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Coordinating Committee
A. Bandy (USA)
T. Bates (USA)
B. Bonsang (France)
A. Delany (USA)
R. Duce (USA)
B. Hicks (USA)
B. Huebert
(USA) ,
Convener
S. Larsen (Norway)
C. Leck (Sweden)
P. Liss (UK)
P. Matrai (USA)
B.-C. Nguyen (France)
W. Oost (Netherlands)
A. Pszenny (USA)
N. Tindale (Australia)
Marine Aerosol and
Gas Exchange (MAGE)
Editor's Note
The description of this Activity is from IGBP Report No. 32 (IGAC: The Operational Plan) published in 1994. An update is awaited. For current information about MAGE contact the Convener(s) listed below.
Introduction/Rationale
The oceans cover about 70% of the Earth's surface and are major sources and major sinks of many trace species that affect the radiative balance of the Earth either directly (e.g., CO2, methane (CH4), nitrous oxide (N2O)) or indirectly (CO, hydrocarbons, halogen and sulphur compounds) by altering the photochemistry of the marine atmosphere. It is essential that these source, sink, and transformation processes be studied in detail.
Understanding the exchange between the atmosphere and oceans requires studies of both emissions from the ocean surface and deposition to it from the atmosphere. Since these studies are limited by the lack of direct measurements of the most important fluxes, MAGE has made the development and testing of new flux-measuring strategies and technologies one of its principal objectives. Furthermore, it is necessary to improve the current understanding and predictive capabilities of the physical and biogeochemical processes controlling the oceanic and atmospheric concentrations of these climatically important trace species. Studies of these processes (e.g., the physical parameters influencing air-sea exchange, the biological production and consumption of climatically important gases and their precursors, marine photochemistry and its effect on chemical concentrations) will require not only measurements of the trace species but also of many ancillary physical, chemical, biological and meteorological parameters. The complexity of these studies requires an integrated, interdisciplinary approach which often includes simultaneous measurements from several research platforms.
Sulphur chemistry is a subject of particular interest to MAGE. Improving the current understanding of the impact of DMS emissions on clouds is fundamental to improving climate models, since some models suggest that a 4% increase in the areal extent of marine stratocumulus clouds could have a cooling effect equivalent to (offsetting) a 30% increase in CO2. Cloud condensation nuclei (CCN) formed from DMS might be one of the significant controllers of cloud radiative properties. Again, such studies are limited by the difficulty in measuring both the emissions of DMS and the deposition of sulphur dioxide (SO2), MSA, and sulphate aerosol.
Continental outflow can affect the chemistry of the marine boundary layer (MBL) through the addition of anthropogenic pollutants. To determine the effect of reducing air pollution on global change, it is essential, for instance, to find out in what regions of the ocean sulfate aerosol formation is dominated by man-made SO2, and where DMS is the major source. The continents can also have a major impact on biota, since the atmospheric transport of nutrients derived from continental sources is thought to limit productivity in some marine regions. How might changes in land-use practices alter the productivity of the oceans? Such uncertainties and questions cannot be answered without much better estimates of fixed-nitrogen and trace metal deposition.
Finally, marine aerosols are important factors in the global radiation budget, independent of their role as CCN. Existing models of aerosol deposition are very sensitive to such factors as size, which, unfortunately, changes dramatically over the normal humidity gradients in the MBL. Improved deposition measurements are needed to estimate the lifetimes of these radiatively important particles, so that the impact of changes in source terms can be modeled.
Scientific Objectives
- To understand the chemical, biological, and physical mechanisms that control the exchange of trace gases and particulate material between the atmosphere and the ocean surface.
- To develop formulations of ocean exchange processes for inclusion in global-scale climate and air chemistry models.
- To extend the experimental knowledge of air-sea interchange to conditions with strong winds, rough seas, and spray.
Implementation
The MAGE Coordinating Committee met in Anaheim, California in February, 1990, and subsequently in Chamrousse, France in September, 1990, to discuss the action plans for this Activity. The Committee identified two areas in which MAGE could contribute to studies of air-sea exchange. The first is to promote the development of new measurement technologies, while the second is to organise field projects both to test the new technologies and to facilitate large interdisciplinary research studies. Opportunities have been identified in both areas. The MAGE Coordinating Committee also met in Hobart, Tasmania, in February, 1993. The Committee decided then to encourage the addition of a biological experiment to the ACE-1 programme (Southern Hemisphere Marine Aerosol Characterisation Experiment; see Task 1.2.2.C and Activity 6.4), and to investigate European interest in a MAGE experiment in the Atlantic or Mediterranean after ACE-1.
Task 1.2.1: Development of Novel Measurement Techniques
At present, air/sea exchange research is seriously limited by technology. Most of the important fluxes have never been measured directly in the field. Improvements in the understanding of marine aerosol properties and gas exchange processes can only be achieved through the development of new measurement technologies.
