy + HO2 and peroxy + peroxy re-

actions. However, more operators need to be introduced with this sequential treatment. Indeed, for three


successive levels of organic peroxy formation, a new operator ((R302)) must be introduced, with the following reaction with NO: ( R 3 0 2 ) + N O ~ NO2 + (R202).

The rate constants associated with the organic peroxy + peroxy reactions can vary by several orders of magnitude depending on the molecular structure of the peroxy involved. Such variations were introduced in MOCA-ref by supposing that the rate constants and the branching ratios depend only on the peroxy class (see above). So, in a first step, this information was kept by introducing a distinction between the different peroxy classes in the chemical operator formulation. These chemical operators, when applied to the n-butane degradation mechanism given in Table 1, lead to the following global equation:

(C) n-C,Hto + OH ~ 0.688 CH3C(O)C2H5

+ 0.110 CH2(OH)CH2CH2CHO

+ 0.033 C3HTCHO

+ 0.186 CH3CHO + 0.005(RtONO2>

+ 0.071(R2ONO2> + 0.033(R102)

+ 0.688(R202) + 0.093(R22_,O2)

+ 0.110(R2,_102).

The subscripts on the chemical operator indicate the peroxy class. The chemical operators in the above equation are then: nitrate formation from a primary alkylperoxy ((R1ONO2)), nitrate formation from a secondary alkylperoxy ((R2ONO2)), NO to NO2 conversion and HO2 production from a primary alkylperoxy ((R102)), NO to NO2 conversion and HO2 production from a secondary alkylperoxy radical ((R202)), NO to NO2 conversion and production of a primary alkylperoxy radical from a primary alkylperoxy radical ((R2,_102 )), NO to NO2 conversion and production of a primary alkylperoxy radical from a secondary alkylperoxy radical ((R22_102)). The rate constants associated to the reactions of these chemical operators are the same as those used in MOCA-ref (Table 2). For example, the reactions associated with the (R2~_102) operator have been written:

Rate constants and branching ratios of reactions (13)--(17) are estimated using the approach given by Madronich and Calvert (1990). {RIO2}, {R202}, {R302}, {RCO3} represent, respectively, the total concentration of primary, secondary, tertiary, peroxyalkyl radicals and peroxyacyl radicals. CHsO formed by equation (13) is supposed to react only with 02, leading to HO2 and HCHO. A total of 11 chemical operators was introduced to treat the peroxyalkyl chemistry involved in the degradation of the 12 HC represented in MOCA-ref. The resulting mechanism includes 470 reactions and 140 species and will be referenced as MOCA-redl here after.

Step 2.2: reduction of the number of chemical operators, loss of the information on the oroanic peroxy class. This level is obtained by dropping the peroxy class information. Only two kinds of organic peroxy are distinguished: peroxyalkyl and peroxyacyl radicals. Under this approximation the previous global equation for n-butane (C) becomes:

(D) n-C4Hto + OH --* 0.688 CH3C(O)C2Hs

+ 0.110 CH2(OH)CH2CH2CHO

+ 0.033 C3HTCHO

+ 0.186 CH3CHO + 0.076(RONO2)

+ 0.721(RO2) + 0.203(R202)

with (RONO2), (RO2) and (R202) being the chemical operators described in the previous general discussion above. The branching ratio proposed by Carter (1990) was used for the radical channel of the (opera tor )+ organic peroxy radical reactions (~ = 0.5 in equations (9), (9'), (10) and (10')) as well as for the rate constant associated with reactions (9) and (9') (k = 1 x 10- t~ cm 3 molec- t s- t). As mentioned above, the only difference between the operators used at this level of reduction and those described by Carter (1990) is in the sequential treatment of the different generations of organic peroxy radicals. At this step of reduction, the mechanism includes 311 reactions and 128 species. It will be referenced MOCA-red2 below.

