DECEMBER 2005|1738Coupling between Land Ecosystems and the Atmospheric Hydrologic Cycle through Biogenic Aerosol PathwaysBY MARY BARTH, JOSEPH P. MCFADDEN, JIELUN SUN, CHRISTINE WIEDINMYER, PATRICK CHUANG, DON COLLINS, ROBERT GRIFFIN, MICHAEL HANNIGAN, THOMAS KARL, SI-WAN KIM, SONIA LASHER-TRAPP, SAMUEL LEVIS, MARCY LITVAK, NATALIE MAHOWALD, KATHARINE MOORE, SREELA NANDI, EIKO NEMITZ, ATHANASIOS NENES, MARK POTOSNAK, TIMOTHY M. RAYMOND, JAMES SMITH, CHRISTOPHER STILL, AND CRAIG STROUDAUTHOR AFFILIATIONS: BARTH, SUN, WIEDINMYER, KARL, KIM, LEVIS, MAHOWALD, MOORE, NANDI, NEMITZ, POTOSNAK, SMITH, AND STROUD—National Center for Atmospheric Research, Boulder, Colorado; MCFADDEN—University of Minnesota, Saint Paul, Minnesota; CHUANG—University of California, Santa Cruz, Santa Cruz, California; COLLINS—Texas A&M University, College Sta-tion, Texas; GRIFFIN—University of New Hampshire, Durham, New Hampshire; HANNIGAN—University of Colorado, Boulder, Colorado; LASHER-TRAPP—Purdue University, West Lafayette, Indi-ana; LITVAK—University of Texas, Austin, Texas; NENES—Georgia Institute of Technology, Atlanta, Georgia; RAYMOND —Bucknell University, Lewisburg, Pennsylvania; STILL—University of Califor-nia, Santa Barbara, Santa Barbara, CaliforniaCORRESPONDING AUTHOR: Dr. Mary Barth, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307E-mail: [email protected]:10.1175/BAMS-86-12-1738©2005 American Meteorological SocietyThe importance of land surface properties in con-trolling the exchanges of energy, water, and mo-mentum with the atmosphere, and thus in infl u-encing local and regional climate, is well recognized. Important land surface characteristics—including albedo, surface roughness, latent and sensible heat fl uxes, and rates of evapotranspiration—aff ect the way in which water is transferred to the atmosphere, processed in the atmosphere, and eventually returned to the surface. Th e amount of water in the atmosphere and returning to Earth, in turn, aff ects many of the key properties of the land surface.Terrestrial ecosystems also modulate aerosols, such as dust and biogenically derived particles. Such aero-sols may play an important role (Fig. 1) in exerting con-trol over cloud development and precipitation through their eff ects on the nuclei on which cloud droplets condense or ice forms. Biogenic and other aerosols, which incorporate organic material, can be eff ective cloud condensation nuclei (CCN) and, thus, infl uence microphysical and optical properties of clouds.Likewise, cloud properties aff ect the quantity and type of biogenic aerosols in the atmosphere. Changes in cloudiness, cloud optical properties, precipitation, and other meteorological variables directly control the emission and formation of biogenic aerosols by altering the availability of radiation for photosynthe-sis and by altering temperature. By aff ecting vapor pressure, temperature also determines the phase distribution of some biogenic aerosols.Conceptually, the potential feedbacks between aerosols originating in the biosphere and the hydro-logic cycle are clear. However, the processes control-ling each step in this coupled system are highly uncer-tain, and the relative importance of these processes to the atmospheric hydrologic cycle is unknown. Th us, these processes have not yet been incorporated into quantitative numerical models describing the interac-tions between the terrestrial biogeochemical aerosol cycle and the hydrologic cycle.We hypothesize that feedbacks between the terres-trial biosphere and the atmosphere by way of biogenic aerosol pathways exist, and that these feedbacks can be important in both direct and indirect radiative process-es. An investigation into this coupled cycle is necessary for a better understanding of the Earth system, including climate change, regional and global atmospheric chem-istry, haze and visibility, weather, and changes in land cover, including biodiversity and ecological changes. Our discussions about these feedback mechanisms led to recommendations for future research.DECEMBER 2005AMERICAN METEOROLOGICAL SOCIETY|1739Energy and water exchange. Land-surface character-istics, such as albedo, land cover, and surface rough-ness, control the transfer of water and energy to the atmosphere. Evapotranspiration is controlled not only by land-surface hydrology, but also by vegeta-tion structure, plant physiology, and atmospheric variables such as solar radiation, air temperature, and humidity. These atmospheric state parameters are modulated by the absence or presence of clouds, the cloud lifetime, the cloud’s optical properties, and precipitation. One example of this modulation is the gradual change of vegetation patterns as soil dries and vegetation returns after heavy rainfall. The importance of aerosols linking the biogeochemical and atmospheric hydrological cycles should be assessed in the context of energy and water exchanges between ecosystems and the atmosphere.Aerosols, CCN, and cloud properties. Clouds and perturbations of clouds affect the CCN concentrations and chemistry (of both aerosols and pre-cursors) by modifying the structure and radiation in the boundary layer. Additionally, biogenic aerosols are linked with cloud formation and rainfall through aerosol growth and cloud drop formation. The growth of aerosols through condensation of semi-volatile and low volatility species and coagulation of aerosols to the size range of CCN has an important effect on the CCN properties that modify cloud drop formation. In traditional theory the droplet formation potential of aerosols is controlled by the hygroscopicity (or, the effective moles of solute released into the droplet solution) and the surface tension of the droplet. Both of these properties vary significantly with dry size, density, and chemical composition. Water soluble organic compounds tend to increase the solution density and decrease the surface tension, therefore increasing CCN number concentrations.The number of CCN is a critical link between aerosols, clouds, and precipitation. When CCN number concentrations are higher, more cloud drops are formed for the same liquid water content. Higher numbers of cloud drops increase the albedo of the cloud, prolong the lifetime of the cloud, reduce pre-cipitation, and aff ect the cloud thickness. Ice nuclei may be equally important in modulating mixed phase clouds and the resulting precipitation, but are less well understood. Th ese eff ects on