Atmospheric deposition measurement sites

Frescholtz 2002). Although ongoing and new planned field and laboratory studies are designed to further test this hypothesis, we feel that it is warranted at this time to develop a pilot-scale network of aimual ecosystem fluxes of THg in TF and LF as indicators of total atmospheric deposition. These fluxes can then be compared with measured wet plus modeled diy deposition based on both inferential and regional-scale models to develop independent estimates of total atmospheric deposition for forested catchments. We also believe that this approach could eventually be applied to a national network, such as the MDN. Although this method is best aimed at forested sites, ongoing research will address methods appropriate for other ecosystems. 

Concentrations at surface sites are often affected by the proximity of the surface. The surface may emit the species, as is the case for NO, or the species may be deposited at the surface, as is the case for HN03. In either case, the atmospheric concentration measured within several meters of the surface may be significantly perturbed from the average for the troposphere as a whole or even for the boundary layer (the lowest layer of the troposphere in direct thermal contact with the surface its thickness varies from 102 m or less at night to 103 m or more during the day, at least over land). Therefore, the surface complicates the interpretation of concentration measurements. However, proper measurements near the surface can be used to determine surface fluxes of the species. 

Atmospheric concentrations of polycyclic aromatic hydrocarbons have been measured since 1991 as part of the Integrated Atmospheric Deposition Network (e.g. [122]). Buehler et al. [35] report PAH concentrations from five US IADN sites (Table 2). 

FIGURE 4-38 Mean pH of precipitation of the United States in 1990, based on weekly measurements at 170 sites (adapted from National Atmospheric Deposition Program, 1990). 

Table 1. Calculated Excess Metal Flux to the Surface of the Farm River Salt Marsh Compared to Measured Atmospheric Deposition Rates at Selected Sites...

Peat bogs constitute historical archives for atmospheric deposition and have been used in this study to investigate the local deposition from the satellite failure. Several vertical profiles were taken at different sites at Madagascar at about 18 S, 4 E. The samples were sliced in 1-2 cm the first 20 cm and then deeper down in 5 cm slices. Samples were dried, homogenized and analyzed for Pu-isotopes. The dating of the profiles was made by analysis of "Pb. Plutonimn isotopes were measured by alpha spectrometry after radiochemical separation. 

Atmospheric deposition of trace metals from urban environments on the perimeter of large water bodies is a potential source to these systems. There is little data, however, to support or refute this notion. Here we discuss a study of Baltimore s urban air and its potential impact on the northern Chesapeake Bay and compare our results to those of other studies. Elevated concentrations of metals, especially lead, zinc and mercury were measured at an urban sampling site compared to a rural location. The difference was most marked for lead with the annual depositional flux almost three times higher in the city. Normalized fluxes at a rural site were mostly similar to those measured previously around the Chesapeake Bay and were also similar to those measured recently at other rural sites in Maryland and around the Great Lakes. The results of our study suggest that local atmospheric inputs from urban sources should be included in any evaluation of atmospheric deposition to lacrustrine, coastal or estuarine systems. 

For estuarine and coastal environments, direct deposition to the water s surface of some pollutants can be as important as runoff from the watershed given that the metals are strongly retained within the terrestrial environment (22, 24). Atmospheric deposition appears to be an important source to the Chesapeake Bay of metals such as Hg, Pb and Cd (3, 5, 20). However, as estimations are based on the measurements of metal deposition at rural sites, they could potentially underestimate the impact of deposition if the urban influence were significant. The present study was aimed at examining the difference in concentration and deposition in an urban and a rural setting to help address this important question. 

This is believed to be a major source of atmospheric HN03. For example, Ljungstrom and Hallquist (1996) report that the calculated rates of N03 formation and the measured rates of wet deposition of nitrate at a site in Sweden were very similar they intepreted this as being due to the formation of N2Os, followed by its hydrolysis (although the direct uptake and reaction of N03 presumably could have contributed as well). 

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