(11) (R22_tO2) + NO --*NO 2 + (RtO2)

(12) (R22_102) + HO 2 ~ ROOH

(13) (R22_102) + CH302 -'* 0.75 HCHO + 0.25 CH3OH

+ 0.5 HO2 + 0.5(R102)

(14) (R22_zO2) + {R102} ~ 0.6(RIO2)

(15) (R22_tO2) + {R202} ~ 0.6(R102)

(16) (R22_~O2) + {R302} ~0 .8(R102)

(17) (R22_tO2) + {RCO3} ~0 .8(RtO2)

k = 4.2 x 10-12 exp(180/T)

k = 4.1 x 10-13 exp(790/T)

k = 3.8 x 10 -14

k = 2.0x 10 -14

k = 1.0x 10 -aS

k = 2.8 x 10-16

k =2 .5x 10 -13

2068

Ethane 7.4 Propane 2.4 Butane l 1.6 Isobutane 18.0 Pentane 20.5

B. AUMONT et al.

Table 3. VOC initial concentration (in ppbC)

Toluene 14.0 2-methyl-2-butene 2.0 m-xylene 8.8 Formaldehyde 2.4 Ethene 8.2 Acetaldehyde 1.2 Propene 6.0 Propanal 1.2 2-butene 4.0 Benzaldehyde 0.7

Step 2.3: lumping of secondary VOC. Most of the products generated by the hydrocarbon degradation are aldehydes and ketones. They can react with OH or through photolysis and form acyl peroxy radicals RCO3. These radicals can react both with NO and NO2 leading to peroxy acyl nitrates PANs. Their evolution depends on the NO/NO2 ratio and therefore it is impossible to write a global equation like (C) or (D) for the degradation of aldehydes and ketones. An explicit treatment of the chemistry would not be appropriate because of the high number of reactions and species involved, especially for the degradation of long carbon chain products. Furthermore, for poly- functional compounds, degradation mechanisms are not well known. A detailed treatment of these compounds is not yet relevant. The methodology em- ployed was to lump aldehydes and ketones into different surrogate species. This lumping approach was applied to MOCA-red2. For HCHO, CHaCHO, and CH2OHCHO the chemistry was kept, while for the higher molecular weight aldehydes C2HsCHO was used as surrogate species. For the ketone family, the full chemistry was kept for acetone, while methyl ethyl ketone was selected as the surrogate species for the higher molecular weight ketones. Nineteen secondary VOCs resulting from the degradation of the alkanes and atkenes were lumped into the six species listed above. The methodology used to decompose each oxygenated compound into the surrogate species was borrowed from Carter (1990). When applied to n-butane, the previous equation (D) becomes:

(E) n-C4Hlo + OH--.0.724 CH3C(O)C2H 5

+ 0.143 C2HsCHO + 0.186 CHaCHO

+ 0.076(RONO2) + 0.721(RO2)

+ 0.203(R202).

Note that, as discussed previously, secondary VOCs resulting from the degradation of aromatic compounds are already lumped. This degradation mechanism involves seven major secondary species (methyl glyoxal, glyoxal, 2 species for uncharacterized ring fragmentation products, benzaldehyde, phenol, cresol) which were totally kept. This last chemical mechanism version includes 150 reactions and 71 species and will be referenced as MOCA-red3 below. The general structure of MOCA-red3 is very close to the one proposed by Carter (1990).

SIMULATION RESULTS

Testing conditions

To evaluate the deviations introduced at each reduction level, the simulation tests are carried out under closed photo-reactor conditions, thus facilitat- ing the interpretation of the results by decoupling chemical effects from physical ones. Constant values are taken for the temperature (300 K), pressure (1 atm), and relative humidity (~45%). Photolytic rate constants are calculated for a zenithal angle of 45 °. The set of initial VOC concentrations is defined to be representative of an intermediate situation, between urban (100-1000ppbC) and rural (10-100 ppbC) (National Research Council, 1991). The distribution of alkanes 55%, alkenes 20%, aromatics 20% and aldehydes 5%, is rather characteristic of urban situations (Stockwell et al., 1990). The detailed speciation is given in Table 3 and corresponds to a VOC initial concentration of about 100 ppbC. As the VOC/NOx ratio is very important for ozone formation, three scenarios are defined to cover continental atmospheric conditions:

• Scenario 1: VOC/NOx = 5, [NOx] ~ 20 ppb • Scenario 2: VOC/NOx = 10, [NO~] ~ 10 ppb • Scenario 3: VOC/NO~ = 20, [NOx] ~ 5 ppb.

The ozone profile and the evolution of the main peroxy radical removal processes are plotted in Fig. 1 for Scenario 2. In these photo-reactor conditions, the ozone profile can be divided into two parts (Fig. 1): a first regime for which the ozone concentration increases linearly vs time and a second one where the concentration of 03 levels off to a plateau. On the basis of smog chamber data, Hess et al. (1992) observed that: (i) in the first regime, 03 formation is proportional to the amount of radiation received (and then to time if hv is constant) and called this regime Light limited. (ii) In the second regime, 03 production is limited by the quantity of NOx available in the system (NOx limited regime).

