Jeffrey W. Grimm

Trends in precipitation chemistry in the United States: A national perspective, 1980–1992

Thirteen years (1980–1992) of precipitation chemistry data from 58 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites are examined for trends using a general linear least squares model. SO42- concentrations are decreasing throughout the United States; significant (p < 0.05) trends are evident at 42 of the 58 sites included in this analysis and the average decrease is 12.026 μeq l−1. The largest concentration of sites with significant decreasing SO42− trends is located in the north central and western regions of the country. Eleven sites exhibit significant NO3− trends, nine of which are decreasing. Ca2+ and Mg2+ concentrations show the most widespread decline (44 and 52 sites, respectively) of all the major cations and anions in precipitation. Ca2+ concentrations have decreased 4.587 μeq l−1 since 1980; Mg2+ concentrations have decreased 2.034 μeq l−1. The most consistent and statistically most significant (p < 0.001) Ca2+ and Mg2+ trends occur in the northeast. Decreasing trends are also evident at 28 sites for Na+ and at 35 sites for K+. NH4+ concentrations exhibit very little change over this 13-year period. The widespread decline in base cations, particularly Ca2+ and Mg2+, appears to have offset the effects that decreasing SO42− concentrations should have on free acidity in precipitation. Only 17 sites exhibit both significant decreasing H+ and SO42− concentration trends, most of which are located in the north central portion of the United States. The average decrease in H+ at the 17 sites is 9.991 μeq l−1. Hawaii is the only site in the NADP/NTN network to exhibit a significant increasing H+ trend.

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

Abstract 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 (SO42−), 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.

Statistical analysis of errors in estimating wet deposition using five surface estimation algorithms

Abstract Wet deposition measurements of H+, SO42− and NO3− from 29 monitoring sites located in (16) and around (13) Pennsylvania, U.S., were analyzed to quantify errors associated with extrapolating point estimates of deposition using five surface-fitting algorithms. The influence of site density on estimation errors associated with each surfacing algorithm was also investigated. The five surfacing differed little in their abilities to predict the concentration or deposition of individual ions found in precipitation in Pennsylvania. However, the size of estimation errors for all parameters, even those based on the densest network, were quite high relative to the variation observed among monitoring sites in Pennsylvania. All monitoring site observations were within 22.8, 17.6 and 23.1 per cent of the median concentration and 33.9, 35.3 and 36.7 per cent of the median deposition for H+, NO3− and SO42−, respectively. Maximum per cent errors indicate that estimation errors may severely obscure actual surface features in at least some portions of the estimated concentration and deposition grids in Pennsylvania. Deposition and concentration estimates based on higher density networks were generally more accurate; however, the improvements afforded by the additional sites were quite modest. Based on the magnitude of estimation errors, kriging produced the most accurate estimates, although no single algorithm consistently yielded the most accurate estimates for all parameters.

Wet atmospheric deposition of organic carbon: An underreported source of carbon to watersheds in the northeastern United States

We measured wet atmospheric deposition of dissolved organic carbon (DOC) over 6 years at a network of 12 monitoring sites across Pennsylvania, quantified rates of wet DOC deposition, and developed the first statewide estimates of inputs of DOC to watersheds via wet deposition. Average annual volume-weighted concentration of DOC was 0.71 mg C L−1. Annual wet deposition fluxes of DOC varied between sites and years, ranging from 3 to 13 kg C ha−1 yr−1, with an average value of 8 kg C ha−1 yr−1 across all sites and years and are of the same order of magnitude as literature values for riverine organic carbon fluxes in the northeastern United States. The rates of wet DOC deposition showed a pronounced seasonality and spatial distribution, with highest deposition rates observed in the summer, especially at the sites located in western Pennsylvania. Significant links between DOC and inorganic constituents in precipitation, such as sulfate and inorganic nitrogen forms, point to the similarity of sources and atmospheric processing and suggest that DOC may potentially affect their atmospheric transport and ecological fate. Observational data resulting from this study underscore the potential significance of atmospheric deposition as an external input of reactive carbon species to watersheds and may be useful for constraining atmospheric carbon models and evaluating atmospheric influences on ecosystems.

Variability of Dissolved Organic Carbon in Precipitation during Storms at the Shale Hills Critical Zone Observatory

Abstract Organic compounds are removed from the atmosphere and deposited to the Earths surface via precipitation. In this study, we quantified variations of dissolved organic carbon (DOC) in precipitation during storm events at the Shale Hills Critical Zone Observatory, a forested watershed in central Pennsylvania (USA). Precipitation samples were collected consecutively throughout the storm during 13 events, which spanned a range of seasons and synoptic meteorological conditions, including a hurricane. Further, we explored factors that affect the temporal variability by considering relationships of DOC in precipitation with atmospheric and storm characteristics. Concentrations and chemical composition of DOC changed considerably during storms, with the magnitude of change within individual events being comparable or higher than the range of variation in average event composition among events. Although some previous studies observed that concentrations of other elements in precipitation typically decrease over the course of individual storm events, results of this study show that DOC concentrations in precipitation are highly variable. During most storm events, concentrations decreased over time, possibly as a result of washing out of the below‐cloud atmosphere. However, increasing concentrations that were observed in the later stages of some storm events highlight that DOC removal with precipitation is not merely a dilution response. Increases in DOC during events could result from advection of air masses, local emissions during breaks in precipitation, or chemical transformations in the atmosphere that enhance solubility of organic carbon compounds. This work advances understanding of processes occurring during storms that are relevant to studies of atmospheric chemistry, carbon cycling, and ecosystem responses.