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Hunt for the missing sources of atmospheric CH3Br and CH3Cl : 

Hunt for the missing sources of atmospheric CH3Br and CH3Cl Robert C. Rhew Department of Earth System Science University of California at Irvine Host: Dr. Eric Saltzman

Outline: 

Background Global atmospheric chemistry problems Importance of CH3Br and CH3Cl Problem of the unbalanced budgets The hunt for “missing” terrestrial sources From dry lands to wetlands Fluxes from salt marshes and shrublands Tropical vegetation Directions for research What plants produce methyl halides? Can we improve the soil sink estimates? What biomes are next? Outline

Trio of global atmospheric chemistry issues: 

Stratospheric ozone depletion Enhanced greenhouse Oxidizing capacity of troposphere (OH) CH3Br CH3Cl HCFCs N2O CFCs O3 CO2 HFCs CH4 H2O CO NMHCs CH3CCl3 Trio of global atmospheric chemistry issues

Revised Venn diagram: 

Land surface  Atmos. Composition  Radiative budget  Surface/ Atm Temp  Reaction Rates  Source and sink strengths  Atmos. Dynamics Enhanced greenhouse Oxidizing capacity of troposphere (OH) Stratospheric ozone depletion Revised Venn diagram

Why do we care about methyl bromide (CH3Br) and methyl chloride (CH3Cl)?: 

Why do we care about methyl bromide (CH3Br) and methyl chloride (CH3Cl)?  Contribution to stratospheric ozone depletion CH3Br: chief source of Br to stratosphere (50x more effective than Cl) CH3Cl: chief source of natural Cl to stratosphere Natural modulation of stratospheric ozone  Economic and political importance of methyl bromide CH3Br widely used and highly effective agricultural fumigant Montreal Protocol to protect the ozone layer 1992 Copenhagen Amendments CH3Br production frozen at 1991 levels; U.S. phase out by 2005  Outstanding scientific questions CH3Br & CH3Cl budgets are poorly understood Known sinks greatly outweigh known sources Natural vs. anthropogenic emissions Role of the terrestrial biosphere? Determine the atmospheric lifetime, ODP Effect of public policy, climate and global change Plant, fungal, and microbial biochemistry

Methyl bromide budget fluxes in Gg/yr (109 g/yr) : 

Methyl bromide budget fluxes in Gg/yr (109 g/yr) Oceanic production 56 41 77 42 86 •OH hv 83 Biomass burning CH3Br fumigation Leaded gasoline Soils Oceans Atmosphere Missing source 20 5 total atmospheric burden = 146 Gg Kurylo, M. and Rodriguez, J., Short-lived ozone-related compounds, in Scientific Assessment of Ozone Depletion: 1998 (eds. Albritton, D., Watson, R., and Aucamp, P.) (Rep #44, World Meteorological Organization, Geneva, 1999).

Methyl chloride budget (1998) : 

Methyl chloride budget (1998) oceans- low lats ? 3.0 •OH hv 1.2 Biomass burning Soils oceans- high lats Atmosphere Missing source Fungi Industry Methyl chloride budget fluxes in Tg/yr (1012 g/yr) total atmospheric burden = 4.3 Tg 1.0 0.5 0.3 0.16 0.2 Kurylo, M. and Rodriguez, J., Short-lived ozone-related compounds, in Scientific Assessment of Ozone Depletion: 1998 (eds. Albritton, D., Watson, R., and Aucamp, P.) (Rep #44, World Meteorological Organization, Geneva, 1999).

Firn air CH3Br and CH3Cl: 

Firn air CH3Br and CH3Cl Butler, Battle, Bender, Montzka, Clarke, Saltzman, Sucher, Severinghaus, & Elkins. A record of atmospheric halocarbons during the twentieth century from polar firn air. Nature 399, 749-755 (1999)

Outline: 

Background Global atmospheric chemistry problems Importance of CH3Br and CH3Cl Problem of the unbalanced budgets The hunt for “missing” terrestrial sources From dry lands to wetlands Fluxes from salt marshes and shrublands Tropical vegetation Directions for research What plants produce methyl halides? Can we improve the soil sink estimates? What biomes are next? Outline

The hunt for “missing” terrestrial sources: 

Identified terrestrial source CH3Br CH3Cl Brassica crops (Gan et al. 1998) 7 ? Freshwater wetlands (Varner et al., 1999) 5 .05 & Irish peat bogs (Dimmer et al., 2001) Coastal salt marshes (Rhew et al., 2000) 14 .17 Rice paddies (Redeker et al., 2001) 1 .006 Shrublands (Rhew et al. 2001) + + Abiotic decomposition (Keppler et al., 2000) + + Tropical plants (Yokouchi et al., 2000; 2002) ? >0.9 From dry lands to wetlands... The hunt for “missing” terrestrial sources