The transition between these two regimes results from a variation of the main removal processes of the peroxy radicals (Fig. 1). In the Light limited regime, the major removal process is the reaction with NO whereas in the NO~ limited regime reactions with HO2 and other organic peroxy radicals become significant.

For the first scenario, the chemical system stays all the time in the light limited regime (see O3 Figs 2a, 3a,


100

i 80

6O

0 0.0

! ' I ' I ' I

= LIGHT UMITED / / • . . . . . . NOx LIMITED

L HPUIMP. . . . . . . . . /

J , (a), , ,

4.0 8.0 12.0 T IME (H)

IO0

8o

0 .

4o

8 20 n-

O 0.0

: RO2+NO => ImP. : 1=102+H02 .> ROOH

I I I

. ( b ) :

4.0 8.0 12.0 T IME (H)

! , ; HOe+H02 => H202 /

- - : H O 2 + R O 2 ,o. ROOH I

0 - = : - : ' : : ~ -

0.0 4.0 8.0 12.0 T IME (H)

Fig. 1. Light limited and NOx limited regime. (a) Ozone concentration versus time for initial condition COV/NOx = 10. (h)Main peroxy radical removal pro-

cesses. (c) Main HO2 removal proceses.

4 and 5) and this simulation is rather representative of the chemistry involved in urban situations. Con- versely, Scenario 3 mainly occurs in the NO~ limited regime and is more representative of the chemistry involved in rural situations. These testing conditions were systematically applied to evaluate the various reduction steps of the chemical mechanism described above.

High NOx approximation

The results of the comparison between the high NOx mechanism and MOCA-ref are presented in Fig. 2a for 03, PANs (~ of all RCOaNO2 compounds), OH, HO2, NOx and H202. As expected, there is a very good agreement between the two mechanisms in the light limited regime, i.e. when the major removal process for peroxy radicals is the reaction with NO. Significant discrepancies only appear in NOx limited regime. If the discrepancy is rather small or O3 (only a few %), it becomes more significant for OH ( - 25%), HO2 ( + 50%), NOx (- - 90%), H20 2 ( + 50%) and PANs ( + 300%) (see Fig. 2a). The NO concentration appears to be the major parameter limiting the validity of the high NOx approximation. This is illustrated in Fig. 2b where the relative error resulting from the high NO~ hypothesis is plotted as a function of the NO concentration calculated with MOCA-ref. The errors do not depend on the selected scenario and seem to be only sensitive to the NO concentration. If we tolerate a 2% error, a NO concentration of about ~ 2 ppb can be determined as a boundary value below which the high NOx approximation generates excessively large errors. Moreover, when the system reaches the threshold NO concentration, the speciation of the remaining HC is significantly different among the three scenarios. Thus, the threshold concentration value should not be sensitive to the initial VOC distribution.

Supposing that the NO-NO2-O3 system is near the photochemical equilibrium, the NO~ concentration limit is calculated to be about 5 ppb (for a zenithal angle of 45 ° and an ozone concentration of 30 ppb). This NO~ concentration limit is too large to permit use of the high NOx approximation in rural areas, where typical NO~ concentrations are 0.2-10 ppb.

Use of the operators

The results of the comparison between MOCA-ref and MOCA-redl are presented in Fig. 3a and b for 03, OH, NO~, PANs, RO2, CHaCHO. For each compound, both concentration profiles and relative errors are plotted as a function of time. Globally, there is a very good agreement between MOCA-redl and MOCA-ref. The relative error does not exceed 0.5% for ozone, and 1% for NOx. For other species, the maximum discrepancy is slightly more significant: OH (8%), RO2 (10%), PANs (35%) and CH3CHO (60%). Most of the differences appear when the system reaches the NOx limited regime. This can be explained by considering the approximations introduced in the determination of the secondary VOC distribution. As said before, the organic products formed by RO2 + RO2 and RO2 + HO2 reactions are represented by the oxygenated products formed when the peroxyalkyls react with NO. Therefore, when these reactions become important the carbonyl species tend to be overestimated (cf. CH3CHO profile Fig. 3b).