MAGE is very interested in supporting the efforts of principal investigators who wish to develop and deploy novel methods for estimating marine surface fluxes. For instance, MAGE has encouraged the development of fast sensors for DMS, SO2, CO2, and other species, in the hopes of making eddy-correlation measurements of fluxes from ships or towers alongside traditional techniques. MAGE is also encouraging the improvement of shipborne eddy - correlation methodology. One of the ASTEX/MAGE ships (see Task 1.2.2, section B below) carried an extensive micrometeorological system, which was primarily used for estimating momentum and moisture fluxes.
Task 1.2.2: Field Measurements (Methodology Intercomparison Tests)
MAGE investigators completed two significant field programmes in 1992 and additional ones are planned for 1995 and beyond.
A. MAGE/JGOFS Equatorial Pacific Experiment
This was the first field programme of the MAGE Activity. It was planned to coordinate with the Joint Global Ocean Flux Study (JGOFS) and to complement the JGOFS oceanographic programme with non-CO2 trace gas and atmospheric chemistry measurements.
Two MAGE ships participated alongside the research vessel Tommy Thompson in the Equatorial Pacific JGOFS experiment during the spring of 1992. Since the primary JGOFS interest is in the carbon cycle and biological productivity, the MAGE observations of atmospheric nutrients and biogenic gases were designed with collaborative interpretations of the phenomena in both phases in mind. NOAA's (National Oceanic and Atmospheric Administration) research vessel, Vickers, carrying investigators from ten U.S. institutions and Russia, conducted a study which emphasised biogenic trace gas fluxes and sulphur chemistry. Scientists on the research vessel Wecoma studied fluxes of atmospheric carbon, nitrogen, and sulphur and the impact of atmospheric iron on productivity.
B. ASTEX/FIRE Experiment in the Tropical Atlantic
FIRE (First ISCCP Regional Experiment) is an on-going multi-agency programme designed to promote the development of improved cloud and radiation parameterisations for use in climate models, and to provide assessment and improvement of ISCCP (International Satellite Cloud Climatology Programme) products. It is primarily a national project of the United States, with important contributions by scientists from the United Kingdom and France. The strategy of FIRE has been to combine modeling activities with satellite, airborne, and surface measurements to study two types of cloud systems -- cirrus and marine stratocumulus -- that have important roles in climate system by virtue of their extensive areal coverage, persistence, and radiative effects. FIRE has been designed to be conducted in two phases. FIRE Phase I (1984-1989) was designed to address fundamental questions concerning the maintenance of cirrus and marine stratocumulus cloud systems. FIRE research over those years has led to major improvements in the current understanding of the role of these clouds in the global climate system. FIRE Phase II (1989-1994) is focusing on more detailed questions concerning the formation, maintenance, and dissipation of these cloud systems.
The Atlantic Stratocumulus Transition Experiment (ASTEX) is a part of the second series of FIRE international cloud climatology experiments. The ASTEX/MAGE experiment was a multinational effort to improve our capability for studying cloud/chemistry interactions and the air/sea fluxes that affect them. Improved analytical techniques and new observational strategies were tested, with the goal of incorporating more realistic chemistry and physics into climate models.
MAGE contributed two ships, two aircraft, and surface measurements on two islands to the ASTEX/MAGE experiment in the eastern subtropical North Atlantic in June of 1992. The aim of ASTEX/MAGE was to understand the factors -- including air/sea fluxes and the formation and loss of aerosols -- which control the life cycles and radiative properties of marine stratocumulus clouds. Extensive sulphur, hydrocarbon, and photochemical experiments were conducted in both Eulerian and Lagrangian reference frames. Constant-density balloons and an inert tracer were twice released from the research vessel Oceanus to tag airmasses, watch their Lagrangian evolution, and evaluate surface sources and sinks. The NCAR (National Center for Atmospheric Research) Electra flew three or four flights into each of these airmasses, while the University of Washington C-131 made one flight. NOAA's research vessel Malcolm Baldrige positioned itself downwind to characterise the tagged air as it passed out of the study area. This effort to develop new experimental strategies for studying air/sea fluxes depended on the combined efforts of scientists from the U.S., France, Germany, England, Spain, and Portugal.
A tremendous amount of experience was obtained from the ASTEX/MAGE experiment about how to conduct Lagrangian studies in the marine atmosphere. One Lagrangian experiment was conducted in extremely clean air, while the other was in polluted air from Europe. Data analyses are underway; it is becoming clear that the budget studies have produced excellent pictures of the evolution of marine sulphur over time. At the conclusion of the ASTEX/MAGE experiment, the science teams met in Santa Maria, Azores in June, 1992, to debrief and discuss plans for data analysis. A data workshop was conducted in February, 1993, in Hilo, Hawaii.
