Atmospheric chemistry

Day 4

Air pollution

Regional ozone formation

Regional air quality – ozone formation

• Ozone is a greenhouse gas. It affects human health, plant growth and materials

• Ozone is a secondary pollutant and is not directly emitted.

• Emission of VOCs and NOx, coupled with sunlight leads to the formation of photochemical smog.

• Major component is ozone. Also aerosols, nitrates …• Need to understand chemical mechanism for formation

in order to develop strategies and legislation for reduction of ozone concentrations.

• The European limit values are linked to these aims• Is it better to control NOx or VOCs – or both?

Chemical mechanism• Initiation: OH formed from ozone photolysis at a rate POH (=

2k3[H2O]J1[O3]/{k2[M] + k3[H2O]} )

• Propagation

OH + RH (+O2) → RO2 + H2O (R4)

RO2 + NO → RO + NO2 (R5)

RO + O2 → R’CHO + HO2 (R6)

HO2 + NO → OH + NO2 (R7)

• Termination

HO2 + HO2 → H2O2 (R8)

OH + NO2 + M → HNO3 + M (R9)

• Ozone formation

O3 is formed by NO2 photolysis with a rate equal to the sum of the rates of reactions 5 and 7 (= v5 + v7)

NOx and VOC control of ozone formation• Under polluted conditions, chain propagation is fast,

so v4 = v5 = v6 = v7

• PO3 = v5 + v7 = 2v7 = 2k7[HO2][NO] A

• Also v4 = v7 [OH] = k7[HO2][NO]/{k4[RH]} B

• Steady state for radicals: rate of termination = rate of initiation, ie POH = v8 + v9

1. Low NOx: v8 >> v9 POH = 2k8[HO2]2; [HO2] = (POH/2k8)

Sub in A: PO3 = 2k7[NO] (POH/2k8).

( PO3 [NO], independent [RH] NOx limited)

2. High NOx: v8 << v9 [OH] = POH/(k9[NO2][M]

Sub in B: [HO2] = POHk4[RH]/{k7k9{NO][NO2][M]

Sub in A: PO3 = 2k4[RH]/{k9[NO2][M]

( PO3 [NO2]-1; [RH]) VOC limited)

DEPENDENCE OF OZONE PRODUCTION

ON NOx AND HYDROCARBONS

HOxfamily

OH

RO2 RO

HO2

HNO3 H2O2O3

O3

O3

PHOx4

5

67

89

1/ 23 7

8

( ) 2 ( ) [ ]HOxPP O k NO

k4

39 2

2 [ ]( )

[ ][ ]HOxk P RH

P Ok NO M

“NOx- saturated” or“hydrocarbon-limited” regime

“NOx-limited” regime

RH

NO

O2

NO

NO2

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed

NOx-saturated

NOx-limitedRidge

Can we determine the relative contributions of different VOCs to ozone formation?Master chemical mechanism (MCM)

• Constructed by University of Leeds, in collaboration with Imperial College and UK Met Office

• Explicit mechanism, based on a protocol which describes the chemistry. Includes reactions of OH, NO3 and O3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., 1997, 31, 81.

• Describes the oxidation of 123 VOCs, based on the UK emissions inventory.

• The MCM is set up to provide input directly to the FACSIMILE integrator.

• It can be accessed via the web:

(http://www.chem.leeds.ac.uk/Atmospheric/MCM/mcmproj.html)

• The MCM is used by Department of the Environment Food and Rural Affairs (DEFRA) to help develop its air quality strategy.

