SOURCES AND SINKS OF ENVIRONMENTALLY IMPORTANT

SUBSTANCES (EXCLUDING CO2) (IAPSO, IAMAS, IAHS, IABO)

Location: Arts Building 120 LT

Thursday 22 July AM

Presiding Chair: Dr D Smythe-Wright

(Southampton Oceanography Centre, Southampton, UK)

JSP21/C/U4/W/03-A4 0910

EVIDENCE FOR VOLCANIC INFLUENCE ON MEXICO CITY AEROSOLS

G. B. RAGA (Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, México DF, México, email: raga@servidor.unam.mx); G. L. Kok (National Center for Atmospheric Research, Boulder, CO 80303, USA, email: kok@ucar.ncar.edu); D. Baumgardner, A. Báez and I. Rosas (all three at: Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, México DF, México, email: darrel@servidor.unam.mx)

In situ measurements of sulphur dioxide (SO2), carbon monoxide (CO) and sulphate mass provide evidence that aerosol composition in Mexico City is affected by emissions from the neighbouring volcano, Popocatepetl. The data suggest that there are two distinct pathways by which SO2 is incorporated into particulates. Periods of high humidity, fog, and rain are accompanied by elevated sulphate mixing ratios, attributed to aqueous chemistry. Similarly, elevated sulphate concentrations in low humidity periods appear to be a result of adsorption onto existing particles. These two mechanisms are important for understanding the processes associated with particle formation in this highly polluted urban area. Under the influence of volcanic emissions, SO2 concentration is more than four times the average value and particulate sulphate is a factor of 2 larger.

JSP21/E/02-A4 0930

CURRENT AND LONG TERM TRENDS OF CFC REPLACEMENT CHEMICALS AT CAPE GRIM, TASMANIA

Georgina STURROCK and Paul Fraser (CSIRO Atmospheric Research/CRC Southern Hemisphere Meteorology, Aspendale, Victoria 3195, Australia, email: g.sturrock@bom.gov.au); Simon O'Doherty and Peter Simmonds (School of Chemistry, University of Bristol, Brsitol, UK); Ben Miller and Ray Weiss (Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA); Ron Prinn (Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA); David Oram (School of Environmental Sciences, University of East Anglia, Norwich, UK)

The refrigeration/air conditioning and foam plastics industries are using HCFCs and HFCs as interim and long-term replacements, respectively, for the ozone depleting chlorofluorocarbons (CFCs). The tropospheric removal of HCFCs and HFCs by reaction with hydroxyl radicals is significant and thus they cause less ozone depletion and global warming than the CFCs they replace. Nevertheless some of the HCFCs reach the stratosphere, resulting in limited ozone destruction, especially over the next 20 years. Both HCFCs and HFCs contribute to global warming - in particular the HFCs.

Monitoring the early accumulation in the background atmosphere of the CFC replacement chemicals is important, as it ensures that any unexpected increase in their levels will be detected early, possibly avoiding a repetition of the CFC pollution problem which was not detected until there had been significant accumulation in the atmosphere, resulting in unavoidable environmental damage, for example the Antarctic ozone hole.

Tropospheric data are being collected for the CFC replacement species using identical instrumentation and methodologies at mid-latitude sites in the Northern (Mace Head, Ireland, 52N) and Southern (Cape Grim, Tasmania, 41S) Hemispheres, as part of the long-term Advanced Global Atmospheric Gases Experiment (AGAGE) program designed to determine global trends and ultimately atmospheric lifetimes or emissions of CFCs, HCFCs and HFCs. In situ measurements are achieved using state-of-the-art instrumentation with high-resolution capillary gas chromatographic (GC) separation and mass spectrometric (MS) detection.

JSP21/W/13-A4 0950

TROPOSPHERIC CONCENTRATIONS OF THE CHLORINATED SOLVENTS, PERCHLOROETHENE AND TRICHLOROETHENE, MEASURED IN THE REMOTE NORTHERN HEMISPHERE.

Claudia H. DIMMER, Peter G. Simmonds, Graham Nickless. (School of Chemistry, University of Bristol. Bristol. BS8 1TS. UK.email: c.dimmer@bris.ac.uk); Archie McCulloch. (ICI Chemicals & Polymers Ltd. P.O. Box 13, The Heath, Runcorn, Cheshire. WA7 4QF. UK.)

A fully-automated twin ECD GC system with sample enriching Adsorption-Desorption front end was deployed on two major field campaigns - Ny-Ålesund, Svalbard, (July to September 1997), and the RRS Discovery CHAOS cruise of the N.E. Atlantic, (April to May 1998). Concentrations of an extensive suite of halocarbons were detected at 50 minute intervals at pptv levels of concentration. The results obtained for the chlorinated solvents, perchloroethene (PCE), and trichloroethene (TCE) will be presented. Trajectory sector analysis sorting, and filtering to remove local pollution events enabled baseline concentrations for each compound to be defined, and ratios between PCE and TCE for polluted air masses to be determined. Background PCE and TCE concentrations of 2.68 and 0.26 pptv respectively were recorded in Ny-Ålesund. During pollution incidences, concentrations rose to 7.41 (PCE) and 2.89 pptv (TCE). The cruise data showed average concentrations ranging from 5.71 (PCE) and 1.43 pptv (TCE) for air masses originating over the N.Atlantic and Arctic open oceans, to 11.46 (PCE) and 5.75 pptv (TCE) for polluted air masses from Northern Europe. The significance of local contamination was highlighted and the concentrations compared to data obtained at Mace Head, Ireland, and to atmospheric concentrations calculated from the audited emissions data.

