Atmospheric deposition across the Central Arizona - Phoenix LTER site

Man is increasingly affecting biogeochemical cycling. This is particularly so in cities where nutrient and material fluxes, as well as the relative importance of transport and storage mechanisms, may be significantly modified with respect to less human-impacted ecosystems. Recent compilation of a nitrogen mass balance for the Central Arizona - Phoenix (CAP) LTER has suggested that atmospheric deposition potentially represents one of the largest inputs of nitrogen to the ecosystem. However few empirical measures of nutrient deposition rates in cities have been made in the US, since most existing national monitoring networks such as the National Atmospheric Deposition Program and NOAA's AIRMON program consist of sites which are located almost exclusively in pristine natural areas, remote from urban influences. Hence atmospheric transport and deposition of nutrients and other ions, along with the effect of that deposition on biogeochemical cycling and ecosystem function in cities and surrounding ecosystems is currently only poorly understood.

The aims of the atmospheric deposition research at the CAP LTER are to i) develop a monitoring network to quantify the spatial variation and overall rates of atmospheric deposition for the major nutrients and ions across the city, ii) determine the role of atmospheric deposition in urban biogeochemical cycling, and iii) understand how atmospheric deposition interacts with and affects the functioning of other urban ecosystem processes, such as primary productivity. A network of deposition monitors at 9 sites across the CAP study area is described. Preliminary data on deposition rates of major nutrients in wet and dry deposition from the first 6 to 9 months of sampling will be presented. A field campaign to characterize the major ion chemistry of aerosols at 3 sites across the valley using Particle Induced X-ray Emission (PIXE) during the summer of 1999 will also be described and the conclusions from this study regarding the main sources of aerosols contributing to dry deposition across the CAP site will be presented.