Environ. Sci. Technol. 1988, 22, 1018-1026
Cloudwater Chemistry from Ten Sites in North America Kathleen C. Weathers,",' Gene E. Likens,' F. Herbert Bormann,* Susan H. B i ~ k n e l lBernard ,~ T. B ~ r m a n n , ~ Bruce C. Daube, Jr.,' John S. Eaton,' James N. Galloway,' William C. Keene,* Kenneth D. Kimball,' William H. McDowell,8Thomas G. Siccama,' Daniel Smlley,' and Robert A. Tarrant" Institute of Ecosystem Studies, The New York Botanical Garden, Millbrook, New York 12545, Yale University, New Haven, Connecticut 065 11, Humboldt State University, Arcata, California 9552 1, U S . Department of Agriculture Forest Service Research, Juneau, Alaska 9980 1, California Institute of Technology, Pasadena, California 91 125, University of Virginia, Charlottesville, Virginia 22902, Appalachian Mountain Club, Gorham, New Hampshire 03581, Center for Energy and Environment Research, San Juan, Puerto Rico 00936, Mohonk Preserve, New Paltz, New York 12561, and Oregon State University, Corvallis, Oregon 97331
Cloudwater and rainwater samples were collected at 2.5 m above ground during 2 years from 10 nonurban sites in North America. On the average, cloudwater collected from sites in the eastern United States was more acidic and had higher concentrations of N0f and SO-: than that at sites in the western United States and Puerto Rico. Of all the sites in the network, cloudwater from Maine had the highest concentrations of these ions. Concentrations of hydrogen ion, nitrate, and sulfate were significantly higher in cloudwater than in rainwater at most sites; however, on a paired-event basis, enrichment factors for cloud vs rain varied greatly. In contrast to distributions of inorganic anions, the concentrations of formate and acetate in cloudwater and rainwater were similar at sites in the western and eastern United States, suggesting that these compounds originated primarily with natural rather than anthropogenic sources.
Introduction Between May 1984 and November 1985, ground-level cloudwater and rainwater samples from 10 nonurban montane and coastal locations in North America were collected and chemically analyzed under the auspices of the Cloud Water Project (CWP). The CWP was initiated because few data were available on cloudwater chemistry and no large-scale, centrally coordinated investigation of cloudwater had been undertaken. The major goal of the CWP was to collect chemical data on cloudwater and rainwater by using standardized protocols and to compare inter- and intrasite differences across a wide geographical range. Previous investigators have assessed the importance of cloud and fog as a source of water for specific ecosystems (e.g., ref 1-4). Indeed, it has been suggested that the geographical distribution of certain vegetation types is correlated with the presence of cloudwater and fogwater (3,5,6).Recently, interest in cloudwater and fogwater as a source of toxic chemicals as well as nutrients to natural ecosystems has increased (e.g., ref 7-15). Extremely acidic cloud and fog events have been documented for sites in the northeastern United States (13, 16), southern California (IO),the United Kingdom (12),and Switzerland (17). Current hypotheses about forest decline in Europe and the eastern United States have suggested that acidic Institute of Ecosystem Studies. 2Yale University. Humboldt State University. 4U.S. Department of Agriculture Forest Service Research. California Institute of Technology. BUniversity of Virginia. 7Appalachian Mountain Club. Center for Energy and Environment Research. Mohonk Preserve. '0 Oregon State University. 1018
Environ. Scl. Technol., Vol. 22, No. 9, 1988
cloudwater, in combination with other pollutants, may contribute significantly to this decline (18). Results of laboratory experiments have indicated that the combination of photochemical oxidants and acidic mists damages some vegetation by altering cell structure in the leaf by ozone and subsequent leaching of ions upon contact with acid fog (19,201. Weathers et al. (13)documented an event in August 1984 at Bar Harbor, ME, where high concentrations of ozone and acidic fog occurred concurrently.
Methods CWP sites were located where cloudwater was expected to be an important input to the ecosystem. (In this study, fog was considered ground-based cloud.) Sites ranged from high-elevation montane (Mt. Washington, NH, at 1534 m msl) to low-elevation coastal areas (Bar Harbor, ME, at 5 m msl) and in vegetation type from elfin forest to alpine tundra (Table I; Figure 1). General criteria for site selection included (1) high frequency of cloud events, (2) distance from local sources of pollution such as power plants, automobiletraffic, and large agricultural areas, and (3) availability of field personnel to operate and clean the equipment and to measure the pH of samples in the field. When possible, sites were located in areas where other ecological studies were being done. Cloudwater was sampled with a CWP active collector, which has been described in detail elsewhere (21). The collector used a fan to draw air and atmospheric droplets through a downward-facing opening and across a removable cartridge of vertical, 0.78 mm diameter Teflon strands. Droplets impacted upon the strands and dripped into a polyethylene bottle. The collector was situated on a stand approximately 2.5 m above the ground. The collector excluded rain in low-wind conditions and reduced rain contamination at moderate wind speeds (510 m/s). Rain samples were collected in a standard Hubbard Brook type bulk precipitation collector (22) during cloudwater collections and, at some locations, during periods of rain without clouds. Accumulation of cloudwater droplets on the surface of the funnel during cloud-only events was not observed. Therefore, cloud deposition to the rain funnels was considered negligible. Both collectors were kept covered between events to exclude dry deposition. Cloud and rain samples were collected on an event basis, although we were not able to sample all cloud events. A cloud event was defined by visibility: collection was initiated when an object approximately 1 km distant was obscured from view consistently for 15 min. The standard collection period lasted approximately 5 h; however, the duration of some evenbs was less than 5 h, and on other occasions sampling times exceeded 5 h because collections were made overnight. Sample pH was measured in the field according to standard CWP protocols (16) and sent
0013-936X/88/0922-1018$01.50/0
0 1988 American Chemical Society
Table I. Site Descriptions
latitude; longitude
site
elevation
dominant, vegetation
25 m rnsl Picea stichensis Tsuga heterophylla 800 m msl Tsuga mertensiana Carex spp. Cassiope spp. 44O26’; 123O38’ 1249 m rnsl Tsuga heterophylla Marye Peak, OR Redwood National Park, 41O15’; 124O02’ 287 m msl Sequoia semipervirens Picea stichensis CA Alnus oregana 18’19’; 65O45’ 1020 m rnsl Tabebuia rigida Pic0 del Este, PR Octoea spathulata Calyptrauthes krugi 38O15’; 78O40’ 500 m msl Quercus pinus Loft Mountain, VA Quercus rubra Acer pennsyluaticum 41O47’; 74O24‘ 467 m msl Quercus ilicifoia Mohonk Mountain. NY Acer rubrum Pinus rigida 1500 m rnsl Abies balsamea Whiteface Mountain, NY 44O29’; 73’54‘ Picea rubens 43O56’; 71O45’ 765 m rnsl Fagus grandifolia Hubbard Brook, NH Acer saccharum 1534 m msl Abies balsamea krumholz Lakes-of-the-Clouds, NH 44O16’; 71’19’ Alpine shrub community 44O24‘; 68O14’ 5 m msl Pinus strobus Bar Harbor, ME Fagus grandifolia Abies balsamea Picea rubens
57O53’; 135O10’ Fresh Water Bay, AK (first collection site) 58O16’; 134’30’ Douglas Island, AK (second collection site)
comments 65 km W of Juneau;