JSP21/L/01-A4 1010

REGIONAL AND BACKGROUND MEASUREMENTS OF HALOMETHANES AT CAPE GRIM, TASMANIA

Michelle COX (CRC for Southern Hemisphere Meteorology, Monash University, Wellington Road, Clayton, Victoria, 3168, Australia, email: michelle.cox@dar.csiro.au); Paul Fraser and Georgina Sturrock (CRC for Southern Hemisphere Meteorology/CSIRO Atmospheric Research, PMB1 Aspendale, Victoria, 3195, Australia, email: paul.fraser@dar.csiro.au); Steve Siems (Mathematics Department, Monash University, Wellington Road, Clayton, Victoria, 3168, Australia,

email: siems@monsoon.maths.monash.edu.au)

Chlorine and bromine species are the major catalysts for the significant, anthropogenically-enhanced, stratospheric ozone destruction that has occurred over the past two decades. The synthetic chemicals that provide the major halogen sources are the chlorofluorocarbons (CFCs) and halons. However approximately 20% of stratospheric chlorine and 50% of stratospheric bromine are derived from halomethanes, largely methyl chloride (CH3Cl) and bromide (CH3Br), which have both natural and anthropogenic sources. The atmospheric behaviour, sources and sinks of these halomethanes are not well understood. In an attempt to reduce the uncertainties associated with these species, an in-situ measurement project for CH3Cl, CH3Br, CH2Cl2 (dichloromethane) and CHCl3 (chloroform) was established at Cape Grim in late 1997, using state-of-the-art GC-MS instrumentation developed at the University of Bristol as part of the global AGAGE (Advanced Global Atmospheric Gases Experiment) program. In this paper, the temporal behaviours of these species in the background atmosphere are described and low level trajectories are used to investigate the pollution episodes and to help identify possible source regions. Trajectories from two high-resolution regional climate models (DARLAM - Division of Atmospheric Research Limited Area Model (CSIRO) - and LAPS - Limited Area Prediction System (BMRC)) are compared.

JSP21/W/11-A4 1050

A FIRN AIR RECORD OF CHLORINATED AND BROMINATED ORGANIC COMPOUNDS AND HCFCS SINCE THE 1950’S IN TWO HEMISPHERES

WT STURGES, DE Oram, H McIntyre, SA Penkett (School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK, email: w.sturges@uea.ac.uk), A Fabre, J Chappellaz, JM Barnola (Laboratoire de Glaciology et Geophysique de l’Environnement, CNRS, 54 Rue Moliere, Domaine Universitaire, 38402 St-Martin-d’Heres Cedex, FRANCE), E Atlas, V Stroud (National Center for Atmospheric Research, Boulder, CO 80307-3000, USA)

R Mulvaney (British Antarctic Survey, High Cross, Cambridge, CB3 0ET, UK)

The histories of a number of HCFCs and chloro-/bromo-carbons, dating back to the 1950’s, have been examined in firn air from Canada and Antarctica. Many of the replacement compounds for CFCs such as HCFC-22, HCFC-141b, HCFC-142b, HCFC-21 HCFC-123 and HFC-152a show trends dating back to the late 1950’s or early 1960’s. Comparisons between hemispheres for the shorter-lived species reveals evidence for changing interhemispheric ratios, suggesting possible changes in global sinks. Trends for methyl chloroform, chloroform, dichloromethane, tetrachloroethylene, 1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane show clear maximum Northern Hemispheric (NH) concentrations in the early 1980’s (early 1990’s for methyl chloroform). The lower levels in the Southern Hemisphere (SH) can be largely attributed to transport from the NH. Current estimates of oceanic sources of these compounds appear to be too high in many instances. Methyl chloride appears to have grown slightly in the SH, possibly due to biomass burning. Organobromines are separated into those showing trends due to anthropogenic sources (methyl bromide, dibromoethane), and those which have no trend and are evidently of entirely natural origin (dibromomethane, chloro-bromo-methanes, etc.). There appears to be a significant source of several organohalogens within the firn itself.

JSP21/W/12-A4 1110

NEW CONSTRAINTS ON GLOBAL AND HEMISPHERIC LOSS RATES OF METHYL CHLOROFORM

Stephen A. MONTZKA, James H. Butler, and James W. Elkins (NOAA/CMDL, ms: R/E/CG1, 325 Broadway, Boulder, CO 80303, email: smontzka@cmdl.noaa.gov, jbutler@cmdl.noaa.gov, jelkins@cmdl.noaa.gov).

