Physik der Atmosphäre IIU. Platt

SS 2009, Tuesday, 16:15-17:45, INF 229 , SR 108/110

31. 3. Introduction - Gas-Phase Reaction Kinetics Pl

7. 4. Tropospheric Chemistry: O3, HxOy, Oxidation Capacity Pl

14. 4. Tropospheric Chemistry: N-, S-, Halogen – Cycles Pl

28. 4. Stratosphere, Radiation and Matter Cycles Pl

5. 5. Strat. Chemistry: Chapman Cycle and Extensions Pl

12. 5. Strat. Chemistry: Halogen Chemistry, The Ozone Hole Pl

19. 5. Isotopes in Atmospheric Research Pl

26. 5. The Carbon Cycle Pl

2. 6. The Hydrological Cycle Pl

9. 6. Liquid Phase Chemistry (atmspheric Aerosol) Pl

16. 6. Aerosol Physics I: Aerosol Mechanics Pl

23. 6. Aerosol Physics II: Particle Formation, Particle Growth Pl

30. 6. Measurement Techniques Pl

7. 7. Modeling Atmospheric Chemistry and Physics Pl

Physics of the Atmosphere Physik der Atmosphäre

WS 2010

Ulrich PlattInstitut f. Umweltphysik

R. 424Ulrich.Platt@iup.uni-heidelberg.de

Last Week

• The carbon cycle has a strong influence on our climate(both, in natural changes and anthropogenic changes)

• Important carbon reservoisrs are the sediments, the ocean, the land biomass, and the atmosphere

• Time constants in the carbon cycle range from years to 105 years

• Oceans get more acidic less carbon uptake

• The role of the biomass is unclear

Contents

Ozone, Hydrogen Radicals (HXOY ), and the Oxidation Capacity

Examples: OH, HO2, as well as the "stable" Molecules

H2O2 (hydrogen peroxyde) and CH3OOH (methyl hydroperoxide).

1st Definition:Radicals are a combination of atoms that have one or several unpaired electrons. (i.e. S  0).This definition, however, includes O2, NO or NO2.

2nd Definition:Radicals are (short lived) combinations of atoms, which play the role of elements and can combine like these with elements and with themselfes.(Justus von Liebig)

First question: What are „Free Radicals“?

Why are Radicals important?Atmospheric mixing ratios of radicals are exceedingly low e.g. about 1 OH-radical for every 1013 air moleculesBut:In chemical reactions with molecules the radical status is „hereditary“ e.g.:OH + CO CO2 + H

Example: oxyhydrogen gas (Knallgas):

H2 + O2 HO2 + H „Start“

H2 + HO2 OH + H2O PropagationH2 + OH H + H2O (inherit radical status)

H + O2 OH + O BranchingO + H2 OH + H (from one radical we get two)

OH + HO2 H2O + O2 Termination

Net: 2 H2 + O2 2 H2O

Consequence: The radikal concentration and thus the speed of the reaction can increase exponentially.

Does that also happen in the atmosphere?

Free Radicals - Which ones are Important in Atmospheric Chemistry?First suggestion of OH reactions in the atmosphere by B. Weinstock 1969.Formation of OH by:

O3 + h O(1D) + O2(1)O(1D) + H2O OH + OH

Role of OH in the formation of CO and formaldehyde suggested [H. Levy 1971].Role of OH in tropospheric ozone formation (P. Crutzen 1974)

Other Radicals (by either definition)? O2(1), Cl-atoms, NO3 ...

HOX - Cycle

OH Degradation of most VOC  Key intermediate in O3 formation 

NOX NOY conversion

HO2

Intermediate in O3 formation 

Intermediate in H2O2 formation

RO2

Intermediate in ROOR´ formation  Aldehyd – precursor  PAN – precursor  Intermediate in O3 formation

XO Catalytic O3 destruction (X = Cl, Br, I)

Degradation of DMS (BrO)  Particle formation (IO)  Change of the Leighton – Ratio

X Degradation of most (some) VOC: Cl (Br)  Initiates O3 formation 

RO2 – precursor

NO3

Degradation of certain VOC NOX NOY conversion (via N2O5) 

RO2 – precursor

Degaradation of Trace Species by Reaction with OH- or other Free Radicals.

Mean Lifetime in Hours against Radical Reaktions

Spezies OH

mittelw. OH

(peak) NO3 (av.)

NO3 (peak)

ClO (av.)

ClO (peak)

Cl (av.)

Cl (peak)

BrO (av.)

