QUESTIONSQUESTIONS

1. If CFC concentrations were to double, how much faster would ozone loss in Antarctica proceed?

2. The ozone hole is of limited vertical extent because PSCs form only in the lowest part of the polar stratosphere. Why don’t they form at higher altitudes too?

CHAPTER 11: TROPOSPHERIC CHEMISTRYCHAPTER 11: TROPOSPHERIC CHEMISTRY

THE ATMOSPHERE: OXIDIZING MEDIUM THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLESIN GLOBAL BIOGEOCHEMICAL CYCLES

EARTHSURFACE

Emission

Reduced gasOxidized gas/aerosol

Oxidation

Uptake

Reduction

Atmospheric oxidation is critical for removal of many pollutants, e.g.• methane (major greenhouse gas)• CO (toxic pollutant)• HCFCs (Clx sources in stratosphere)

THE TROPOSPHERE WAS VIEWED AS THE TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970CHEMICALLY 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, Science, 1971, 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?WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(Production of O(11D) in troposphere takes place in narrow band [290-320 nm]D) in troposphere takes place in narrow band [290-320 nm]

solar flux I

ozone absorptioncross-section

O(1D)quantumyield

I

O3 + hO2 + O(1D) (1)

O(1D) + M O + M (2)

O(1D) + H2O 2OH (3)

Primary source:

Sink: oxidation of reduced species

CO + OH CO2 + H

CH4 + OH CH3 + H2O

HCFC + OH H2O + …

Major OH sinks

GLOBAL MEAN [OH] = 1.0x106 molecules cm-3

~tropopause

10 ppmv

40 ppbv

TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHEREGLOBALLY IS IN THE TROPOSPHERE

• 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 = 2 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 UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work.FROM STRATOSPHERE…but that cannot work.

FO3 = 2x1013 moles yr-1

SCH4 = 3x1013 moles yr-1

SCO = 10x1013moles yr-1

SCO+ SCH4 > 2FO3 OH would be titrated!

Recycling of OH involving NOx is critical, and this recycling drives tropospheric ozone production

CHAIN MECHANISM FOR OCHAIN MECHANISM FOR O33 PRODUCTION: PRODUCTION:

CO OXIDATIONCO OXIDATION

Initiation: source of HOx (OH production)

Propogation: CO + OH CO2 + HH+ O2 + M HO2 + M

HO2 + NO OH + NO2

NO2 + hv (+O2) NO + O3

NET: CO + 2O2 CO2 + O3

Termination: by loss of HOx (self reaction of HO2) Propagation efficiency of the chain determined by the abundance of NOx

NOTE: HOx and NOx catalyze O3 production in the troposphere, and O3 destruction in the stratosphere! The key difference is that [O3] and [O] are much lower in the troposphere, thus NO2 does not react with O, and OH is far more likely to react with CO, HC, etc. than with O3

RADICAL CYCLE CONTROLLING TROPOSPHERIC OH RADICAL CYCLE CONTROLLING TROPOSPHERIC OH AND OZONE CONCENTRATIONSAND OZONE CONCENTRATIONS

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, CH4

NO2

h

STRATOSPHERE

TROPOSPHERE

8-18 km

SURFACE

CARBON MONOXIDE IN ATMOSPHERECARBON MONOXIDE IN ATMOSPHERE

Source: incomplete combustionSink: oxidation by OH (lifetime of 2 months)

SATELLITE OBSERVATION OF CARBON MONOXIDESATELLITE OBSERVATION OF CARBON MONOXIDE

MOPITT CO (2000)

SATELLITE OBSERVATIONS OF BIOMASS FIRES SATELLITE OBSERVATIONS OF BIOMASS FIRES (1997)(1997)

GLOBAL DISTRIBUTION OF COGLOBAL DISTRIBUTION OF CO

NOAA/GMD surface air measurementsNOAA/GMD surface air measurements

SPACE-BASED METHANE COLUMN OBSERVATIONSSPACE-BASED METHANE COLUMN OBSERVATIONS

by solar backscatter at 2360-2385 nm

GLOBAL DISTRIBUTION OF METHANEGLOBAL DISTRIBUTION OF METHANENOAA/CMDL surface air measurementsNOAA/CMDL surface air measurements

Sink: oxidation by OH (lifetime of 10 years)

HISTORICAL TRENDS IN METHANEHISTORICAL TRENDS IN METHANE

The last 1000 years

The last 30 years

CHAIN MECHANISM FOR OCHAIN MECHANISM FOR O33 PRODUCTION: PRODUCTION:

CHCH44 OXIDATION OXIDATIONInitiation: source of HOx (OH production)Propogation:

CH4 + OH CH3 + H2OCH3 + O2 + M CH3O2 + M

CH3O2 + HO2 CH3OOH + O2

CH3O2 + NO CH3O + NO2

CH3OOH + OH CH2O + OH + H2O CH3OOH + OH CH3O2 + H2OCH3OOH + hv CH3O + OH

CH3O + O2 CH2O + HO2

CH2O + OH CHO + H2OCH2O + hv + O2 CHO + HO2 CH2O + hv CO + H2

CHO + O2 CO + HO2

(…then CO oxidation…)

Oxidation from C(-IV) in CH4 through to C(+IV) in CO2

*

*

**

*

Ozone production from NO2 photolysis following peroxy+NO rxns (where peroxy radicals generated by reactions above)

High NOx: CH3O2 and HO2 react only with NO, and CH2O removed only by photolysisCH4 + 10O2 CO2 + H2O + 5O3 + 2OH

Low NOx: CH3O2 reacts with HO2, CH3OOH reacts with OH and CH2O reacts with OHCH4 + 3OH + 2O2 CO2 + 3H2O + HO2