The atmospheric residence time for many CFC-replacements and other reduced gases is determined primarily by the abundance of the hydroxyl radical. Accordingly, an accurate understanding of the burden of OH on global and hemispheric scales is essential for gauging the environmental effects of these gases. Estimates of the global mean hydroxyl radical concentration have been made in the past by inferring residence times or lifetimes for CH3CCl3 and 14CO from consideration of global measurements and sources. As emissions of methyl chloroform become insignificant, owing to the limits outlined in the Montreal Protocol on Substances that Deplete the Ozone Layer, atmospheric measurements of this gas will allow for refined estimates of its lifetime on global and hemispheric scales. >From our measurements of CH3CCl3 at 7-9 remote sampling locations, the decrease observed between 1997 and 1998 corresponds to an e-folding time of 5.7 ± 0.3 yr. Because the data suggest that industrial emissions are not yet zero, this e-folding time represents only an upper limit to the global atmospheric lifetime of CH3CCl3. This upper limit, however, is independent of accuracy associated with calibration gas standards and industrial emission estimates. Additional consideration of our data allows us to estimate an upper limit of approximately 4.8 yr to the lifetime of CH3CCl3 in the Southern Hemisphere. This limit is also quite insensitive to calibration accuracy, emission estimates, and hemispheric exchange rates. Finally, we explore the utility of our measurements to discern relative differences

JSP21/W/09-A4 1130

CHANGES IN STRATOSPHERIC OZONE INDUCED BY CHANGES IN NATURAL ORGANOHALOGEN EMISSIONS

K. KOURTIDIS, C. Tourpali, C. Zerefos and D. Balis (Aristotle University of Thessaloniki, Laboratory of Atmospheric Physics, 54006 Thessaloniki, Greece, email: kourtidi@ccf.auth.gr)

We present here model simulations with a 2-D CTM that deal with the effect on stratospheric ozone of changes in the tropospheric abundance of the natural organohalogens methyl chloride (CH3l) and methyl bromide (CH3Br). The simulations include a 5-fold increase in the tropospheric CH3Cl abundance, a 5-fold increase in the tropospheric CH3Br abundance, and combined 5-fold increases in the tropospheric levels of both substances in both the unperturbed pre-industrial atmosphere and the industrial atmosphere. Thirty-fold increases in the tropospheric levels of both substances in the unperturbed pre-industrial atmosphere have also been simulated. The simulations for a combined 5-fold increase in both substances in a pre-industrial atmosphere show significant depletion of ozone at high latitudes during the spring season, that are mostly confined in the 30-55 km altitude region, while 30-fold increases are shown to significantly affect the global ozone layer. The results for 5-fold increases are also discussed in view of the effect of the calculated ozone changes in the radiation reaching various atmospheric levels in the 10-40 km altitude region. Possible mechanisms that could have lead to changes of the simulated magnitudes in the tropospheric natural organohalogen abundance in the past are also discussed.

JSP21/W/02-A4 1150

THE INFLUENCE OF DIURNAL VARIATIONS IN THE CONCNETRATION OF HALOCARBONS ON AIR-SEA FLUX CALCULATIONS

Katarina ABRAHAMSSON, Anja Ekdahl and Anders Lorén (Department of Analytical and Marine Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden)

Diurnal variations in concentrations of volatile halogenated organic compounds have been studied in a rock pool and in open ocean. The concentrations were measured every hour or every other hour at six different occasions during a cruise in the Atlantic sector of the Atlantic Ocean, as well as every hour during one occasion in a rock pool at the Canary Islands. We will present results that show that the variations in concentrations of halocarbons correlate with photosynthesis and respiration. The levels of halocarbons vary considerable during a 24 hour period even in the open ocean, for example for diodomethane, bromoform and trichloroethylene the concentrations varied with a factor of 90, 8 and 10 respectively. The diurnal variations are characterised by rapid increases and decreases in concentrations, just within a few hours, and calculations of the air-sea flux cannot account for the observed losses.

Thursday 22 July PM

Presiding Chair: TBA

JSP21/W/08-A4 1400

ON THE FLUXES OF SHORT-LIVED ORGANOCHLORINE COMPOUNDS BETWEEN THE OCEAN AND ATMOSPHERE.

Robert M. MOORE, (Department of Oceanography, Dalhousie University, Halifax, N.S. Canada, B3H 4J1, email: rmoore@is.dal.ca)

Measurements of dichloromethane, chloroform, trichloroethylene and tetrachloroethylene were made, at the same time as several naturally-produced trace gases, in the ocean water column during cruises in the western Atlantic in 1997, and, as part of the GAS EX98 experiment, north of the Azores in 1998. While it is evident that the gases were supersaturated at the ocean surface, and that the ocean was therefore acting as a source to the atmosphere, it will be shown that it is more difficult to establish whether the gases are produced within the ocean or are, at least in part, anthropogenic. All of these compounds are expected to have seasonally varying concentrations in the atmosphere as a result of their short lifetimes (ca. 7 – 150 days) with respect to loss by reaction with the OH radical. This can, depending on the lifetime of the gas in seawater, drive a wintertime flux of a gas from atmosphere to ocean, and a reverse flux during summer. The effects of any biological production or consumption of the gas in the ocean will be superimposed on the physically driven fluxes.

JSP21/W/03-A4 1420

SOURCES AND SINKS OF HALOGENATED GASES IN THE NORTH ATLANTIC OCEAN

Denise SMYTHE-WRIGHT, Stephen Boswell and Russell Davidson (Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH)

Following the Montreal Protocol on ozone depleting gases and continuing concern over greenhouse warming, attention is now being focused on the new anthropogenic CFC ‘replacements’ and other radioactively-active and ozone-depleting gases which initially were given little regard; for example, hydrohalomethanes, haloforms and methylhaloforms. Evidence suggests that the ocean is both a source and a sink of such compounds but the specific sources (and sinks) within the ocean are poorly understood. While it is possible to measure variations in the seawater concentrations, little is known about seasonal and geographical variation, or about biological production or release (ie during the growth and mortality of planktonic organisms). Results from 3 RRS Discovery cruises covering winter, summer and spring biological conditions will be presented to show that there is a direct relationship between the distribution of methyl chloride, methyl bromide and methyl iodide and biological production, depending on season and geographical location. For example up to 40% methyl bromide supersaturation (with respect to the atmosphere) has been observed off the coast of Greenland and in excess of 1000% methyl iodide supersaturation in the subtropical gyre. Speciation studies indicate that the methyl bromide production may be related to a bloom of Nitzchia ssp while the methyl iodide may result from a prochlorophyte source, but the mechanism by which the compounds are produced is still unclear.

