Biogeochemical Cycles of Methane and Nitrous Oxide

AOSC 433/633 & CHEM 433/633

Ross Salawitch

Class Web Site: http://www.atmos.umd.edu/~rjs/class/spr2013

Goals :

• CH4

sources and sinks

lifetime

recent reduction in d(CH4)/dt

• N2O

sources and sinks

• Connection of CH4 and N2O to stratospheric O3

Lecture 06

12 February 2013

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Radiative Forcing of Climate, 1750 to 2005

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CH4 & N2O have contributed

about 38% of the RF of CO2

IPCC 2007 FAQ

What do these time series resemble ?

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CO2, CH4, & N2O

Chapter 3, Chemistry in Context

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CH4 is the most reduced form of carbon

CO2

Carbon dioxide

+4

CO

Carbon Monoxide

CH2O

Formaldehyde

CH4

Methane

+20-4

CO2

Carbon dioxide

+4

CO

Carbon Monoxide

CH2O

Formaldehyde

CH4

Methane

+20-4

Decreasing oxidation number (reduction reactions)Decreasing oxidation number (reduction reactions)

Increasing oxidation number (oxidation reactions)Increasing oxidation number (oxidation reactions)

Oxidation state represents number of electrons:

added to an element (negative #) or removed from an element (positive #)

H

H

C HH

C in CH4: has received an electron from each H atom.

All electrons are paired and hence this compound

is relatively stable

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Atmospheric Time Series of CH4 – Recent Data

Data shown in ORANGE are preliminary

http://www.esrl.noaa.gov/gmd/dv/iadv/graph.php?code=MLO&program=ccgg&type=ts

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Biological Production of CH4

CH4 produced by “methanogenic” bacteria:

• grow only in low O2 environments

• fermentation of cellulose and other organic material

• swamps, marshes, rice paddy fields

• rumina of cows and sheep.

Warneck, Chemistry of the Natural Atmosphere, 2000

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Sources and Sinks of CH4

Human

Evans, New Phytologist, 2007.

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Sources and Sinks of CH4

Chapter 3, Global Warming, Houghton

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Latitudinal Distribution of CH4

Warneck, Chemistry of the

Natural Atmosphere, 2000

A nice animation of CH4 vs latitude, as time evolves, is at

http://www.esrl.noaa.gov/gmd/ccgg/globalview/ch4/ch4_intro.html

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CH4 Lifetime

We have stated (e.g., Lecture 1, Slide 19) that CH4 has a lifetime of ~9 years.

Chemistry in Context (Table 3.2) and Houghton (page 44) give 12 years for the

lifetime of CH4 and Jacobson (page 71) states “the e-folding lifetime of CH4 due

to chemical reaction is about 8 to 12 years”.

What is meant by lifetime?

Which value is correct?

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CH4 is lost by reaction with OH

Computer models indicate tropospheric OH maximizes in tropics because:

a) overhead O3 column lowest in the tropics, allowing more UV light

to reach lower atmosphere

b) warmer air holds more H2O: OH produced by O(1D)+H2O

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CH4 is lost by reaction with OH

CH4 + OH H2O + CH3

44

CHProduction Loss = Production [OH][CH ]

dk

dt = − −

Arrhenius Expression for rate constant:

12 1775 / 3 12.45 10 cm secTk e− − − = × ×

44

4

[CH ]Abundance 1Lifetime of CH

Loss [OH][CH ] [OH]k k = = =

4 15 3 1 6 3

9 1

1Lifetime of CH

10 cm s 1 10 molecules cm

yrs

− − −

− −

= = 3.59 × ⋅ ×

1 = 8.9 3.59 × 10

Commonly T = 272 K and [OH] = 1 × 106 molec cm 3 are used (see Box 1-3 of

http://www.unep.ch/ozone/Assessment_Panels/SAP/Scientific_Assessment_2010/03-Chapter_1.pdf)

yielding :

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CH3CCl3 (methyl chloroform) is lost by reaction with OH

& its atmospheric abundance / industrial production are well known

CH3CCl3 + OH CH2CCl3+H2O

3 33 3

CH CClProduction Loss = Production [OH][CH CCl ]

dk

dt = − −

Fig 1-1, WMO/UNEP (2010) Fig 1-2, WMO/UNEP (2010)

Methyl chloroform (1,1,1-trichloroethane) is an excellent solvent and one of the least toxic of the chlorinated hydrocarbons. Prior to the

Montreal Protocol, it was widely used for cleaning metal parts and circuit boards, as an aerosol propellant, and as a solvent for inks, paints,

and adhesives. It was also the standard cleaner for photographic film (movie/slide/negatives, etc.). Methyl chloroform was also used as a

thinner in correction fluid products such as Liquid Paper.