C. First Aerosol Characterisation Experiment
The Southern Hemisphere Marine Aerosol Characterisation Experiment (ACE-1) is being planned in collaboration with IGAC Activity 6.4 (MAC, Multiphase Atmospheric Chemistry). ACE-1 is to take place in late 1995 in the vicinity of Cape Grim, Tasmania, and will involve scientists from New Zealand, Australia, the U.S., Europe, and southeast Asia. The intent is to study the DMS flux and its conversion to aerosols in a region where anthropogenic influences are minimal.
This experiment will also be coordinated with the IGBP/JGOFS Southern Ocean experiment, which was originally planned for the same area and time period. Although the U.S. JGOFS Southern Ocean field work has been delayed until 1996, MAGE will coordinate its measurements with Australian JGOFS. IGAC scientists hope to have one research vessel (NOAA's Discoverer) on which atmospheric sampling and studies of sulphur and nutrient chemistry and biology in the water column can be conducted. Italian tracer technology will be used to confirm the Lagrangian trajectories identified by both constant-density and constant-altitude balloons released from the ship. NCAR's new C-130 will be the primary airborne sampling platform. There are opportunities for investigators to make related measurements at surface sites at Cape Grim, in New Zealand, and on Macquarrie Island (where IGAC scientists will be erecting a sampling tower for the experiment), as well as from Antarctic supply ships which will be transiting the study area. ACE-1 offers an excellent chance for investigators to observe the relationship between gradients in water column and atmospheric properties in the Southern Ocean, in the context of a very comprehensive atmospheric chemistry experiment.
D. Second Aerosol Characterisation Experiment
The second in this series of experiments, Radiative Forcing due to Aerosols over the Polluted North Atlantic Region (ACE-2), is planned for boreal summer, 1997. A multi-national effort built around a core of European scientists will conduct studies similar to those planned for ACE-1 (see Activity 6.4).
E. Flux Measurement Methods Development and Testing Experiments
Reliable methods for measuring fluxes of trace species directly are still lacking, so significant work is needed on technique development. Although it is likely that so-called "two-layer" models will be used for parameterising exchange in models, confirming techniques based on different physical principles are needed for each important species. Micrometeorological techniques are particularly important, since they measure fluxes directly on relatively short time scales. Accordingly, two future experiments have been conceived.
The first would be a tower-based field campaign which the two-layer method (TLM) and micrometeorological approaches would be intercompared. The Dutch research platform Meetpost Nordwijk is an ideal site for such an experiment, since its flow distortion has already been well characterised. Ideally the experiment would include eddy-correlation of all species which could be measured rapidly such as O3, DMS, NOY, N2O, total sulphur, CO2 and, perhaps, CH4. Eddy-accumulation and conditional-sampling methods would be used for a variety of species such as hydrocarbons, alkyl halides, DMS, and, perhaps, NH3. Several fluxes (DMS, some hydrocarbons) could also be measured by gradient methods. Finally, a ship would be used to do the surface measurements for the TLM comparison, and a dual-tracer experiment might be included. A small aircraft might profitably be added to the experiment to confirm the tower and shipboard measurements. An October, 1994, planning workshop for this experiment is envisioned.
Once this experiment has characterised the most promising techniques, they then could be used in a second experiment which would be designed to assess fluxes over a wide range of forcing factors. It has been suggested that a transect across the Atlantic at about 200S in approximately 1998 would work well for this. This area is relatively convenient logistically, with airfields on both ends and near the middle of the region. It would allow studies of productivity and fluxes in rich upwelling Argentinean waters, then yellow river water, then blue, non-productive waters, and finally in rich, upwelling waters near Africa. The contrast would provide the kinds of data needed to create expert systems which will convert satellite measurements of chlorophyll into reliable biogenic trace gas flux estimates. The possibility of joint work with the JGOFS North Atlantic Process Study in 1998 or 1999 is being explored as well.
Data Archival and Availability
Information not supplied.
Timetable
1992: MAGE Equatorial Pacific Experiment in the spring.
ASTEX/MAGE experiment in June.1993: IGAC MAGE/MAC Planning Meeting for Southern Hemisphere Marine Aerosol Characterisation Experiment, in Hobart, Australia in February. 1994: Meeting of the MAGE Coordinating Committee in September in Japan (in conjunction with the CACGP Meeting). Articulation of other MAGE objectives, such as studying the impact of severe storms on fluxes; discussion of novel approaches for understanding air/sea fluxes.
Planning workshop in October in the Netherlands for Meetpost Nordwijk flux measurement intercomparison experiment.1995: Field experiment (ACE-1) in Southern Ocean in October-December. 1996: Meetpost Nordwijk flux measurement intercomparison experiment. 1997: Field experiment (ACE-2) in North Atlantic in June-August. 1998: South Atlantic flux measurements transect cruise.
Relationships to Other Activities
Information not supplied.