JSP21/L/02-A4 1440

METHYL BROMIDE AND METHA CHLORIDE EMISSIONS FROM COASTAL SALT MARSHES

ROBERT C. RHEW, Benjamin R. Miller, and Ray F. Weiss (Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0220; 619-534-2599,

email: rob@gaslab.ucsd.edu)

Field measurements show that coastal salt marshes are large natural sources of methyl bromide and methyl chloride and may contribute significantly toward balancing the known global budgets of these compounds. We chose to investigate salt marshes for methyl halide emissions because they are known to be regions of high primary productivity and high halide concentrations. Flux chamber deployments were carried out between June and December 1998, in two southern California coastal salt marshes. Measured net fluxes show that methyl bromide and methyl chloride are emitted from all vegetation zones of both salt marshes, with greater emissions in the growing season than in the non-growing season. The greatest emissions of these methyl halides are from the middle to upper-middle zone of the salt marsh and are associated with high carbon dioxide respiratory emissions and high densities of above-ground green biomass. Furthermore, there is a strong correlation between the molar fluxes of methyl bromide and those of methyl chloride, pointing to a common mechanism of production and release. The average molar ratio of chloride ion to bromide ion in seawater is about 600, while the average ratio of emissions of methyl chloride to methyl bromide from the salt marsh is roughly 20. Therefore the mechanism appears to favor the production of methyl bromide over methyl chloride, an observation consistent with the biosynthesis of these compounds by enzymes isolated from certain plants and wood-rot fungi. Estimated global extrapolations indicate that coastal salt marshes represent the largest natural terrestrial source of methyl bromide and methyl chloride identified thus far. Although salt marshes comprise less than 0.1% of the global surface area, they may produce roughly 10% and 5% of the total annual fluxes of methyl bromide and methyl chloride, respectively.

JSP21/W/17-A4 1500

PRODUCTION AND EMISSION OF METHYL CHLORIDE IN THE SURFACE OCEAN

Daniel B. KING (NOAA/CMDL, Boulder, CO 80303, USA and CIRES, University of Colorado, Boulder, CO 80309, USA, email: dking@cmdl.noaa.gov); James H. Butler (NOAA/CMDL, Boulder, CO 80303, USA, email: jbutler@cmdl.noaa.gov); Jürgen M. Lobert (Center for Clouds, Chemistry, Climate, SIO/UCSD, La Jolla, CA 92093, USA); Shari A. Yvon-Lewis (NOAA/AOML/OCD, Miami, FL 33149, USA); Stephen A. Montzka and James W. Elkins (NOAA/CMDL, Boulder, CO 80303, USA)

Methyl chloride (CH3Cl) is the primary natural source of organic chlorine in the atmosphere. As anthropogenic sources of atmospheric, organic chlorine are reduced in the future, the relative contribution of CH3Cl to chlorine in the stratosphere will increase. However, there are currently significant uncertainties associated with the global budget of atmospheric CH3Cl. The oceans are a substantial source, as is biomass burning, and perhaps wood-rotting fungi. The oceanic contribution alone is difficult to rectify, and extrapolations of the known oceanic sources of CH3Cl cannot currently account for the estimated oceanic emission to the atmosphere.

In an attempt to understand the oceanic processes driving the production and emission of CH3Cl, we examined the relationship between its air-sea flux and a variety of physical and chemical oceanic properties. A total of four field campaigns was conducted in the Atlantic, Pacific, and Southern Oceans, in open ocean, upwelling and coastal environments. CH3Cl was supersaturated in most of the waters, with higher degrees of saturation in warmer waters. Data from these campaigns are used to derive a correlation between the flux and temperature, along with other physical and chemical properties. The contributions of the chemical degradation of CH3Br and CH3I to the production of CH3Cl are estimated for the four field campaigns and compared to the total air-sea flux of CH3Cl.

JSP21/W/05-A4 1520

THE OCEANIC CONTRIBUTION TO ORGANIC BROMINE IN THE ATMOSPHERE

Shari YVON-LEWIS (NOAA/AOML, Miami, FL 33149, USA, email: Shari.Yvon-Lewis@noaa.gov); James Butler, Daniel King, Stephen Montzka (NOAA/CMDL, Boulder, CO 80303, USA,

email: jbutler@cmdl.noaa.gov); Jose Rodriguez (RSMAS, Univ. of Miami, Miami, FL 33149, USA); Jürgen Lobert (C4/SIO, Univ. of California San Diego, La Jolla CA 92093, USA); Malcolm Ko (AER Inc, Boston, MA 02139, USA)

The ocean delivers considerable organic bromine into the troposphere. Although many of the oceanic, bromine-containing compounds are short-lived in the troposphere, they can be convected periodically into the stratosphere, where they contribute to the destruction of stratospheric ozone. Methyl bromide (CH3Br) and the halons are recognized as being the primary carriers of bromine into the stratosphere, but contributions from other compounds may be significant. Some of these other trace gases include CH2Br2, CHBr3, CH2BrCl, CHBr2Cl, and CHBrCl2. All of these gases are produced to some extent in the ocean and their collective sea-air flux is of the same order as the total flux of CH3Br from all sources.