Br (av)

Typ. Konz. in Molek.

cm-3 6105 1107 1.5108 11010 106 108 103 105 108 5106

DMS 110a 6.3a 1.7a 0.03a 3104 c 280c 870f 8.7f 10c 26g

n-Butane 190a 11a 2.8104

a 430 a - 1400h 14h

C2H4 56b 3.4b 1104 h 160h 2800b 28b

Isoprene 4.6h 0.3h 1.9h 0.03h - <1100d <11d 0.9e

Toluene 77h 4.6h 3.1104

h 460h -

CH4 7104 a 4103 a >5104 h >700h >7107

b >7105

b 3106 a 3104 a

A X A,XA X

Degradation of species A by radical X :

1d kA X A tk Xdt

OH Formation in the Atmosphere

The most important source of OH-radicals is the photolysis of ozone by UV-radiation with wavelengths < ca. 320 nm (R1a) (UV-B region) or  411nm (R1b):

O3 + h O(1D) + O2(1) (R1a)

O3 + h O(1D) + O2 (3) (R1b)

O3 + h O(3P) + O2 (3) (R1c)

Reaction R1b ist spin forbidden, thus unlikely.

O (1D) + M O (3P) + M (R2)

O (1D) + H2O OH + OH (R3)

OH - yield:

3 2

32 22a 2b 2

k H OV

k k kN H OO

Photochemical OH - Formation

2

1

J I d

Photolysis Frequency:

Annual Variation of the Photochemical OH – Formation Rate at Mid-Latitudes

Month

OH

Rat

e of

For

mat

ion

cm-3s-1

The HXOY - Cycle

CH2O

HO2OH

NO2 NO

H2O2HNO3

HONO

OH(O2)

NO(O2)

CH3O2

O3(O2)

O3

h

O3

O3

CO(O2)

H2(O2)

CH2O(O2)

H2O

h,

OH

CH4(NMHC)

H2O

h(O2)OH(O2)

[HOX] P(OH) „Final“ OH Sink

„Primary“OH

Source

P(OH)

[HOX] P(OH)

CH3O2

(CnH2n+1-O2)

HCHO

O3

Olefins

O3

Diurnal Variation of the Various

OH – Sources in an Urban Region

(Berlin)(Alicke 2000)

103

104

105

106

107

103

104

105

106

107

103

104

105

106

107

103

104

105

106

107

time [GMT]

from J(O1D) J(HONO) VOC's + O

3

J(HCHOrad

)

OH

pro

duct

ion

[mol

ec./(

cm3 s)

]

00:00 04:00 08:00 12:00 16:00 20:00 24:00

00:00 04:00 08:00 12:00 16:00 20:00 24:00

21.7.1998

20.7.1998

00:00 04:00 08:00 12:00 16:00 20:00 24:00

Nitrous Acid (HONO)

H2O(g) H2O(ads)(a)

NO2(g) NO2(ads) (b)

H2O(ads) + NO2(ads) H2O.NO2(ads) (c)

NO2(ads) + H2O.NO2(ads) HNO3(ads) + HONO(g)(d)[ HNO3(ads) + HONO(g) 2 NO2(g) + H2O(g) ]

The OH – Cycle as Function of the NOX -

Concentration

[NO ] = 0X

86 %

14 %

O H

O H

4* 1 0 c m6 -3

96 %

+ HO 2 4 %H O

H O(Pe ro xid e )

(Pe ro xid e )

HO 2

2

2 2

[NO ] = 2 p p b

3

2

2

2

3

X20 %

80 %

se c

se c

O H

O H

14 * 10 c m6 -3

+ N O

H O

17 %

1 %

82 %

+ HO2

2

2

2 %

HN O 3

H O

2

H O 2

2

80 %

[NO ] = 100 p p bX

3 70 %

30 %

20 %C H O + h(+ O )

2

2

O H

3* 1 0 c m6 -3

se cO H

HO

2

90 % HN O 3

+ N O

2

10 %

+ C O+ KW

O + h+ H O

O + h+ H O

O + h+ H O

+ NO

+ NO

82%

+ O3

Model Calculations of the OH - Concentration as Function of the NOX (Oxides of Nitrogen) - Level

(D. Poppe)

Concentrations of OH and HO2 as a function of the NOX level

Conditions: 37.6 ppb ozone, 88.7 ppb CO, 0.92 ppb CH2O, J(O3)=9.110‑6 s‑1, J(NO2)=

9.110‑6 s‑1. The HO2 concentration and

therefore H2O2 production rate are highest in

unpolluted air at NOX levels below a few

100ppt. Figure from Ehhalt [1999]

Observed Diurnal Variation of the OH - Concentration

Red Circles: Measurements by Laser-Induced Fluorescence, LIF by A. Hofzumahaus et al., Jülich)