The Montreal Protocol banned the use of methyl chloroform and is use was rapidly phased out, throughout the world.

See http://en.wikipedia.org/wiki/1,1,1-Trichloroethane

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Ozone Depletion and Halocarbons

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ODP (species " ") =i

3

3

global loss of O due to unit mass emission of " "

global loss of O due to unit mass emission of CFC-11

i

where :is the global atmospheric lifetimeτ

is the molecular weightMW

is the number of chlorine or bromine atomsn

is the effectiveness of ozone loss by bromineαrelative to ozone loss by chlorine α = 60 12 1775 / 3 1

OH+CH4

15 3 1

12 1520 / 3 1

OH+CH3CCl3

15 3 1

4

2.45 10 cm sec

3.59 10 cm sec @ 272 K

10 cm sec

6.14 10 cm sec @ 272 K

6.14CH lifetime 5 yr

3.59

T

T

k e

k e

− − −

− −

− − −

− −

= × × =

×= 1.64× × =

×

≈ × = 8.6 yr

Br Cl CFC-11

CFC-11

( + )

3

i

i

n n MW

MW

α ττ

≈

So Why Does IPCC Give a Lifetime for CH4 of 12 Years?

http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/134.htm

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CH4 and Stratospheric Ozone

• Oxidation of CH4 in the stratosphere produces CO2 and H2O:

Tropospheric CH4 is an important source of stratospheric H2O

Dessler, The Chemistry and Physics of Stratospheric Ozone, 2000

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CH4 and Stratospheric Ozone

• Oxidation of CH4 in the stratosphere produces CO2 and H2O:

Tropospheric CH4 is an important source of stratospheric H2O

• If tropospheric CH4 continues to rise, stratospheric H2O should increase:

As stratospheric H2O rises:

a) polar stratospheric clouds easier to form larger O3 hole

b) more HOx radicals more efficient loss of O3 most of strat

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CH4 and Stratospheric O3

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CH4 constant @ year 2000 level

CH4 doubling by year 2100

Portmann and Solomon, GRL, 2000

CH4 and Stratospheric Ozone

• As tropospheric CH4 rises, stratospheric CH4 will also rise,

reducing efficiency of chlorine catalyzed loss of O3

due to acceleration of Cl+CH4 sink of chlorine radicals:

O3

Production : CFCs +hν→ Inorganic chlorine

Reservoir : HCl

Cl ClO

O

CH4

O3

Production : CFCs +hν→ Inorganic chlorine

Reservoir : HCl

Cl ClO

O

CH4

ClO & Cl catalyze loss of stratospheric O3:

Cl + O3 → ClO + O2

ClO + O → Cl + O2

Net: O3 + O → 2 O2

ClO & Cl catalyze loss of stratospheric O3:

Cl + O3 → ClO + O2

ClO + O → Cl + O2

Net: O3 + O → 2 O2

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CH4 and Stratospheric Ozone

• As tropospheric CH4 rises, stratospheric CH4 will also rise,

reducing efficiency of chlorine catalyzed loss of O3

due to acceleration of Cl+CH4 sink of chlorine radicals

• Since stratospheric CH4 is most abundant in the lower

part of the stratosphere where CO is still high, the larger

abundance of HOx radicals associated with rising CH4 produces

O3 via this set of reactions:

OH + CO → CO2+ H

H + O2+ M → HO2 + M

HO2 + NO → OH + NO2

NO2 + hν → NO + O

O + O2

+ M → O3

+ M

Net: CO + 2 O2

→ CO2

+ O3

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CH4 and Stratospheric Ozone

• Oxidation of CH4 in the stratosphere produces CO2 and H2O:

Tropospheric CH4 is an important source of stratospheric H2O

• If tropospheric CH4 continues to rise, stratospheric H2O should increase:

As stratospheric H2O rises:

a) polar stratospheric clouds easier to form larger O3 hole

b) more HOx radicals more efficient loss of O3 most of strat

• As tropospheric CH4 rises, stratospheric CH4 will also rise,

reducing efficiency of chlorine catalyzed loss of O3

due to acceleration of Cl+CH4 sink of chlorine radicals

• Since stratospheric CH4 is most abundant in the lower

part of the stratosphere where CO is still high, the larger

abundance of HOx radicals associated with rising CH4 produces

O3 via traditional “smog” chemical reactions

Computer models project stratospheric column O3 will increase as CH4 rises

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Atmospheric CH4 and Energy from Gas, Rice, Cattle Trends

Data from http://cdiac.ornl.gov/ftp/trends/atm_meth

Ice Cores

Modern

Air

Dlugokencky et al., GRL, 2009

(kindly updated)

Data from

Data from http://cdiac.ornl.gov

http://faostat.fao.org

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Recent trends in CH4

Two recent papers offer conflicting views recent reduction in d(CH4)/dt:

• Aydin et al. (Nature, 2011) suggest “rising economic value of natural gas”

and “development of cleaner technologies” have led to a sharp decline in

unintentional release of CH4 by the petroleum industry, based on the

temporal evolution of CH4 and ethane (C2H6)

• Kai et al. (Nature, 2011) suggest changes in agricultural practices,

particularly in China, including new high yield rice species, use of more

fertilizer, and most importantly shorter water inundation periods have led

to a sharp decline in microbial release of CH4, based on the temporal

evolution of the isotopic composition of CH4

Can learn more from the Heimann News & Views piece in supplemental

reading

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CH4 Hydrates and Permafrost

Chapter 3, Chemistry in Context

We describe and quantify an important source of CH4 - ebullition (bubbling) from northern

lakes - that has not been incorporated in previous regional or global methane budgets…

Extrapolation of measured fluxes from 16 sites to all lakes north of 45°N suggests emission

of 24.2 ±10.5 Tg CH4 yr 1. Thermokarst lakes http://en.wikipedia.org/wiki/Thermokarst

have particularly high emissions because they release CH4 produced from organic matter

previously sequestered in permafrost.

Schematic depicting future of CH4 emissions from

northern lakes, as the north changes from a permafrost-

rich landscape to a landscape free of surface permafrost.

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http://www.sciencenews.org/view/feature/id/343202/description/The_Facts_Behind_the_Frack

CH4 (or natural gas) production

from fracking

Fracking

Airplane measurements of CH4 obtained by Anna Karion, Colm Sweeney, et al. over

Uintah Basin, Utah indicate fugitive emission of CH4 is ~9% of total fracking yield.

Break even point for “climate” is 3.2 % (we’ll explore this later)

EPA projects fugitive emission = 2.4% of total U.S. natural gas production

Can learn more from Tollefson News In Focus piece in supplemental reading

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Nitrous Oxide: N2O

N2O Lifetime ≈ 114 yrs

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Oxidation state represents number of electrons:

added to an element (negative #) or removed from an element (positive #)

HNO3

Nitric acid

NO3-

Nitrate

+5

NO2

Nitrogen

dioxide

+4

HONO

Nitrous acid

NO2-

Nitrite

NO

Nitric

oxide

N2O

Nitrous

oxide

N2NH3

Ammonia

+3+2+10-3

HNO3

Nitric acid

NO3-

Nitrate

+5

NO2

Nitrogen

dioxide

+4

HONO

Nitrous acid

NO2-

Nitrite

NO

Nitric

oxide

N2O

Nitrous

oxide

N2NH3

Ammonia

+3+2+10-3

Decreasing oxidation number (reduction reactions)Decreasing oxidation number (reduction reactions)

Increasing oxidation number (oxidation reactions)Increasing oxidation number (oxidation reactions)

See http://guweb2.gonzaga.edu/faculty/cronk/chemistry/L00-index.cfm?L00resource=Lewis_structures

for Lewis Dot Structure of N2O … please note we will not ask questions

about Lewis Dot Structures on exams !

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Sources and Sinks of N2O

http://www.esrl.noaa.gov/gmd/hats/combined/N2O.html

Chapter 7, IPCC 2007

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N2O and Stratospheric O3

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N2O constant @ year 2000 level

N2O rising above 400 ppb in year 2100

Portmann and Solomon, GRL, 2000

N2O and NOy

Chapter 6, WMO 1998 Ozone

Assessment Report.

Loss of N2O occurs mainly in the stratosphere, due to:

photolysis − main sink

reaction with electronically excited O(1D) − minor sink

Minor sink for N2O loss has a path that results in “fixed

nitrogen”:

N2O + O(1D) NO + NO

This is critical: source of stratospheric total fixed

nitrogen (NOy) is crucial to stratospheric chemistry

We’ll later see that nitrogen oxides catalyze loss of O3 &

participate in a series of chemical reactions that

affect partitioning of chlorine radicals, etc.

Minschwaner, Salawitch, and McElroy, JGR, 1993

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Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance

[to be] Emitted in the 21st Century

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http://www.sciencemag.org/content/326/5949/123.full