We have been measuring CH3Br, CH2Br2, CHBr3, and other bromine-containing compounds in air and surface seawater samples on research cruises in the Pacific, Atlantic, and Southern Oceans since 1994. Results show CH3Br undersaturations throughout much of the ocean, suggesting that the ocean is a net sink for this brominated trace gas. However, results for CH2Br2, CHBr3, and the other bromine-containing compounds show these trace gases to be supersaturated throughout much of the ocean, often with higher degrees of saturation in the tropics and subtropics. These data are presented here and will be used in conjunction with a 2-D model to examine the role that the oceans play in the cycling of atmospheric organic bromine.

JSP21/W/15-A4 1600

SEASONAL AND TEMPORAL VARIABILITY IN THE DISTRIBUTION OF METHYL BROMIDE IN THE SURFACE OCEAN

James H. BUTLER (NOAA/CMDL, 325 Broadway, Boulder CO, 80303, USA;

email: jbutler@cmdl.noaa.gov; +01-303-497-6898); Daniel B. King (University of Colorado/NOAA, CIRES; Boulder CO 80309, USA); Shari A. Yvon-Lewis (NOAA/AOML, Miami, FL 33149, USA); Jürgen M. Lobert (C4/SIO, Univ. of California San Diego, La Jolla CA 92093, USA); Stephen A. Montzka, James W. Elkins (NOAA/CMDL, Boulder CO, 80303, USA)

The ocean is both the largest known source and the second largest known sink for methyl bromide (CH3Br) in the atmosphere, yet we still don’t fully understand how the ocean regulates the atmospheric burden of this gas. Consequently, we cannot predict accurately how oceanic fluxes of methyl bromide will respond to global change. First-order calculations suggest that the steady-state, net flux of CH3Br from the ocean will act in opposition to changes in the atmospheric burden. The distribution of the oceanic sources and sinks of this gas, however, lacks uniformity on both small and large scales. The open oceans are typically undersaturated in CH3Br, with coastal waters commonly supersaturated. Warm waters tend to be undersaturated and cooler waters can be super- or undersaturated, but there are significant exceptions to these generalities. Although the chemical degradation of dissolved CH3Br is predominantly a function of sea-surface temperature, production appears to be mainly biological, and there is strong evidence for biological degradation as well. Here, we examine the distribution of methyl bromide from recent studies of its saturation in the Pacific, Atlantic, and Southern Oceans to examine spatial and temporal dependencies upon its net flux and, consequently, its production and degradation.

JSP21/W/07-A4 1620

DEGRADATION OF METHYL BROMIDE IN SURFACE WATERS OF ATLANTIC AND PACYFIC OCEAN (8N TO 45N).

Ryszard TOKARCZYK, Eric Saltzman (RSMAS, Univ. of Miami, FL-33149, USA,

email: rtokarczyk@rsmas.miami.edu, esaltzman@rsmas.miami.edu)

Methyl bromide is both produced and destroyed in the surface ocean, at rates which significantly impact its tropospheric burden and lifetime. Recycling of methyl bromide within the water column limits emissions from this source into the atmosphere and enhances the removal of atmospheric methyl bromide derived from anthropogenic or terrestrial biospheric emissions. Methyl bromide is destroyed in the ocean by chemical and biological processes. In this study seawater samples were incubated with 13C-labelled methyl bromide in order to measure the degradation rate, with the goal of determining the relative importance of chemical and biological processes. Degradation rates were measured during the GASEX-98 cruise across the Atlantic and the western Pacific coast (May-July 1998). Total measured degradation rates averaged from 0.17 to 0.31 per day. Methyl bromide degradation was faster in the Caribbean Sea and the coastal Pacific waters, than in the Atlantic. Temperature seems to be the primary factor controlling degradation rates in the mid-latitude ocean. On average, biological degradation contributed only 1-5% to the total degradation rates in these waters, but its contribution increases with decreasing temperature.

JSP21/W/01-A4 1640

ARE SOILS A GLOBALLY SIGNIFICANT SINK FOR ATMOSPHERIC CARBON TETRACHLORIDE AND METHYL CHLOROFORM?

James D. HAPPELL (Department of Marine and Atmospheric Chemistry, University of Miami, 4600 Rickenbacker Cswy, Miami, Florida, 33149, USA, email: jhappell@rsmas.miami.edu); Douglas W.R. Wallace (Abteilung Meereschemie, Institut fuer Meereskunde, Duesternbrooker Weg 20, 24105 Kiel, Germany, email: dwallace@ifm.uni-kiel.de)

Evidence will be presented that suggests that soils are a globally significant sink for atmospheric carbon tetrachloride (CT) and methyl chloroform (MC). Soil gas profiles of CT and MC obtained from temperate and sub-tropical forests and lawns indicate rapid removal of CT and MC in the top 30 cm of soil. The concentration of CT and MC in the soil gas decreased by up to 75 % and 50 % relative to the respective atmospheric concentration in the top 30-cm of the soil. CFC-11, CFC-12 and CFC-113 were also measured and there was no evidence for the removal of these compounds. These data and the changes in the soil gas profile during and after a large rain storm at one site will be used to speculate on likely removal mechanisms for CT and MC.

