14 Perspectives on IGAC
Alex Pszenny
IGAC Core Project Office
MIT, Cambridge MA
One of IGAC's three overall goals is to understand interactions between atmospheric chemical composition and biospheric and climatic processes. Research in pursuit of this goal in the North Atlantic region is underway or planned within four IGAC Activities (see Annex X). The cycling of sulfur in the marine atmosphere and upper ocean is a prominent theme common to all. This prominence is due in large measure to the potential for sulfurous aerosols to affect Earth's radiative balance both directly through the backscatter of solar radiation (Charlson et al., 1994) and indirectly as cloud condensation nuclei (Charlson et al., 1987). The total global mean radiative forcing due to anthropogenic aerosol particles is calculated to be of the same magnitude (as much as 1 W m-2) but opposite in sign to the forcing due to CO2 and other "greenhouse" gases (e.g., Penner et al., 1993). Although aerosols have this potential climatic importance, they are hardly considered by global climate models in their treatment of radiative transfer and clouds. This is a result of the lack of data on the required time and space scales, and of a clear understanding of the processes linking emissions and effects. At present, the effects of tropospheric aerosols are among the largest uncertainties in quantifying climate forcing due to man-induced changes in atmospheric composition (Hansen and Lacis, 1990). Considerable attention must be focused both on quantifying the processes controlling the natural and anthropogenic aerosol and on defining and minimizing the uncertainties in the calculated climate forcings.
Biological processes are a major source of sulfur to the marine atmosphere, mainly as dimethylsulfide (DMS). IGAC/MAGE researchers are working to quantify and to develop a process understanding of this source (see Figure 14.1). Photochemical oxidation of DMS in the atmosphere is thought to yield primarily sulfuric acid which, as sulfate, is often the dominant component of total aerosol mass. It is therefore essential that we understand DMS production mechanisms and rates in the ocean if we are to develop realistic climate prediction capabilities.
The nitrogen cycle is another theme common to MAC, MAGE, NARE, and PASC. NARE studies in the Bermuda area have shown that deposition to the ocean surface can be an important source of fixed nitrogen for the Sargasso Sea when waters are oligotrophic and when the mixed layer is shallow (Owens et al., 1992; Michaels et al., 1993). Evidence from recent work in the Pacific suggests that there may be a net upward flux of ammonia (NH3) across the sea-air interface of magnitude similar to that of DMS (Quinn et al., 1988; 1990). Zhuang and Huebert (manuscript in preparation) obtained similar results over the eastern subtropical North Atlantic. Ammonia is of particular interest because it is the major neutralizing species for sulfuric acid aerosols in the marine atmosphere. In addition, evasion of NH3 from surface waters followed by atmospheric transport and redeposition would be a means of redistributing biologically utilizable N on a regional scale.
A third theme of common interest is mineral aerosol. In the marine atmosphere "dust" can attenuate infrared radiation thus affecting radiative transfer (Penner et al., 1993). This effect is so strong in the "Saharan plume" region that specific corrections to satellite sea surface temperature data are required (May et al., 1992). A link between the atmospheric sulfur and iron cycles has been hypothesized (Zhuang et al., 1992). Atmospheric deposition of mineral dust also can be an important source of trace metals, including the essential micronutrient Fe, to the oceans (Donaghay et al., 1991; Duce and Tindale, 1991).
Few specific plans for field work within these IGAC activities after 1997 have been formulated. Given this, and the long lead time available, a continuing joint planning process may serve to minimize problems with funding limitations and ship availability that have severely limited previous efforts to conduct IGAC studies of the marine S, N, and "dust" cycles jointly with JGOFS C cycle process studies.
Annex X. IGAC Activities Conducting or Planning Research in the North Atlantic Region, Their Coordinating Committee Conveners, and Published Goals
Multiphase Atmospheric Chemistry (MAC) Co-Conveners: T.S. Bates, NOAA/PMEL/OCRD, Bldg. 3, 7600 Sand Point Way NE, Seattle, WA, 98115, USA; Tel: (206) 526-6248; Fax: (206) 526-6744; E-mail: bates@pmel.noaa.gov and J. L. Gras, CSIRO Division of Atmospheric Research, Private Bag No. 1, Mordialloc, Victoria 3195, Australia; Tel: (+61-3) 586-7666; Fax: (+61-3) 586-7600; E-mail: jlg@dar.csiro.au
* Identification: To develop a chemical and physical characterization of aerosol particles in the key air masses (i.e., clean vs. polluted marine air, clean vs. polluted continental air, and upper tropospheric air)
* Dynamics: To understand the physical and chemical factors that produce aerosol particles and that control their evolution and relevant physical properties
* Modeling: To develop and use improved aerosol and cloud parameterizations in order to relate the factors controlling the radiative and cloud nucleating properties of aerosol particles to sources of aerosols and aerosol precursors and large-scale variables used in climate model calculations
* Spatial extrapolation: To evaluate the means to utilize and interpret remote sensing to extend local aerosol concentration measurements to larger geographical areas
Marine Aerosol and Gas Exchange: Atmospheric Chemistry and Climate (MAGE) Convener: B.J. Huebert, Department of Oceanography, University of Hawaii, Honolulu, HI, 96822, USA; Tel: (808) 956-6896; Fax: (808) 956-9225; E-mail: huebert@okika.soest.hawaii.edu
* To understand the chemical, biological, and physical mechanisms that control the exchange of trace gases and particulate materials 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
North Atlantic Regional Experiment (NARE) Co-Conveners: F.C. Fehsenfeld, NOAA Aeronomy Laboratory (R/E/AL7), 325 Broadway, Boulder, CO, 80303-3328, USA; Tel: (303) 497-5819; Fax: (303) 497-5126; E-mail: fcf@aztec.al.bldrdoc. gov and S.A. Penkett, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom; Tel: (+44) 603 56 161; Fax: (+44) 603 52 420; E-mail: m.penkett@uea.ac.uk
* To assess the long-range transport of photochemically active compounds and/or their products and determine the impact of this transport on hemispheric air quality
* To ascertain the effects of these photochemically active compounds on the oxidative properties and radiation balance of the atmosphere
* To estimate the amounts of these photochemically active compounds that are deposited in this marine environment and to determine the impact of this deposition on surface seawater chemistry and marine biological processes
Polar Atmospheric and Snow Chemistry (PASC) Co-Conveners: L.A. Barrie, Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4; Tel: (416) 739-4868; Fax: (416) 739-5704; E-mail: lbarrie@dow.on.doe.ca and R.J. Delmas, Laboratoire de Glaciologie et Geophysique de l'Environnement, B.P. 96, 38402 St. Martin d'Heres Cedex, France; Tel: (+33) 76 82 42 65; Fax: (+33) 76 82 42 01; E-mail: delmas@glaciog.grenet.fr
* To understand the role of tropospheric chemistry in the polar regions in global change
* To establish the relationship between atmospheric chemical composition and that of glacial snow and ice in the polar regions
* To document present and planned studies of the polar troposphere and of snow chemistry relevant to global change
* To identify knowledge gaps in polar atmospheric and snow chemistry and encourage coordinated efforts to fill those that cannot be addressed by individual studies