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

~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 = 0.7 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 = 3x1013 moles yr-1

SCH4 = 3x1013 moles yr-1

SCO = 8x1013moles yr-1

SCO+ SCH4 > 2FO3 OH would be titrated!

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

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

GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, VOC

NO2

h

STRATOSPHERE

TROPOSPHERE

8-18 km

Chem prod in troposphere,

Tg y-1

4300

1600

Chem loss in troposphere,

Tg y-1

4000

1600Transport from stratosphere,

Tg y-1

400

400

Deposition,

Tg y-1700

400Burden, Tg 360

230

Lifetime, days 28

42

Present-day Preindustrial

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 columns(Mar-Apr 01)

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

SHORT QUESTIONSSHORT QUESTIONS

1. How does a thinning of the stratospheric ozone layer affect tropospheric OH concentrations?

2. 2. If the CO source to the atmosphere were to double, would the CO concentration (a) double, (b) less than double, (c) more than double?

3. Methylperoxy radicals produced from methane oxidation can self-react to form methanol:    CH3O2 + CH3O2 CH3OH + CH2O + O2

What is the effect of this reaction on OH levels?

METHANE: #2 ANTHROPOGENIC GREENHOUSE GASMETHANE: #2 ANTHROPOGENIC GREENHOUSE GASGreenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]

Referenced to concentration Referenced to emission

GLOBAL METHANE SOURCES, Tg a-1 [IPCC, 2007]

ANIMALS80-90

LANDFILLS40-70

GAS50-70

COAL30-50RICE

30-110

TERMITES20-30

WETLANDS100-230

BIOMASSBURNING10-90

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 20 years

IPCC [2007]

600

800

700

Scenarios

A1BA1TA1F1A2B1B2IS92a

900

Year

IPCCIPCC [2001] Projections of Future [2001] Projections of Future CHCH44 Emissions (Tg CH Emissions (Tg CH44) to 2050) to 2050

2000 2020 2040

NONOxx EMISSIONS (Tg N a EMISSIONS (Tg N a-1-1) TO TROPOSPHERE) TO TROPOSPHERE

FOSSIL FUEL 23.1

AIRCRAFT 0.5

BIOFUEL 2.2

BIOMASSBURNING 5.2

SOILS 5.1

LIGHTNING 5.8

STRATOSPHERE 0.2

USING SATELLITE OBSERVATIONS OF NOUSING SATELLITE OBSERVATIONS OF NO22 TO MONITOR NO TO MONITOR NOxx EMISSIONS EMISSIONS

SCIAMACHY data. May-Oct 2004

(R.V. Martin, Dalhousie U.)

detectionlimit

NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006)NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006)

March 2006

LIGHTNING FLASHES SEEN FROM SPACE (2000)LIGHTNING FLASHES SEEN FROM SPACE (2000)

DJF

JJA

PEROXYACETYLNITRATE (PAN) AS RESERVOIR PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NOFOR LONG-RANGE TRANSPORT OF NOxx

NOAA/ITCT-2K2 AIRCRAFT CAMPAIGN IN APRIL-MAY 2002 NOAA/ITCT-2K2 AIRCRAFT CAMPAIGN IN APRIL-MAY 2002 Monterey, CAMonterey, CA

Asian pollution plumes transported to California

CO

O3

PAN

HNO3

May 5 plume at 6 km:High CO and PAN,no O3 enhancement

May 17 subsidingplume at 2.5 km:High CO and O3,PAN NOxHNO3

Hudman et al. [2004]

NOx

NOx

HNO3

PAN

O3

CO

TROPOSPHERIC OZONE COLUMN DATA FROM SPACETROPOSPHERIC OZONE COLUMN DATA FROM SPACE

June-August 2006 observations

TROPOSPHERIC OZONE: TROPOSPHERIC OZONE: #3 ANTHROPOGENIC GREENHOUSE GAS#3 ANTHROPOGENIC GREENHOUSE GAS

Greenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]

Referenced to concentration Referenced to emission

IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC OZONE (0.35 W mOZONE (0.35 W m-2-2) RELIES ON GLOBAL MODELS) RELIES ON GLOBAL MODELS

Preindustrialozone models

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Observations at mountain sites in Europe [Marenco et al., 1994]

…but these underestimate the observed rise in ozone over the 20th century

Fitting to observations would imply a radiative forcing of 0.8 W m-2

1996-2005 NO1996-2005 NOxx EMISSION TREND SEEN FROM SPACE EMISSION TREND SEEN FROM SPACE

Van der A et al., 2008

RECENT TRENDS IN TROPOSPHERIC OHRECENT TRENDS IN TROPOSPHERIC OHinferred from methylchloroform observationsinferred from methylchloroform observations