The soil gas profiles will also be used to calculate the flux of CT and MC into soils, and the flux estimates will be scaled up to give a preliminary estimate of the global soil sink strength for these compounds. Because estimates of the ODP and GWP for CT and MC are based on their atmospheric lifetimes, which in turn are based on estimates of their sources and sinks, a globally significant soil sink would require a reevaluation of their ODPs and GWPs. Changes in the atmospheric lifetime of MC will also affect the deduced concentration of tropospheric OH, because the global mean OH concentration is estimated from the MC lifetime. If the deduced OH concentration changes so will the estimated lifetime, ODP and GWP of other radiatively and/or chemically important gases (CH4, HCFCs, HFCs, CO, NOx, SO2, etc) oxidized by OH.

JSP21/E/03-A4 1700

AN INVESTIGATION INTO THE RELEASE OF HALOCARBONS BY PHYTOPLANKTON CULTURES.

Cristina F. PECKETT+*, Denise Smythe-Wright*and Duncan A. Purdie+. *George Deacon Division for Ocean Processes, +School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, Empress Dock, European Way, Southampton, SO14 3ZH,

email: crfp@soc.soton.ac.uk.

Halogenated methanes are known to contribute significantly to ozone destruction in the stratosphere and to play an important part in tropospheric chemistry. The oceans act as both a source and a sink for these halogenated methanes, and marine organisms are possibly a major oceanic source. This paper describes the results of an investigation into the release of a suite a halogenated methanes by a number of different phytoplankton species in culture.Cultures were grown for about four weeks in specifically designed, gas-tight glass vessels, which hold 1 litre of culture, leaving over 1 litre of headspace for gas exchange. The headspace was sampled every two days and analysed using a GC-ECD or GC-MS. Culture growth was monitored by cell counts and chlorophyll analysis. Species tested include Isochrysis galbana, Phaeocystis pouchetii, Chaetoceros sp., Emiliania huxleyi, Thalassiosira gravida and Prochlorococcus marina. Methyl chloride, methyl bromide and methyl iodide production has been detected for most species and some negative production, i.e. apparent uptake of halocarbons by the culture has also been observed. DMS production has also been detected. The mechanisms behind halocarbon production by phytoplankton are poorly understood. Further experiments are planned involving nutrient manipulation, the use of axenic cultures and grazing experiments. It is anticipated these will allow the mechanism of halocarbon production by marine phytoplankton to be investigated and results from some of these experiments will be discussed. The pattern of halocarbon production during the growth cycle will be compared between species and the significance of this halocarbon production on a global scale will be discussed.

JSP21/L/03-A4 1720

THE QUANTIFICATON OF DIMETHYLSULPHIDE

ARCHER SD, Stelfox CE , Burkill PH, (Centre for Coastal and Marine Science, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK); Malin G, Steinke M, Liss PS. (School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK)

Microzooplankton herbivory is one process by which dimethylsulfoniopropionate (DMSP) contained in phytoplankton, is converted to dimethylsulfide (DMS). Several previous studies have illustrated the extent of conversion of algal DMSP to DMS as a result of grazing by microzooplankton in laboratory cultures. However, few if any studies have demonstrated or quantified this process in natural waters. We have developed a new approach in the laboratory that has been applied to natural waters to quantify DMS production by microzooplankton. The model is based on the dilution technique that is used routinely to determine the impact of microzooplankton grazing on phytoplankton. The modified dilution approach provides simultaneous estimates of microzooplankton grazing rate, phytoplankton specific growth rate, grazing-mediated production of DMS and dissolved DMSP and the production/loss of DMS and dissolved DMSP in the absence of grazing.

During field studies in the southern North Sea and Iceland Basin in 1998, we found that microzooplankton were active grazers in waters containing DMSP-rich Phaeocystis spp. and autotrophic dinoflagellates. Our modified dilution technique was used to quantify this grazing impact and to estimate the production of DMS and dissolved DMSP that resulted from the grazing. Variations in the production of DMS and dissolved DMSP due to grazing reflect the differences in plankton composition between experiments. The experiments confirm the importance of bacterial processes to dissolved DMSP and DMS turnover and provide an indication of these rates in relation to grazing-mediated production. An impression of the relative importance of microzooplankton grazing to the DMS budget in each region was possible. We suggest that the technique provides a useful means by which some of the complex processes involved in the production of DMS in natural waters can be quantified and modelled.

Friday 23 July AM

Presiding Chair: Dr J Butler (NOAA/CMDL, Boulder, Colorado, USA)

JSP21/L/04-A5 0930

OBSERVATIONAL CONSTRAINTS ON THE GLOBAL BUDGET OF ATMOSPHERIC NITROUS OXIDE

James W. ELKINS, James H. Butler, and Thayne M. Thompson (Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA,

email: jelkins@cmdl.noaa.gov, jbutler@cmdl.noaa.gov, thompson@cmdl.noaa.gov)

Nitrous oxide (N2O) is a significant contributor to the radiation budget of the atmosphere and to the depletion of stratospheric ozone (O3). N2O is a strong greenhouse gas and it is the major source of ozone-depleting nitric oxide in the stratosphere. Increased levels of atmospheric N2O may delay the recovery of the O3-layer even though mixing ratios of the chlorofluorocarbons (CFCs) are decreasing. The relative effect of N2O in depleting stratospheric ozone also could increase in the future because methane has now reached steady state in the atmosphere; methane is the primary source of water that aids in tying up free chlorine. Our group has been monitoring N2O from ground-based stations since the mid-1970's and, in recent years, has developed small gas chromatographs for sampling N2O in the atmosphere from airborne and shipboard platforms. While trends over the past 20 years tend to be relatively constant, there have been some significant variations over short periods. This talk will focus on the observational constraints on the global budget of atmospheric N2O from measurements made from tall towers, baseline stations, aircraft, balloons, and ships.