Blue, drawn Line: Ozone – Photolysis Frequency

BERLIOZ81% of OH dependent on J(O3)

0.0 1.0 2.0 3.0 4.0 5.0

0

2

4

6

8

10

12

14

J(O1D) / 10

-5 s

-1

[ O

H ]

/

1

06 cm

-3

NOX Dependence of OH -Observation

0.01 0.1 1 10 1000

2

4

6

8POPCORN 94

BERLIOZ 98

ALBATROSS 96

NOx / ppb

OH

/

106 c

m-3

Forschungszentrum

Jülich

OH (LIF) Summer & Winter

0.E+00

1.E+06

2.E+06

3.E+06

4.E+06

00:00 06:00 12:00 18:00 00:00

[OH

] / m

olec

ule

cm-3

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

j(O1 D

) / s

-1

OH summer winter

j(O1D) summer winter

D.E.Heard, J.D.Lee, D.J. Creasey, University of LeedsL.J.Carpenter (University of York) c.f. George et al., 1999

Average ROX diurnal profile (Chemical Amplifier) during

BERLIOZ (July 14 to 15, 17 to 18, 24 to 26, Aug. 3, total of 8 days in 1998)

NO3 Data from Dieter Perner, in Platt et al. 2002

00:00 06:00 12:00 18:00 24:00

0

2

4

6

8

10

12

14

RO

2 [pp

t]

NoontimeRO2 Peak

NighttimeRO2 Peak:

NO3 + VOC

Radical Gap:no J(O3) but

still J(NO3)

HOX in the Upper Troposphere

Too much HOX ?

--> No, have to include other sources Photolysis of acetone, aldehydes, peroxides, ...

However, do these additional sources always explain observed HOX?

OH in the UT controlled by P(HOx) and NOx

Jaeglé et al., Atmos. Env. 2001

HOX Source Gases in the Free Troposphere(Pacific Ocean, spring 1999, Singh et al., Nature 2001)

SH,0-30oS, 165oE-100oW

NH,0-30oN, 170o-120oW

Nighttime Peroxy Radicals

0

2

4

6

8

10

12

14

0 3 6 9 12

Hour of night /GMT - 18 h

HO

2 +

RO 2

, N

O3 /p

ptv

0

5

10

15

20

25

30

35

40

45

O3 /p

pb

v

HO2 + RO2

NO3

O3

Salisbury et al. 2001

12:00 18:00 24:00 30:00 36:000

2

4

6

8

III

NO 3

[ppt]

0.0

0.2

0.4

0.6

0.8

1.0

II

J(NO3)

J(1OD)

arb

itra

ry u

nits [

s-1]

0

2

4

6

8

10

12

14

12:00 18:00 24:00 30:00 36:00

I

06:00 12:00

12:0006:00

RO

2 [p

pt]

NO3 Data from Dieter Perner, Platt et al. 2002

NO3 + olefins

O3 + olefins

Observed Diurnal Variations of OH-, HO2-,

RO2- and NOX- Concentrations During

BERLIOZ 1998 (Platt et al. 2002)

hohes NOX niedriges NOX

What is the Evidence for OH in theAtmosphere ?

What is the Evidence for HO2 (RO2) in the Atmosphere ?

Simplified Outline of the NOX- (=NO + NO2) and NOY- Cycles

N O 3 N O N O 2 E m is s io n R O 2

N 2 O 5

N 2 O

H N O 2 3NO

A e ro s o l

H N O 3

H O 2 N O 2 P A N

o x id a t io n s ta g e : 2 3 4 5 6

R O 2

< 1 0 % > 9 0 %

H 2 O liq .

H 2 O liq .

O H

O H

h

h h

O 3 O 3

N O , N O 2 , N O 3

?

V O C s

Temperature dependent!

Photolysis of the Nitrate Radical

The NO3 Spectrum Product Yield of NO3 Photolysis

NO3 Field Measurements

Cavity Ringdown(Boulder, CO)Brown et al. 2001 - Ravishankara

DOAS(Edwards AFB)Platt et al. 1984

NO3 Vertikal Profile - Schematic

z

NO2

NO3

NO, olefins

Low NO3 production(little NO3)

high NO3 destruction(lot of NO, terpenes)

1 km

0

Distance

Direction of Observation

SZA

OdenwaldMountains

Terminator

Institut für Umweltphysik

Z

NO3 - Vertical Profile Measurements

Geyer et al. 2001

Relative Importance of the Different NO3 - Loss Processes During BERLIOZ 1998

VOCsOOHNOfOOHNOVOCsdtd

oc 33 //33 ]//[:

