TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY

Troposphere

Stratosphere:90% of total

The many faces of atmospheric ozone:

In stratosphere: UV shield

In middle/upper troposphere: greenhouse gas

In lower/middle troposphere: precursor of OH, main atmospheric oxidant

In surface air: toxic to humans and vegetation

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes

at different temperatures

THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES

EARTHSURFACE

Emission

Reduced gas Oxidized gas/aerosol

Oxidation

Uptake

Reduction

Atmospheric oxidation is critical for removal of many pollutants, e.g.• methane (major greenhouse gas)• Toxic gases such as CO, benzene, mercury…• Gases affecting the stratosphere

Example: Biogeochemical cycle of mercury

Hg(0) Hg(II)

particulateHg

burial

SEDIMENTS

uplift

volcanoeserosion

oxidation

Hg(0) Hg(II)reduction biological

uptake

ANTHROPOGENIC PERTURBATION:fuel combustion

mining

ATMOSPHERE

OCEAN/SOIL

VOLATILE WATER-SOLUBLE

(months)

CO and methane account for most of reduced gas flux to atmosphere

• Methane observed from space: 1650-1800 ppb

• CO observed from space: 50-200 ppb

THE TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970

• “The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane and CO are chemically quite inert in the troposphere” [Cadle and Allen, Atmospheric Photochemistry, Science, 1970]

• Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967]

FIRST BREAKTHROUGH:

• Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969]

SECOND BREAKTHROUGH:• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, J. Geophys. Res. 1973]

THIRD BREAKTHROUGH:• Methylchlroform observations provide indirect evidence for OH at levels of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]

…but direct measurements of tropospheric OH had to wait until the 1990s

WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux I

ozone absorptioncross-section

O(1D)quantumyield

I

MEAN VERTICAL DISTRIBUTION OF ATMOSPHERIC OZONE:only 10% is in the troposphere

OZONE CHEMISTRY IN STRATOSPHERE

2

2 3

13 2

1

2

( )

( )

O h O OO O M O M

O h O O D

O D M O MXO O X O

O2+hvO3+hv

By contrast, in troposphere:

• no photons < 240 nmno oxygen photolysis;

• neglible O atom conc.no XO + O loss

• Estimate ozone flux FO3 across tropopause (strat-trop exchange)– Total O3 col = 5x1013 moles– 10% of that is in troposphere– Res. time of air in strat = 1.4 yr

• Estimate CH4 source SCH4:– Mean concentration = 1.7 ppmv– Lifetime = 9 years

• Estimate CO source SCO:– Mean concentration = 100 ppbv– Lifetime = 2 months

UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work.

FO3 = 3x1013 moles yr-1

SCH4 = 3x1013 moles yr-1

SCO = 9.7x1013moles yr-1

SCO+ SCH4 > 2FO3 OH would be titrated!

We need a much larger source of tropospheric ozone

CONSTRAINT ON CROSS-TROPOPAUSE OZONE FLUXFROM OBSERVED OZONE-NOy CORRELATION

IN LOWER STRATOSPHERE

EN2O = 13 Tg N yr-1 (±17%)

FN2O = EN2O

12 ( ) 2N O O D NO

Oxidation products (HNO3, etc.)

2/ 0.073 ( 14%)yNO N O

3in lower strat.: / 0.0033 ( 12%)yNO O

2 2 -13 3

3

( / )540 140 Tg O yr

/N O y

Oy

F NO N OF

NO O

tropopause

NOy chemical family

OZONE LOSS IN TROPOSPHERE

tropopause-1

3 3540 140 Tg O yrOF 1

2

3 2 2

2 3 2

( ) 2

2

O D H O OHOH O HO OHO O OH O

Chemical loss:

-13 31000 200 Tg O yrOD

-13 34600 700 Tg O yrOL

deposition

Ozone chemical loss is driven by photolysis frequency J(O3 O(1D)) at 300-320 nm:

0

( ) ( ) ( )J q I d

dJ/d

, 1

0-6 s

-1 n

m-1

Closing the tropospheric ozone budget requires a tropospheric chemical source >> FO3

OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)

catalyzed by hydrogen oxide radicals (HOx) and nitrogen oxide radicals (NOx)

HOx = H + OH + HO2 + RO + RO2

NOx = NO + NO2

Oxidation of CO:

2

2 2

2 2

2

2 3

2 2 3Net: 2

CO OH CO HH O M HO MHO NO OH NONO h NO OO O M O M

CO O CO O

2

2

2 2

2 2

2 3

2 2

2 2

2 3 2

'

Net: 4 ' 2

O

RH OH R H OR O M RO MRO NO RO NO

NO h NO ORO O R CHO HOHO NO OH NO

RH O R CHO O H O

Oxidation of VOC:

RO can also decompose or isomerize; range of carbonyl products

Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)

OH can also add to double bonds of unsaturated VOCs

GLOBAL BUDGET OF TROPOSPHERIC OZONE (Tg O3 yr-1)

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, VOC

NO2

h

STRATOSPHERE

TROPOSPHERE8-18 km

Chem prod in troposphere 4700

±700Chem loss in troposphere 4200

±500Transport from stratosphere 500

±100Deposition 1000

±200

IPCC (2007) average of 12 models

Ozone lifetime: 24 ± 4 days

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation

NOx-saturatedregime

NOx-limited regime Ridge