JSP21/W/04-A5 0950

FACTORS GOVERNING THE OCEANIC NITROUS OXIDE SOURCE AND DISTRIBUTION

P. SUNTHARALINGAM (Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA, email: pns@europa.harvard.edu); J. L. Sarmiento (Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, NJ 08544, USA,

email: jls@splash.princeton.edu)

N2O is a radiatively important trace gas and also plays a significant role in stratospheric ozone depletion. The major contributors to the atmospheric budget are biological processes in soils and water. The oceanic source displays a high degree of spatial and temporal variability, making it difficult to derive reliable flux estimates from sparse surface measurements. Furthermore, the main formation mechanisms responsible for marine N2O remain subject to some uncertainty. In this study, ocean general circulation model (OGCM) simulations of the marine N2O distribution are employed to evaluate current understanding of the dominant formation processes and to estimate the magnitude of the global sea-air flux. N2O is modelled as a non-conserved tracer in a global OGCM and subject to biological sources and sinks in the ocean interior and gas-exchange at the surface. A comparison of two source presentations is presented; the first is based on observed correlations between excess N2O and AOU, and models N2O production as a linear function of organic matter remineralisation. The second parameterization incorporates, in addition, behaviour suggested in recent studies, namely, enhanced N2O production at low oxygen concentrations. Simulation results indicate that both parameterizations are able to reproduce the large-scale features of the observed distribution, i.e., high surface supersaturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic sub-tropical gyres. However, inclusion of a dependence on a depth varying quantity in the relationship between N2O production and remineralisation (as, for example, in the oxygen dependent case) is more successful in reproducing the structure of the observed deep N2O distribution. These simulations yield an estimate for the global open ocean water column N2O source of 3.8 Tg N per year (range 2.7 - 8.0 Tg N).

JSP21/L/05-A5 1010

NEW SOURCES AND SINKS OF NITROUS OXIDE

Sheo S. PRASAD (Creative Research Enterprises, P.O. Box 174, Pleasanton, CA, USA,

email: ssp@CreativeResearch.org)

Nitrous oxide (N2O) is an important greenhouse gas, since it is about 300 times more efficient than CO2 on per molecule basis and its atmospheric loading is rising. The current belief is that N2O has no atmospheric sources. However, recent observations of N2O isotopic anomalies and reviews of earlier laboratory experiments suggest the existence of significant atmospheric photochemical production of N2O. The talk will review these breaking developments and the experimental study, which shows that vibrationally highly excited O3 reacting with N2 in the gas phase produces N2O at an atmospherically significant rate. Kinetically, this result closely parallels the recently reported N2O production from vibrationally highly excited ClO. If we accept the 1990 IPCC position on the N2O source-sink budget, then the new atmospheric source bridges the source deficit. On the other hand, if the current IPCC position of a nearly balanced source-sink budget is accepted, then the new source implies that either the current methodology for estimating the surface emission is over-estimating this emission, or there is a significant missing sink. It will be further argued that this missing sink, if it is really there, must be a surface sink. Laboratory experiment based speculations about its nature, and suggestions for new field and laboratory experiments will be presented.

JSP21/W/06-A5 1050

LIGHT NON-METHANE HYDROCARBONS IN THE OCEAN - AN OVERVIEW

J. RUDOLPH (Centre for Atmospheric Chemistry and Chemistry Department, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada, email: rudolphj@yorku.ca)

Emissions from the ocean into the atmosphere are a source of a substantial number of different non-methane hydrocarbons (NMHC). However, the magnitude of this source is still under debate with estimates of global emission rates ranging from a few Tg/yr to over 50 Tg/yr. Since the atmospheric residence time of most non-methane hydrocarbons does not allow for long-range transport, oceanic emissions will determine the abundance of NMHC in the remote marine atmosphere. The oceanic emission rates of light alkenes, derived from oceanic concentrations and air-sea transfer formulations are in the range of several Tg/yr. The oceanic production rates derived from laboratory experiments and field observations are somewhat higher, suggesting that emission to the atmosphere is not the sole loss process for hydrocarbons produced in the ocean, although the difference between both estimates is statistically not significant. Total oceanic emissions of all non-methane hydrocarbons are estimated to be somewhat less than 10 Tg/yr. Consequently oceanic NMHC emissions have only a minor impact on the chemistry of the remote marine atmosphere. However, oceanic emissions have to be considered if measurements of reactive in the marine atmosphere are used to study atmospheric processes.

JSP21/E/04-A5 1110

ON INTERPRETING THE SEASONAL CYCLE IN AGAGE METHANE OBSERVATIONS AND VARIATIONS IN THE METHANE COLUMN.

D.CUNNOLD (School of Earth and Atmospheric Sciences, Georgia Tech, Atlanta, GA 30332-0340, USA Email: cunnold@eas.gatech.edu); P.Steele, P.Fraser, R.Prinn, P.Simmonds, R.Weiss and L.Emmons

Measurements of methane at the AGAGE sites are reported and compared against results from the CMDL network. Simultaneous measurements of the CFCs and CH3CCl3 are used to constrain the OH distribution and transport in models in order to better isolate the effects of regional sources on the observed seasonal cycle in methane. Results from a 2D model are reported and the 3D MOZART model is now being used to test the conclusions. The MOZART model is also being used together with the ground-based observations to investigate likely sources of variability in observations of vertical column methane which will be made by upcoming satellite experiments such as MOPITT.

