A study of night-time chemistry of A study of night-time chemistry of HOHOxx radicals in the Northern and radicals in the Northern and

Southern HemisphereSouthern Hemisphere

R. Sommariva1, L. Whitehouse1, M.J. Pilling1, W.J. Bloss1, Z. Fleming2, D.E. Heard1, J.D. Lee3, P.S. Monks2

1) University of Leeds, Leeds, UK2) University of Leicester, Leicester, UK3) University of York, York, UK

Field CampaignsField Campaigns

SOAPEX-2SOAPEX-2 took place in January–February 1999 at Cape Grim, on the north-western tip of Tasmania (40°41’ S, 144°41’ E).

Cape Grim frequently experiences unpolluted air from the Southern Ocean (baseline sector: 190°-280°).

Late evening and early morning measurements of HO2 and night-time measurements of total peroxy radicals were taken on 15 – 16 February.

NAMBLEXNAMBLEX took place in July–September 2002 at Mace Head, on the west coast of Ireland (53°19’ N, 9°54’ W).

Air masses arriving at Mace Head from the Atlantic Ocean are considered representative of the background conditions in the Northern Hemisphere (clean sector: 180°-300°).

During the night between the 31 August and 1 September measurements of HO2 by FAGE and total peroxy radicals by PERCA were taken.

MeasurementsMeasurements

0.0E+00

5.0E+08

1.0E+09

1.5E+09

2.0E+09

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (AEST)

[NO

2] /

mo

lec

ule

cm

-3

0.0E+00

1.0E+08

2.0E+08

3.0E+08

4.0E+08

5.0E+08

[NO

] /

mo

lec

ule

cm

-3

NO2

NO

0.0E+00

4.0E+11

8.0E+11

1.2E+12

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (AEST)

[O3]

, [C

O]

/ m

ole

cu

le c

m-3

4.3E+13

4.3E+13

4.3E+13

4.3E+13

4.4E+13[C

H4

] /

mo

lec

ule

cm

-3

O3 CO CH4

0.0E+00

6.0E+09

1.2E+10

1.8E+10

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (GMT)

[NO

2] /

mo

lec

ule

cm

-3

0.0E+00

5.0E+08

1.0E+09

1.5E+09

2.0E+09

[NO

] /

mo

lec

ule

cm

-3

NO2

NO

0.0E+00

1.0E+12

2.0E+12

3.0E+12

4.0E+12

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (GMT)

[O3]

, [C

O]

/ m

ole

cu

le c

m-3

4.5E+13

4.6E+13

4.7E+13

4.8E+13

[CH

4] /

mo

lec

ule

cm

-3

O3 CO CH4

ObjectiveObjective

QuestionsQuestions? What determines the

radical concentration at night?

? What are the main reactions driving the radical cycle?

? What determines the loss from the radical pool?

The aim of this work was to study the night-time chemistry of HOx in the Northern and Southern Hemisphere under unpolluted conditions with no significant night-time radical sources.

OH

HO2

O3, NO

HO2

CH4, (NMHC)

RO2

ROOH, H2O2

HO2, RO2

CO

NO

ModellingModelling

Two models based upon the Master Chemical Mechanism (http://mcm.leeds.ac.uk/MCM/) with different chemical complexity have been used to study night-time chemistry in unpolluted conditions.

The models were built using the procedure outlined in Carslaw et al., 1999. They were constrained to measured H2, O3, NOx, photolysis rates, HCHO, H2O and Temperature. Additionally they were constrained to:

Only the clean model was used for SOAPEX-2, while both models were used for NAMBLEX.

1. CO and CH4 (clean model).2. CO, CH4, 23 NMHCs and 3 Oxygenates (fulloxy model).

Model ResultsModel Results

0.0E+00

5.0E+07

1.0E+08

1.5E+08

2.0E+08

2.5E+08

3.0E+08

3.5E+08

18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00

Time (AEST)

[RO

2] /

mo

lecu

le c

m-3

PERCA modelled

HO2

RO2

PERCA measured

HO2 measured

SOAPEX-2SOAPEX-2

15-16 February15-16 February

0.0E+00

5.0E+07

1.0E+08

1.5E+08

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (GMT)

[HO

2]

/ m

ole

cule

cm

-3

HO2 [clean]

HO2 [fulloxy]

HO2 measured

NAMBLEXNAMBLEX

31 August-1 September31 August-1 September

The agreement with the measurements is much better with the fulloxy model (NAMBLEX) than with the clean model showing the importance of organic compounds even under unpolluted conditions. 0.0E+00

5.0E+07

1.0E+08

1.5E+08

2.0E+08

2.5E+08

18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

Time (GMT)

[HO

2+R

O2]

/ m

ole

cu

le c

m-3

HO2+RO2 [clean]

HO2+RO2 [fulloxy]

PERCA measured

NAMBLEXNAMBLEX

31 August-1 September31 August-1 September

HOHO22 Rate of Production/Destruction Analysis Rate of Production/Destruction Analysis

On both campaigns main sources of HO2 were the reaction of OH with CO and CH4 and main sinks were the recycling reactions with O3 and NO.

Radicals were lost via peroxy-peroxy reactions (mainly HO2+HO2) and aerosol uptake. Radical sources in these unpolluted conditions were negligible.