Shrubland study sites: 

Shrubland study sites

Flux chamber design: 

heat exchanger with fan sampling flask fan pressure vent BASE LID filter abutting joints overlap soil surface thermocouple removable top cover Flux chamber design

Shrublands are sources and sinks of CH3Br and CH3Cl: 

CH3Br emissions (note log scale): primarily during growing season B. juncea, C. edulis, L. tridentata CH3Cl emissions (note log scale): primarily during dry seasons B. juncea, C. edulis, A. californica CH3Br uptake higher in wetter seasons, but much smaller than other estimates: -47 to -58 nmol/m2/d CH3Cl consumption: Higher in wetter seasons than drier seasons Shrublands are sources and sinks of CH3Br and CH3Cl (Rhew et al., 2001)

Net uptake rates of CH3Cl and CH3Br are correlated: 

Net uptake rates of CH3Cl and CH3Br are correlated (Rhew et al., 2001)

Salt marsh study sites: 

Pacific Ocean 5 km San Dieguito Lagoon (32° 58’ N, 117° 15’ W) Mission Bay Marsh (32° 47’ N, 117 ° 13’ W) SIO Salt marsh study sites

Results from coastal salt marshes: 

Large sources of CH3Cl & CH3Br  large spatial variability  500-750  freshwater wetlands  ~10% of the global source Seasonal and diurnal cycles CH3Br : CH3Cl fluxes correlated  1 : 20 ratio  same mechanism of production  methyltransferase kinetics These results were not predicted Results from coastal salt marshes

Models and measurements point to a large tropical source of CH3Cl: 

Models and measurements point to a large tropical source of CH3Cl Mangroves may emit CH3Cl, but do not appear to emit CH3Br (2001)

Nature Vol 416, Mar 14, 2002, p. 163-165: 

Nature Vol 416, Mar 14, 2002, p. 163-165

Outline: 

Background Global atmospheric chemistry problems Importance of CH3Br and CH3Cl Problem of the unbalanced budgets The hunt for “missing” terrestrial sources From wetlands to drylands Fluxes from salt marshes and shrublands Tropical vegetation Directions for research What plants produce methyl halides? Can we improve the soil sink estimates? What biomes are next? Outline

What plants produce methyl halides?: 

What plants produce methyl halides? Project 1: Survey of emissions from a variety of plant species Shrubland plants isolated from soils: Carpobrotus edulis (sea fig) Artemisia californica (sage) Larrea tridentata (creosote) Class Magnoliopsida / Subclass Dilleniidae Includes Brassicaceae, Bataceae, Frankeniaceae Determine variability and controls on emissions

Preliminary measurements from: 

Preliminary measurements from

Can we improve the soil sink estimates?: 

Can we improve the soil sink estimates? Project 2: Develop a method to measure soil consumption at near-ambient concentrations Soil sink components Tropical forest/savannah 6.5 ± 0.2 Temperate grassland 9.7 ± 3.2 Boreal forest 1.7 ± 0.1 Cultivated land 7.5 ± 0.6 (Varner et al., 1999) Temperate forest, woodland, and shrubland 21.7 ± 12.1 (Shorter et al., 1995) (38.2 ± 30.3) (Serça et al., 1998) Soil studies estimate a sink of 42 ± 32 and 143 ± 70 Gg/yr for CH3Br. The soil sink for CH3Cl is not currently estimated.

What processes do we measure when we measure fluxes?: 

What processes do we measure when we measure fluxes? CH3X loss by chemical & biological degradation in soils and on plant surfaces In situ soil production: abiotic degradation, subsurface fungi, roots, soil bacteria? Uptake from atmosphere Return Above surface production by plants and fungi Net fluxes measured by flux chamber Gross uptake measured by using high concentration spikes Plants and fungi measured by growing under controlled conditions

Use stable isotopes to separate production and consumption: 

Use stable isotopes to separate production and consumption CH3X loss by chemical & biological degradation in soils and on plant surfaces Inject 13CH3Br and 13CH3Cl to yield near ambient concentrations. Measure the loss rate of the isotopically labeled gases and the net fluxes too.

What biomes are next?: 

What biomes are next? Shortgrass steppe Long Term Ecological Reserve Near Fort Collins, Colorado Photo credits: http://sgs.cnr.colostate.edu *Shorter et al., 1995 †Serca et al., 1998 …or are they a net source? Temperate grasslands: 9.7* to 31.4† Gg/yr CH3Br sink

Trace gas emissions and global change: 

Trace gas emissions and global change What are the ecophysiological / biogeochemical controls on trace gas emissions? How much does the heterogeneity of terrestrial ecosystems regulate the trace gas fluxes? How do you scale up from the plant to the ecosystem level? What are the effects of climate change on biogeography? What are the effects of land cover and land use change on trace gas emissions?

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