JSP21/W/16-A5 1130

METHANE EMISSION REDUCTION IN WASTE MANAGEMENT

Percival THOMAS (Department of Environmental Management and Ecology, La Trobe University, P.O. Box 821, Wodonga, Victoria 3689, Australia)

Methane is considered as one of the main greenhouse gases because of its ability to trap much of the outgoing earth's radiated energy forming a thermal blanket around the globe, raising its temperature. It is produced in large quantities under anaerobic conditions in garbage dumps, swamps and cattle feedlots. Methane which is more potent than carbon dioxide in causing global warming has an atmospheric residence time of 10-12 years as compared to carbon dioxide's residence time of about 100 years, and reduction of methane from the atmosphere should produce an immediate beneficial effect. Solid waste landfills are found in every country where tonnes of garbage are disposed of each year. Gas produced from these sites consists of about 45-55% methane, which is also a useful source of energy, and therefore, control of atmospheric emission can have an added benefit if the energy content is utilised. In many developing countries the level of income of the people is so low that they cannot afford to pay for the conventional energy sources, and methane collected from garbage dumps can be a cheap source of energy. In most cases revenues from the sale of methane will offset the cost associated with the operation of the landfill.

JSP21/W/10-A5 1150

NON-CONTROLLED EMISSION OF METHANE, CARBON DIOXIDE, AND OTHER TOXIC GASES FROM LANDFILLS

Noemi LIMA, José M. L. Salazar, Nemesio Pérez (Environmental Research Division, ITER, 38594 Granadilla, Tenerife, Canary Islands, Spain, email: nlima@iter.rcanaria.es); Candelaria Perdigón, Ana Martinez-Zubieta (Faculty of Chemistry, Univ. La Laguna, 38206 La Laguna, Tenerife, Canary Islands, Spain); and Pedro A. Hernández (Laboratory for Earthquake Chemistry, Univ. Tokyo, Tokyo 113, Japan).

Landfills work as chemical and biological reactor where biogas and leachates are the products. CH4 and CO2 are major components of biogas, which also contains other trace gas components. These gas species are atmospheric contaminants which must be collected and either flared or utilized for the production of energy. Landfill gas extraction system recover just 35 to 50 % of the total production of biogas at landfills, but significant amounts of non-controlled emissions of major and trace biogas components could be released to the environment in the form of diffuse degassing. The goal of this study is to evaluate the non-controlled emission of biogas in Arico's landfill, where biogas extraction system is not operative yet. Arico's landfill has an extension of 350,000 m2 and 1,170 tons of urban solid wastes, which contents 48 % of organic mater, are daily deposited. Diffuse emission of CO2 levels were measured at 525 sites by means of a NDIR spectrometer and ranged from 0.8 to 32,868 g·m-2·d-1. Spatial distribution of diffuse emission of CO2 indicates that biogas degassing is not uniform all over the landfill. The total output of CO2 released to the atmosphere from Arico's landfill is about 150 t·d-1. The total output of biogas can be estimated by taking into consideration the CH4/CO2 molar ratio of the biogas; this amount is about 423 t·d-1. Trace components emission levels are 11 t·d-1 of H2O, 153 t·yr-1 of VOCs, 0.9 t·yr-1 of C2HCl3, 2.5 t·yr-1 of C2Cl4, and 1.1 kg·yr-1 of As. Significant amounts of non controlled emission of toxic gases to the surrounding environment suggest important environmental and engineering implications. In addition, generation of methane from numerous anaerobic wastewater treatment ponds, sludge digesters and artificial wetlands designed to treat wastewater needs attention, if a country has to maintain its commitment to the United Nations Framework on Climate Change.

JSP21/W/14-A5 1210

ESTIMATION OF THE METHANE EMISSIONS FROM THE WEST SIBERIA WETLANDS AND OIL AND GAS DEPOSITS BY THE 3D REGIONAL CHEMICAL TRANSPORT MODEL

Svetlana JAGOVKINA, Igor Karol, Vladimir Zubov (Main Geophysical Observatory, Karbyshev str. 7, 194021, Russia, email: karol@main.mgo.rssi.ru); Victor Lagun (Arctic and Antarctic Research Institute, Bering str. 38, 199397, Russia, email: lagun@aari.nw.ru); Eugeny Rozanov (University of Illinois at Urbana- Champaign, IL 61801, USA, email: rozanov@uiatma.atmos.uiuc.edu)

Methane is one of the most important greenhouse gases, but some its source intensities are not well known. The 3-D regional transport chemical model of the troposphere with the calculated PBL and wind fields of the above layers from data bases is used for estimation of methane fluxes from West Siberia Region (58-730N x 62-810E). The model has 0.5x10 resolution, 10 layers up to 1 km and 10 layers in the troposphere. Databases of the wind and temperature fields are created from the data from Russian meteorological stations located in the considered and adjacent areas. The estimations of the methane fluxes from gas deposits, as well as from the wetland are made on the base of the data for three periods (July 1993, June 1996 and May 1997) of airborne and ground surface methane concentration measurements. The results of the model estimations are compared with published data of the methane fluxes from Canadian and Alaska wetlands and the input of West Siberia Region into the global methane budget is evaluated.