Wet deposition from clouds and precipitation in three high-elevation regions of the Eastern United States

Three regions are identified in the Eastern United States (US) that contain substantial land area at high elevations: the mid-Appalachian region (MAR), Eastern New York state (ENY), and the New England region (NER). Approximately 75% of the land cover in these areas is forested, with 5.6-29% of the total acreage above 600 m and subject to cloud deposition. Measurements of cloud deposition are scarce. A 6-year data record of measurements at two high-elevation locations is considered, and scaling factors are developed to enable the rough estimation of area-wide cloud deposition at various elevations in each region. Estimates of precipitation and associated ion deposition are made at 12 arc-second resolution for the Eastern US and are used to obtain elevation-resolved precipitation-mediated deposition for the three study regions. At high elevations, clouds account for a substantial proportion of wet deposition (i.e., the sum of that from clouds and precipitation). For the total land area above 600 m, clouds may account for 20-60% of the total wet ion deposition, with the exact proportion depending on both location and ion species. At elevations above 600 m, but below the climatic tree line, the ratio of cloud-to-precipitation-mediated deposition is higher in NER and ENY than in MAR. At the highest elevations of each study region, clouds may account for over 80% of the wet ion deposition. Although the wet deposition of ammonium (NH₄⁺), sulfate (SO₄^2-), nitrate (NO₃^-), and hydrogen (H⁺) ions is enhanced at higher elevations by clouds over precipitation, this enhancement is the largest for ammonium. This study illustrates the major and perhaps dominant role that clouds may play by delivering considerable ion loads to montane ecosystems in selected elevation ranges where these ecosystems may be especially vulnerable.