NAMBLEX, 31 August-1 SeptemberNAMBLEX, 31 August-1 September

-2.0E+05

-1.5E+05

-1.0E+05

-5.0E+04

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

19 20 21 22 23 24 25 26 27 28 29 30

Time (GMT)

mo

lecu

le c

m-3

s-1

OH + CO -> HO2

CH3O + O2 -> HO2 + HCHO

OH + HCHO -> HO2 + CO

OH + H2 -> HO2

OH + O3 -> HO2

HO2NO2 -> HO2 + NO2

HO2 + NO2 -> HO2NO2

HO2 + O3 -> OH

HO2 + NO -> OH + NO2

HO2 + HO2 -> H2O2

HO2 + CH3O2 -> CH3OOH

HO2 -> aerosol

SOAPEX-2, 15-16 FebruarySOAPEX-2, 15-16 February

-1.2E+04

-8.0E+03

-4.0E+03

0.0E+00

4.0E+03

8.0E+03

1.2E+04

19 20 21 22 23 24 25 26 27 28 29 30

Time (AEST)

mo

lec

ule

cm

-3 s

-1

OH + CO -> HO2

CH3O + O2 -> HCHO + HO2

OH + H2 -> HO2

OH + HCHO -> HO2 + CO

OH + O3 -> HO2

HO2NO2 -> HO2 + NO2

HO2 + NO2 -> HO2NO2

HO2 + O3 -> OH

HO2 + NO -> OH + NO2

HO2 + HO2 -> H2O2

HO2 + CH3O2 -> CH3OOH

To get a better insight into the dynamics of the kinetic system a perturbation analysis was performed on a simplified version of the clean model. A perturbation c was applied to the set of ODEs representing the kinetic system and the propagation of the perturbation was studied.

The variation of the perturbation in time can be written as:

If Λ is the diagonal matrix of eigenvalues of the Jacobian matrix J and U the matrix of the eigenvectors, the solution of the system is:

Perturbation AnalysisPerturbation Analysis

.)( 01 cUUetc t

dt

dc

dc

d

dc

dfJ i

jj

iijc

dt

cd

J where

For species i the evolution of the perturbation in time is given by:

The perturbation propagates through the system and then relaxes on different timescales (). Each eigenvalue () is related to a different timescale (i = -i

-1) and thus determines the decay of the perturbation on that timescale.

To understand the physical meaning of the eigenvectors a new set of variables (modes) z = U-1c are defined:

The matrix of the eigenvectors U-1 can be considered as the linear transformation of the species into the modes showing how each species contributes to the mode associated with that eigenvalue or timescale.

Eigenvalue/ Eigenvector DecompositionEigenvalue/ Eigenvector Decomposition

.)( 0zetz t

tnin

ti

tii

neaeaeac uuu 212211 where

kkjkij cua )0(1

Results (I)Results (I)

A version of the clean model including simplified inorganic, CO, CH4 chemistry was used for the perturbation analysis.

The system consists of three radicals (OH, HO2, CH3O2). The other species can be considered invariant on these timescales.

The system relaxes on three timescales described by three eigenvalues (1, 2, 3).SOAPEX

-6.2E-01

-6.0E-01

-5.8E-01

-5.6E-01

-5.4E-01

0 100 200 300 400 500

Time (min)

λ 1 /

s-1

-3.0E-03

-2.0E-03

-1.0E-03

0.0E+00

λ2,

λ3

/ s

-1

λ1

λ2

λ3

NAMBLEX

-1.10E+00

-1.08E+00

-1.06E+00

-1.04E+00

-1.02E+00

-1.00E+00

0 100 200 300 400 500

Time (min)

λ 1 /

s-1

-4.0E-03

-3.0E-03

-2.0E-03

-1.0E-03

0.0E+00

λ2,

λ3

/ s-1

λ1

λ2

λ3

Results (II)Results (II)

1. The fastest timescale is described by 1 (~ -1 s-1). This time-scale corresponds to the recycling of radicals through the system and is controlled mainly by OH+CO and OH+CH4 reactions.

2. The second timescale is described by 2 (~ -1×10-3 s-1). In the associated eigenvector the CH3O2 contribution is of the same order of magnitude as the HO2 contribution indicating a strong influence of the OH+CH4 route to HO2 formation. The timescale described by 2 then corresponds to the slow conversion of CH3O2 to HO2 by reaction with NO.

3. The slowest time-scale is described by3 (~ -5×105 s-1). All three radicals contribute equally to the associated eigenvector. The process involves loss of the coupled radical pool via peroxy-peroxy reactions (mainly HO2+HO2). The long and increasing timescale is a result of the small and decreasing peroxy radical concentrations.

While 2 and 3 are similar in both campaigns, 1 is about 40% higher in NAMBLEX. 1 is approximately equal to the reciprocal of OH lifetime. The difference can be attributed to higher [CO] in the Northern Hemisphere.

A small radical source (O3+butenes) results in a steady radical concentration at night. This does not affect the perturbation analysis.

ConclusionsConclusions

AcknowledgementsAcknowledgementsSOAPEX-2 and NAMBLEX participants (Aberystwyth, Birmingham, Cambridge, East Anglia, Leeds, Leicester, UMIST, York Groups), Australian Bureau of Meteorology, Cape Grim and Mace Head staff, BADC, CSIRO-Melbourne, DIAC.

Propagation reactions of OH with CO and CH4

Slow conversion of CH3O2 to HO2 via reaction with NO

Removal from the radical pool via radical-radical reactions (HO2+HO2)

1. The agreement between the modelled and measured HO2 and RO2 at night is reasonably good when the model is constrained to hydrocarbons and oxygenates.

2. Under unpolluted conditions and in the absence of significant radical sources, the radical concentrations remain non-zero at night because loss via second-order radical-radical reactions becomes increasingly small.

3. The system relaxes in three timescales corresponding to: