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The Global Methane Cycle. CH 4 in soil & atmosphere. Topics. General Methane Information Sources & Sinks (general) CH 4 in the soil CH 4 in the atmosphere Conclusions. General Methane Information. Ins & Outs. Most abundant organic trace gas in the atmosphere

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the global methane cycle

The Global Methane Cycle

CH4 in soil & atmosphere

topics
Topics
  • General Methane Information
  • Sources & Sinks (general)
  • CH4 in the soil
  • CH4 in the atmosphere
  • Conclusions
ins outs
Ins & Outs
  • Most abundant organic trace gas in the atmosphere
  • Concentrations have doubled since pre-industrial times (now ~1700 ppbv)
  • After CO2 and H2O most abundant greenhouse gas
  • 20 to 30 times more effective greenhouse gas than CO2(carbon dioxide)
ch 4 what does it do
CH4, what does it do?
  • Helps control amount of OH (hydroxyl) in the troposphere
  • Affects concentrations of water vapor and O3 (ozone) in the stratosphere
  • Plays a key-role in conversion of reactive Cl to less reactive HCl in stratosphere
  • As a greenhouse gas it plays a role in climate warming
ch 4 through time
CH4 through Time
  • Record of CH4 from air bubbles trapped in polar ice (Antarctica and Greenland)
  • CH4 levels closely tied to glacial-interglacial records
  • CH4 ‘follows’ temperature
  • Unprecedented rise since industrial revolution: CH4 emissions
ch 4 geographically
CH4 Geographically
  • 150 ppb Pole-to-pole gradient, indicating consistently large emissions in the northern hemisphere
natural sources

+

Total : 30% (~100-200 TgCH4/year)

Natural Sources
  • Wetlands
  • Oceans
  • Hydrates
  • Wild ruminants
  • Termites
anthropogenic sources

+

Total : 70%

Anthropogenic Sources
  • Agriculture (ruminants)
  • Waste disposal
  • Biomass burning
  • Rice paddies
sinks for tropospheric ch 4
Sinks for tropospheric CH4
  • Reaction with hydroxyl radical (~90%)
  • Transport to the stratosphere (~5%)
  • Dry soil oxidation (~5%)

+

Total : ~560 TgCH4/y

general information
General Information
  • Atmospheric CH4 is mainly (70-80%) from biological origin
  • Produced in anoxic environments, by anaerobic digestion of organic matter
  • Natural and cultivated submerged soils contribute ~55% of emitted CH4
  • Upland (emerged) soils responsible for ~5% uptake of atmospheric CH4
methanogenesis in soils
Methanogenesis in Soils
  • Produced in anoxic environments, by anaerobic digestion and/or mineralisation of organic matter:

C6H12O6 3CO2 + 3CH4

(with low SO42- and NO3- concentrations)

  • Formed at low Eh (< -200mV)
  • Formed by ‘Methanogens’ (Archaea)
methanotrophy in soils
Methanotrophy in Soils
  • 2 Forms of oxidation recognized in soils:
  • I) ‘High AffinityOxidation’ in soils with close to atmospheric CH4 concentrations (<12ppm), upland/dry soils
  • II) ‘Low AffinityOxidation’ in soils with CH4 concentrations higher than 40 ppm, wetland/submerged soils
low affinity oxidation
Low Affinity Oxidation
  • Performed by methanotrophic bacteria
  • Methanotrophs in all soils with pH higher than 4.4 in aerobic zone
  • Methane oxidation in methanogenic environments is Low Affinity Oxidation
  • Methane oxidation is Aerobic  the amount of oxygen is the limiting factor
low affinity rice fields
Low Affinity & Rice Fields
  • More than 90% of methane produced in methanogenic environments is reoxidised by methanotrophs
  • Variations in CH4 emissions from ricefields mostly due to variations in methanotrophy
  • Emission of CH4 mostly through rice aerenchyma (‘pipes’)
  • Soil oxidation through aerenchyma
more general info
More General Info
  • Methanotrophy is highest in methanogenic environments
  • Both methanogens and trophs prevail under unfavorable conditions (high/low water etc)
  • Methane emission is larger from planted rice fields than from fallow fields, due to higher C availability and aerenchyma
high affinity
High Affinity
  • Upland forest soils most effective CH4 sink
  • Temporarily submerged upland soils can become methanogenic
  • Arable land much smaller CH4 uptake than untreated soils
water
Water
  • Soil submersion allows methanogenesis
  • Reduces methanotrophy
  • Short periods of drainage decreases methanogenesis in ricefields dramatically (Fe, SO4)
ph and temperature
pH and Temperature
  • Methanogenesis most efficient around pH neutrality
  • Methanotrophs more tolerant to variations in pH
  • Methanogenesis is optimum between 30 and 40 oC
  • Methanotrophs are more tolerant to temperature variations
rice and fertilizers
Rice and Fertilizers
  • Goal: High yield and less methane emission
  • Organic fertilizers increase CH4 (incorporation org. C)
  •  Reduce CH4 by raising Eh and competition (e.g. SO4)
rice up ch 4 down
Rice UP, CH4DOWN
  • Fertilizers containing SO4 may poison the soil
  • Ammonium and urea decrease methanotrophy/CH4 oxidation, especially in upland soils
  • Calcium carbide significantly reduces CH4 emission and increases rice yield by inhibiting nitrification
major atmospheric ch 4 sink oh
Major atmospheric CH4 sink: OH
  • Reaction with hydroxyl (OH) radical (~90%) in the troposphere
  • OH is formed by photodissociation of tropospheric ozone and water vapor
  • OH is the primary oxidant for most tropospheric pollutants (CH4, CO, NOx)
  • Amount CH4 removed constrained by OH levels and reaction rate
source of oh
Source of OH
  • Formed when O3 (ozone) is photo-dissociated:

O3 + hv O(1D) + O2

which in turn reacts with water vapor to form 2 OH radicals:

O(1D) + H2O  OH + OH

(OH is also formed in Stratosphere by oxidation of CH4 due to high concentrations of Cl)

sink of oh
Sink of OH
  • CH4 mainly removed by reaction

CH4 + OH•  CH3• + H2O

  • OH concentrations not only affected by direct emissions of methane but also by its oxidation products, especially CO
  • Increase in methane leads to positive feedback; build-up of CH4 concentrations
projections

 Urban areas: NOx increase

 NOx results in O3 formation

 O3 dissociates to OH

Projections
  • OH loss rates may increase due to rising anthropogenic emissions
  • OH loss rates may be balanced by increased production through O3 and NOx::
projections 2
Projections 2
  • Stratospheric ozone decreases as seen in recent years
  • Due to decrease of stratospheric O3, ultraviolet radiation in troposphere increases  increase OH
  • Water vapor through temperature rise may either increase or decrease OH
projections 3 tropics
Projections 3: Tropics
  • Tropics: high UV, high water vapor  High OH
  • High CH4 production due to rice fields, biomass burning, domestic ruminants
  • Future changes in land use / industrialization
no x and oh
NOx and OH
  • Polluted areas High NOx OH production(temperate zone Northern hemisphere, planetary boundary layer of the tropics)
  • Unpolluted areas  Low NOx  OH destruction (marine area`s, most of the tropics, most of the Southern hemisphere)
o 3 in tropo and stratosphere
O3 in Tropo- and Stratosphere
  • Ozone (O3) absorbs ultraviolet radiation, but is also a greenhouse gas
  • 90% of O3 in the Stratosphere
  • Stratospheric production by photo- dissociation of O2 and reaction with O2
  • 10% of O3 in the Troposphere, through downward transportfrom the stratosphere and photolysis of NO2 in the troposphere
stratospheric ozone
Stratospheric Ozone
  • O3 destroyed by catalytic mechanisms involving free radicals like NOx, ClOx, HOx
  • CH4 acts as source and sink for reactive chlorine:
  • Sink: direct reaction with reactive Cl to form HCl (main Cl reservoir species)
  • Source: OH (oxidation of CH4 in stratosphere) reacts with HCl to form reactive Cl
stratospheric ozone 2
Stratospheric Ozone 2
  • OH from the dissociation of methane can react with ozone (especially in the upper stratosphere)
  • Conclusively: increasing CH4 leads to net O3 production in troposphere and lower stratosphere and net O3 destruction in the upper stratosphere
ch 4 impact on climate
CH4 impact on Climate
  • CH4 absorbs infrared radiation  increases greenhouse effect
  • Globally-averaged surface temperature 1.3oC higher than without methane
  • Dissociation of CH4 leads to CO2: additional climatic forcing
slide37
CH4 has increased dramatically over the last century and continues to increase
  • Causal role of human activity
  • Climate forcing by CH4 confirmed, though not fully understood
  • Future developments uncertain