- a ve ra g e o f a ll B E R LIO Z -d a ta- 2 4 h - m e a n va lu e s :

oc [cm -3s -1]N O 3 4 .51 0 5

O H 8 .11 0 5

O 3 2 .41 0 5

21

)()( 3

OHocNOoc

O H5 4 %

O 3

1 6 %

N O 3

3 0 %

Contribution of NO3 to the Atmospheric Oxidation Capacity in Pabstthum (BERLIOZ 1998)

Geyer et al. 2001

The Oxidation Capacity by Radical X

.]X][Y[k1i

iXYi

OC

Ozone Formation in the Troposphere

Initial Idea: Stratosphere

Troposphere

No Chemistry, since(  242 nm) = 0

O3 – Flux

5-81010

Molec.cm-2s-1

O3 – Depos.

3-61010

Molec.cm-2s-1

Stratosphere

Troposphere

CO, HC O3

O3 + h + H2O OH

O3 – Flux

5-81010

Molec.cm-2s-1

O3 – Depos.

3-61010

Molec.cm-2s-1

Today:

30-501010

Molec.cm-2s-1

Penetration-Depth of UV - Radiation

DeMore et al., 1997

Intensity at 0, 20, 30, 40, 50 km altitude)

NOX- and HOX - Katalysis of the Photochemical

Ozone Formation in the Troposphere

Ozone formation rate P(O3) :P(O3) = [NO]·(k1[HO2]+ki·[RO2]i)

Disturbance of the Leighton Ratio and the rate of O3 Formation

There is no net ozone production. However, if other reactions, in particular by Peroxy radicals oxidise NO to NO2:

RO2 + NO NO2 + RO (R4)

Ozone will be formed at the rate PO3. Also the Leighton Ratio will be reduced:

12 3 R 2

[NO] JL

[NO ] k [O ]+k [RO ]

O3 11 0

1 1P j NO

L L

The rate of ozone formation, PO3 in a given airmass can be calculated from

measurements of the Leighton Ratio [NO]/ [NO2]=L1 together with [O3], and J:

O3 R 2P k RO

Since PO3 relates to the concentration of peroxy radicals (RO2), there concentration can also be inferred from these measurements.

Smog Chamber Experiments Ozone IsopletsLines of constant O3 mixing ratios (ppb)

NOX-limited

Hydrocarbon-limited

Ozone Formation as a Function of NOX Level

Ehhalt 1998, Science 279. No. 5353, 1002 –

1003, DOI: 10.1126/science.279.5353.1002

Averaged O3 production rates PO3 calculated from simultaneous observations of NO, NO2, O3, OH, HO2,

H2CO, actinic flux, and T. Data were placed into three PHOx bins: high (0.5 < PHOx < 0.7 ppt/s, circles),

moderate (0.2 < PHOx < 0.3 ppt/s, squares), and low (0.03 < PHOx < 0.07 ppt/s, triangles), and then averaged as

a function of NO. All three PHOx regimes demonstrate the expected dependence on NO: PO3 increases linearly

with NO for low NO (<600 ppt NO), PO3 becomes independent of NO for high NO (>600 ppt NO). Crossover

between NOX - limited and NOX - saturated PO3 occurs at different levels of NO in the three PHOx regimes.

Observed O3 Production

Rates 'Southern Oxidant Study‘ June 15 to July 15, 1999 at Cornelia Fort Airpark,

Nashville, Tennessee [Thornton et al. 2002]

Diurnal variation of O3 levels in different air masses during August 11-14, 2000: Forrest (Weltzheimer Wald) and city of

Heilbronn (southern Germany).

Landesanstalt für Umweltschutz Baden-Württemberg and UMEG)

Vertical profiles of the concentrations of ozone, NO, H2O2, and humidity (dew point).

The maximum H2O2 concentration occurs in an altitude range layer (ca. 1.1 - 1.4 Km) with high humidity but low NO [Tremmel et al. 1993].

The Evolution of Tropospheric Ozone

Monks et al. 2009

Ozon – Annual Variation at Different Altitudes

Isobars 200, 300, 500, 800 hPa

Logan, J. Geophys. Res., 16115-16149, 1999.

Ozon – Annual Variation at Sea Level

Logan, J. Geophys. Res., 16115-16149, 1999.

Summary

•OH is the most important free radical in the Atmosphere, since it inititates the degradation of most oxidisable air pollutants it is sometimes called the „cleansing agent of the atmosphere“

•However, there are several other relevant Radical in the Atmosphere

•Ozone is a central species, in the troposphere it is mostly formed by reactions catalysed by OH