A ,.Q

z o

E p- Z LIJ O Z O O

1 ~ . 0 , i , i

~ . 0 0 3

~.0 ~

40.0

20.0

0.0- 0.0 4 . 0 8 .0 12 .0

0.50 A

0.40 Q . v

z o 0 . 3 0

0 . 2 0 LU

z o 0.10 (3

0.00 0.0

TIME (H)

O .

I.- ,<

L) z 0 o

1 . 5

1.0 ~ ,~

0.5 [~~,

0.0 0.0

t I 1

-./- " -. PANs \

I , I , L

4.0 8.0 12.0

TIME (H)

, i i , j . 60.0 'iO0 20.0

OH

, t , I , I 0.0 4 . 0

I ' I ' t /

8.0 1 2 . 0 0 . 0 4 . 0 8.0 12.0

TIME (H) TIME (H)

102

• ,Q !

101(

10 °

z I11 o 10-1 0

I I I

"- ,,,-~

v

z

,< tr

w 0

0

, I , I L I

0.0 4.0 8.0 12.0

(a) TIME (H) TIME (H)

10.0

8.0

6.0

4.0

2.0

0 . 0 - 0.0

i'l H202 ,- ,,

/ /

/ / / /

// / /

/ /

/ / /

/ / /

/ L z

4.0 8.0 12.0

Fig. 2a. Comparison of high NO~ approximation mechanism (---) with MOCA-ref. ( ): Concentration profiles for 03, PANs, OH, HO2, NOx, H202. (o): Scenario 1, (#): scenario 2, (&): scenario 3.

This occurs in the NO~ limited regime and consequently modifies the photo-stationary equilibrium of some of the radical species by decreasing OH, increasing peroxy radicals, and consequently increasing their degradation products (particularly the PANs and organic hydroperoxides).

The relative errors associated to ROz, PAN and HCHO have been plotted as a function of the NO

concentration calculated with MOCA-ref(see Fig. 3c). If we tolerate ~ 2 % error, MOCA-redl does not induce significant errors as long as the NO concentration stays greater than 200 ppt. Below this limit, more significant errors appear. However, since the NO concentration decreases regularly, the relative errors at a given NO concentration are cumulative errors. To avoid this problem, the derivative of the relative error


20.0

0.0

[~ -20.0

re " ' -40.0 UJ >

-60.0 5 m re -80.0

-100.0

. . . . . . . I . . . . . . . . ! 'I . . . . . . . I

NOx .~+ ~..#- 4~H-*HH--H-

-t,H-

/ +

+

+

i ,l,lul , , ,,,,,,l i I ii,i,I I

10" 10 ° 101 NO CONCENTRATION (ppb)

400.0

~" 300.0

r e m 200.0 W

100.0 UJ re

0.0 10 =

•l.•lHH[ ' ' , ,HH [ ' ' 1 ' ~ ' ' I ' r ~ 'HH

PANs +

÷ -I- +

I lllllll l"l'l~¢ll~dLi Jll.,l==,~llll~J ,I l |III

1o -I 1 0 ~ . . . . ~ i i ~ " 1# NO CONCENTRATION (ppb)

10.0

re 0.0

re m -10.0 . I

-20.0

-30.0

(b)

. , H I . I , , , , , . , , I , , ,H . j

4-++

+ ÷÷

/ +

+

+

I I llllll I I I llllll I I IIIIII l I II

10-' 100 10' NO CONCENTRATION (ppb)

10 =

60.0

40.0 re 0 r e n" LU 20.0 UJ >

'~ 0.0 W IX:

-20.0

, , , , , , , I , 1 , , , , . . ]

+++

+ +

+ ~ + ~

, , ,H . I , , , , . , .

H02

J I J i l l l l = I I l IH , [ I I I l lH [ I I I , , ,

10-' 10 ° 101 10 = NO CONCENTRATION (ppb)

Fig. 2b. Comparison of high approximation mechanism with MOCA-ref.: Relative error as a function of the NO concentration calculated with MOCA-ref for NOx, PANs, HO, HO2. Results are plotted for the

three scenarios.

vs time has also been plotted for RO2, PAN, HCHO as a function of NO concentration (Fig. 3c). For these three species, the error production rates are also inde- pendent of the selected scenario. As mentioned above, at a given NO concentration, the HC speciation of the system is significantly different. Thus the relative error derivative should be relatively insensitive to HC speciation. If we now tolerate an error growth rate of 1% per hour, it appears that MOCA-redl does not introduce significant errors as long as the NO concentration stays greater than 100 ppt. This threshold concentration value is low enough to allow the use of the operator concept for most diurnal situations encountered in the lower continental troposphere. Moreover, since the chemistry of the organic hydroperoxides is not treated in MOCA-ref, the introduction of the operator concept leads to an over- production of organic carbon, these species being represented in MOCA-redl by those produced via the RO, + NO reactions (see above). The treatment of the organic hydroperoxide chemistry would allow organic carbon conservation and should decrease the observed error. Thus, the error introduced by the chemical operator concept is somewhat overestimated in our study and the threshold NO concentration value should be lower.