Master chemical mechanism (MCM) A specific, explicit implementation

(http://Mcm.leeds.ac.uk/MCM

Navigational Features: Extract

Use Mark List as primaryspecies

Choose output format- HTML- FACSIMILE- FORTRAN- XML- KPP

Navigational Features: Extract Listing

Navigational Features: Source information

● Information based on Protocol papers

● Hyperlinked citations

Mechanism testing using chamber experiments

Developing and testing the MCM using chamber experiments

• Double outdoor chambers at Valencia, Spain.• Carry out experiments under atmospheric conditions, but under defined

conditions.• Heavily instrumented. Measure NOx, O3, VOCs, oxygenates, CO, particles,

radicals (OH, HO2) vs time.• Applications:

– Biogenics – pinenes– aromatics

Photo-oxidation of -pinene / NOX: gas-phase simulation

[-pinene]0 = 97 ppb; [NO]0 = 9.7 ppb; [NO2]0 = 0.85 ppb

Jenkin – OSOA project

0

2

4

6

8

10

12

10:00 12:00 14:00 16:00

NO

, N

O2 (

pp

b)

0

20

40

60

80

100

120

-p

inen

e ,

ozo

ne

(pp

b)

27-Sep-00 Chamber A

Comparison of MCM3.1 to Toluene Chamber Experiment (27/09/01)

9 10 11 12 13 14 15 16 17 18 19150

200

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500

9 10 11 12 13 14 15 16 17 18 19

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100

9 10 11 12 13 14 15 16 17 18 19

0

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40

60

80

100

120

140

Tol

uene

[pp

b]

Time [h]

Experiment MCM3.1

O3 [

ppb]

Time [h]

NO

2 [pp

b]

Time [h]

NO

[pp

b]

Time [h]

Also possible to measure radicalsOH, HO2. Provides A sensitive test of the mechanisms

The discrepancies show that there are significant deficienciesin the mechanismespecially related to radical formation

C. Bloss et al Atmospheric Chemistry & Physics, 2005, 5, 623 – 639.

Photochemical ozone creation potentials (POCPs)

• Is there a way in which we can quantify the differential impact of different VOCs on ozone formation?

• The UK DEFRA uses POCPs to assess differences between VOCs and hence to develop policy.

• The method is based on the use of a photochemical trajectory model (PTM), in which the chemical evolution of an air parcel is followed as it travels, under anticyclonic conditions, from central Europe to the UK, over a period of 5 days.

• Details:– air parcel extends from surface to top of boundary layer. It is

10kmx10km (horizontal dimensions) and has a height,h, of 300 m at 06.00 h, rising to 1300m at 14.00h;

maintained at 1300 m till early evening, then 300 m again. – Rate equation:

dCi/dt = Si –Li(Ci )-viCi/h - wiCi/h -{wv(Ci-Ci0)/h}

POCP II

• Emissions (VOCs and NOx) estimates utilise 3 emissions inventories, UN ECE EMEP; EC CORINAIR and UKNAEI. These give total VOC emissions, which are speciated into 135 organic compounds + methane, using the UK emissions inventory.

• The master chemical mechanism is used to describe the chemistry and photochemistry.

• The coupled differential equations are integrated using the FACSIMILE integrator. Most concentrations are set initially to zero, except for NO, NO2, SO2, CO, methane, HCHO, ozone and hydrogen.

• The air parcel is carried on a straight line trajectory at 4 m s-1

Calculation of POCP values:‘Photochemical Trajectory Model (PTM)’

VOC and NOX

sunlight

chemistry and transport

emissions

calculate ozone along pre-selected trajectories

over Europe

well-mixedboundarylayer box

POCP III( see Derwent et al , Atmos Environment, 1996, 30, 181-199)

• The POCP is calculated by incrementing the emissions of each of the VOCs in turn by 4.7 kg km-2 across the entire domain. (corresponds to an increase in total VOC emissions of 4%)

• The ozone formed over the 5 day trajectory is increased as a result and by different amounts for each VOC. The POCP of the ith VOC is given by:

POCPi = 100x(ozone increment with the ith VOC)

(ozone increment with C2H4)