Reduction of the number of chemical operators: loss of the information about the peroxy class

Results of MOCA-red2 are very similar to those of MOCA-redl and are consequently not presented. The relative errors between MOCA-red2 and MOCA-ref are only slightly higher than those obtained with MOCA-redl: 03 (1%), OH (9%), PANs (40%) compared with 03 (0.5%), OH (8%) and PANs (35%) for MOCA-redl. As discussed above, MOCA-red2 differs from MOCA-redl only in the treatment of the RO2 + RO2 and RO2 + RCO3 reactions. The slight increases of the observed discrepancies underline the fact that, over the studied time scale and for the scenario used (NO concentration is always greater than 30 ppt), the reaction of alkylperoxy radical with other organic peroxy radicals contribute only weakly to the general evolution of the chemical system.

Lumping of secondary VOC

In order to specifically address the errors introduced by the use of surrogate species, the results obtained with MOCA-red3 were compared to those obtained with MOCA-red2. The concentration profiles for Oa, PAN and PANs calculated with these two mechanisms are plotted in Fig. 4. Good agreement is observed for Oa, the error being lower than 5%.


z 0 I- ,,~ or I-- z LU O Z O O

100.0

80.0

60.0

40.0

20.0

0.0 0.0

' I ' I ' I -

03

4.0 8.0 12.0

TIME (H)

v

n- O rr nt" uJ LU _> F -

5 t.U n -

0.4

0.2

0.0

- 0 .2 0.0

03

4.0 8.0 12.0

TIME (H)

0.50

0.40

0.30

0.20

0.10 (.)

0.00 0.0

' I T I ~ I

, I i I I

4.0 8.0 12.0

TIME(H)

n- O

LU

I.U _>

..-I LU rr

. 0 - - 1

0 . 0

-2 .0

~ . 0

~ . 0

~ . 0 ' I , I , =

0.0 4.0 8.0 12.0

TIME (H)

30.0

20.0

l o .o

0.0

2.0 I I I

NOx

0.0 4.0 8.0 12.0

TIME (H)

~ 1.0 n- O 1I: n.- tu 0.0 LU _>

'~ -1.0 lie

I I I

NOx

- - 2 . 0 = I , I t I

0.0 4.0 8.0 12.0

(a) TIME (H)

Fig. 3a. Comparison of MOCA-red 1 (---) with MOCA-ref (--): Concentration profiles and relative errors for 03, OH, NOx. (e): Scenario 1, (e): scenario 2, (&): scenario 3.

However, the lumping introduces a loss of information on the secondary products and their chemistry. The use of surrogate species modifies the aldehyde speciation and consequently their reaction products. For example, the comparison of PAN concentrations calculated with and without surrogate species shows that such a methodology introduces an important error (300 discrepancy) that is directly linked to the fact that what is called PAN does not represent the

same quantity in each mechanism. Thus, the only relevant comparison is the one associated with PANs. Good agreement between the two mechanisms is then observed in spite of an underestimation of the maximum PANs concentration ( - 10%) for Scenario 3. The discrepancy observed for 03 can be linked to a modification of the initiation rate of the radical chain due to variations of the VOC + hv reactions after lumping.


1.5

i 1.0

0.5

0 . 0 - 0.0

I I I | 40.0 I I I /

30.0 PANs

20.0

~> 10.0

0.0 = •

I , J , = I -10.0 ' 4.0 8.0 12.0 0.0 4.0 8.0 12.0

TIME (H) TIME (H)

25.0 ' I I I ~ 15.0 | I I I

200 _- - - - . - - - - - I lO.O[ ~: 15.0 wn-"

" ' 5.0 f 10.0 ~,

oo

0.0 - .0 ' ' - -j 0.0 4.0 8.0 12.0 0.0 4.0 8.0 12.0

TIME (H) TIME (H)

i 3.0

- 2.0

1.0

0.0

(b)

' I I I

t

0.0 4.0 8.0 12.0

TIME (H)

6 0 . 0 I I I

50.0

g 4o.o 30.0

i 20.0

10.0

0 . 0 - : - - - ; . . . . . 0.0 4.0 8.0 12.0

TIME (H)

Fig. 3b. Comparison of MOCA-redl (---) with MOCA-ref(--): Concentration profiles and relative errors for PANs, RO,, CH3CHO. (o): Scenario 1, (O): scenario 2, (&): scenario 3.

Finally, MOCA-red3, the most reduced mechanism, was compared to MOCA-ref to estimate the global errors introduced by the three successive levels of reduction. The results are presented for 03, OH and H20 , in Fig. 5. Very good agreement is observed between the two mechanisms, especially for Oa (3%) and OH (10%) but also for H202 (20% maximum discrepancy).

CONCLU~ON

This study has proposed a methodology for evalu- ating the errors introduced by the reduction of a chemical reaction scheme and their application domain.

In a first step, the errors introduced by the high NO~ approximation were investigated. We observed


A

15.0

10.0

5.0

0.0

-5.0

' ' ' " " 1 ' ' ' ' t ' " l ' ' ' ' " " 1 ' ' ' ' ' " '

RO2 +

+ +

4"

10-' 100 10 ~ 10 z

NO concentration (ppb)

A a-

®

" 0

2.0

1.0

0.0

-1.0

' ' " " 1 , , , , r l l , I t , l l , , , r I I f

+ + +

+ + + + ÷

+~ ÷ +

+

+

RO2

+

+

+ + + ÷

++ +++ +++ + +

,l,,,,J , , lll,,d , , ,,,,,,I

10-' 10 0 10' NO concentration (ppb)

i i i

10 2

$

40.0

30.0

20.0

10.0

0.0

-10.0

' ' ' " " 1 ' ' ' ' ' " ' 1 ' ' ' ' " " 1 , , , , T ,

÷ PAN +

++

4.

, , , , , , , I , , , , , , , , I , , , , , , , d


, , , , , ,

10 2

2"

®

O >

6.0

5.0

4.0

3.0

2.0

1.0

0.0

-1.0

' ' " " 1 ' ' ' ' " " 1 ' ' ' ' " " 1

+

PAN

++

+ +

+# +

+++ ÷~-** + +

, , , , 1 , 1 . . . . . . . . 1 , , , , , , , d


10 2

1 - 4 )

4 ) 1>

=e

(c)

20.0

15.0

10.0

5.0

0.0

-5.0

' ' ' " " 1

+

+, +,

I I i l l l r r I I t t l ~ [ I r I

HCHO

' ' . . . . t

r q

+ ~ + 4 % + ~ + + + - I t + 4 4 t + + , # + + ~ +-*+

, , , , , , d I t , , , , , , I , . . . . . . . I , , , , 1 1

10-' 10 o 101 10 2

NO concentra~on (ppb)

4.0

3.0

2.0

1.o

=0 0.0

- I .o

' ' ' ' " 1 ' ' ' ' " " 1 . . . . . " q

HCHO +

+

+ +

+ +

+

÷÷

*f "~+ ~ ' + +- 'k- + + + +

, , , , , d i , , , , , . I , J i J i l . I , , , , , ,

10 -1 100 10' 10 2 NO concentration (ppb)

Fig. 3c. Comparison of MOCA-redl with MOCA-ref: Relative error and d(relative error)/dt in % per hour as a function of NO concentration calculated with MOCA-ref for PAN, RO2, HCHO. Results are plotted

for the three scenarios.

that this approximation is perfectly justified as long as the NO concentration stays greater than 2 ppb. Below this limit, important errors were observed.

In a second step, three successive levels of reduction were tested:

• use of the chemical operator concept to treat the organic peroxy chemistry,

• loss of the information on the organic peroxy class,

• lumping of secondary VOCs into surrogate species.

The use of chemical operators to reduce the organic peroxy chemistry is satisfactory for NO concentra-

tions down to about 100 ppt. Below this limit, some significant errors are observed. However, this threshold concentration value, which is certainly overestimated in our study, is low enough to allow the use of this approach for most diurnal situations encountered in the lower continental troposphere.