• Examples (ethene = 100):

methane = 3; ethane = 14, propane = 41, butane = 60

isoprene = 118

benzene = 33; toluene = 77; m-xylene = 109; 1,2,4 TMB = 130

VOC POCP VOC POCP

ethene (reference VOC) 100.0

benzene 20.3 1,2,4,-trimethylbenzene 113.0

toluene 51.0 1,3,5,-trimethylbenzene 106.2

ethylbenzene 52.5 o-ethyltoluene 69.4

o-xylene 84.1 m-ethyltoluene 74.0

m-xylene 85.6 p-ethyltoluene 73.2

p-xylene 77.5 1-ethyl-3,5-dimethylbenzene 106.4

propylbenzene 42.7 1,3-diethyl-5-methylbenzene 101.0

i-propylbenzene 35.3 styrene 14.5

1,2,3,-trimethylbenzene 108.2 benzaldehyde -10.4

Notes a POCP values are quoted to one decimal place, not as an indication of inherent precision, but to facilitate comparisons. The precision in an individual POCP value is estimated to be 2 POCP units.

MCM v3 POCP values

Global budget for ozone (Tg O3 yr-1)

• Chemical production 3000 – 5000

HO2 + NO 70%

CH3O2 + NO 20%

RO2 + NO 10%

• Transport from stratosphere 400 – 1100

• Chemical loss 3000 – 4200

O1D + H2O 40%

HO2 + O3 40%

OH + O3 10%

others 10%• Dry deposition 500 - 1500

GLOBAL BUDGET OF TROPOSPHERIC OZONE – recent calculations

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, VOC

NO2

h

STRATOSPHERE

TROPOSPHERE

8-18 km

Chem prod in troposphere

4920 Chem loss in troposphere

4230

Transport from stratosphere

475 Deposition 1165

GEOS-CHEM model budget terms, Tg O3 yr-1

-0.5

0

0.5

1

1.5

2

2.5

HCHO JULY 1996 (molec cm-2)

Biogenic

Biomass Burning

Quantifying emissions of natural VOCs using HCHO

column observations from space

Paul I. Palmer

GOME

HCHO columns – July 1996HCHO columns – July 1996

[1016molec cm-2]

GEOS-CHEM HCHO GOME HCHO

[1012 atoms C cm-2 s-

1]

GEIA isoprene emissions

BIOGENIC ISOPRENE IS THE MAIN SOURCE OF HCHO IN U.S. IN SUMMER

GOME footprint320X40 km2

Cumulative HCHO yield per C atom from isoprene oxidation. ([O3] = 40 ppb, [CO] = 100 ppb,

[isoprene] = 1ppb. CO, NOx, O3 held constant.)

• Full MCM mechanism.

• Final yield increased from GEOS-CHEM by 16% for high NOx, 65% low NOx

0 20 40 60 80 100 120 1400.00

0.05

0.10

0.15

0.20

0.25

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0.45

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Cum

mul

ativ

e H

CH

O Y

ield

from

isop

rene

oxi

datio

n (p

er C

)

TIME (HOURS)

NOX = 0.1 PPB

NOX =1 PPB

Vertical lines denote midnight of each day

HCHO formation from pinene

acetone, which has a long atmospheric lifetime, is an

intermediate in HCHO formation

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

200

400

600

800

1000[A

PIN

EN

E] P

PT

DAYS

NOX = 1 PPB NOX = 100 PPT

0 2 4 6 8 10 12 14 16 18 20 220.00

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0.40

HC

HO

YIE

LD

PE

R C

RE

AC

TE

D

DAYS

NOX = 100 PPT NOX = 1 PPB

Decay of pinene

CONCENTRATION OF CH3COCH3 FORMAED FROM 1PPB APINENE

0

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900

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

DAYS

[CH

3CO

CH

3] P

PT

CH3COCH3 NOX1PPB

CH3COCH3 NOX100PPT

Relating HCHO Columns to VOC Emissions (Palmer)

VOC source

Distance downwind

HCHO Isoprene

-pinenepropane

100 km

VOC

HCHOhours

OH

hours

h, OH

Ultimate Yield Y (per C)

Approx. Time to Y

isoprene ~0.5 2-3 hrs

pinene ~0.3 3-4 days

pinene ~0.25 3-4 days

MBO ~0.4 3-4 days

Master Chemical Mechanism