Only minor errors were introduced by the approximation performed by the loss of the alkyl peroxy radicals class. This approximation appears to be relevant for the simulation of diurnal continental troposphere. No major error was observed after lumping of oxygenated products into surrogate species. How- ever, the errors induced by this approximation are dependent on the initial condition (especially the VOC


100.0

80.0

- 6 0 . 0

I 40.0

20.0

0 . 0 - -

0,0

I I I

03 ~

4.0 8.0 12.0 TIME (H)

1.0

~ 0.0

~ - 1 . 0

-2.0

~ - 3 . 0

-4.0 0.0

I l I

I J I , I

4.0 8.0 12.0 TIME (H)

1.5 I I I

PANs

1.0

~ 0.5

o

0 . 0 -

5.0

0.0 4.0 8.0 12.0 TIME (H) TIME (H)

I I I

PANs rr o o o

~ - 5 . 0

-10.0 0.0 4.0 8.0 12.0

1.0 , , , 30.0

0.8 ~ PAN

z ~ 20.0

I 0 . 6 ~ " - " mm

0.4 ~> 10.0

0.2

0.0 - 0.0

I I I

. aw'T" I , I , I 0.0 4.0 8.0 12.0 0.( 4.0 8.0 12.0

TIME (H) TIME (H)

Fig. 4. Comparison of MOCA-red3 (---) with MOCA-red2 (--): Concentration profiles and relative errors for 03, PANs, PAN. (.): Scenario 1, (0): scenario 2, (A): scenario 3.

speciation) and should be specific to each situation. More general tests should be performed to character- ise precisely the errors introduced by this approximation. By definition, the use of surrogate species modifies the VOC speciation and a particular attention should be g0ven on the interpretation of such lumped mechanisms.

The above error evaluations are closely linked to our current knowledge of the atmospheric chemistry,

i.e. to our ability to develop relevant reference mechanisms for a complete set of representative primary species. Many aspects of the reference mechanism are pure speculations and the rate constants or the branching ratios of several reactions could be in error by more than an order of magnitude. In addition, the considerable lack of experimental data leads to the use of generic rate constants and justifies, for example, the use of chemical operators to treat the organic


A

"8.

Z o I - ,,( n- I - Z ILl

o

100.0

80.0

6 0 . 0

40.0

20.0

b.b 0.0

l ' I ' I 1 I

j

4.0 8.0 12.0 TIME (H)

Q .

I -

I.- Z I, LI O Z O o

0.50

0.40

0.30

0.20

0.10

0 . 0 0 . 0.0

I r I

I J I , I

4.0 8.0 12.0 TIME (H)

1.0

o.o

~" -I.0 w

i 2.0

~ -3 .0

--4.0 0.0

03

4 . 0 8 , 0 1 2 . 0

TIME (H)

rr O n- n" I L l

kU _> i.- 5 ILl n-

5.0

0.0

-5.0

-10.0 0.0

I I I

= i , I J I

4.0 8.0 12.0 TIME (H)

5.0 I I I 10.0 I I I

. ,O 4 . 0

Z --- 3.0

i- 2.0 Z LU O Z o ¢O

A

~) 0.0 i

uJ - 1 0 . 0

- -20.0

H202 /,,

4.0 8.0 12.0

1 .0 w n-"

0.0 - - 3 0 . 0 , I , l , J

0.0 0.0 4.0 8.0 12.0

H202

TIME (H) TIME (H)

Fig. 5. Comparison of MOCA-red3 (---) with MOCA-ref(--): Concentration profiles and relative errors for 03, OH, H202. (*): Scenario 1, (0): scenario 2, (&): scenario 3.

peroxy chemistry. Another particular limiting point is the representation of the degradation scheme of aromatic compounds for which numerous research is still in progress. Furthermore, in order to limit the number of reactions and species, many intermediates were omitted in the reference mechanism, especially hydroperoxides and alkyl nitrates. The chemistry of these species may become important after several days. Another limitation in the

tests is that no organic nighttime chemistry was included in the mechanism. Thus, additional tests must be performed in order to evaluate the range of validity of the different reduction hypotheses over longer time scale.

Acknowledoements--The authors would like to thank the French Ministry for Environment and ADEME for their financial support.


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Pergamon - Paris-Est Créteil Val-de-Marne Universityaumont/contenu/recherche/paper/1996... · 2006. 3. 28. · representation of the organic peroxy chemistry for NO concentration