Urban areas currently constitute one of the least understood terrestrial surfaces with regard to land-atmosphere exchange. Despite their small areal extent (2% of the land surface), cities and urban regions have an enormous impact on the atmosphere, as well as on regional and global biogeochemical cycles (Grimm et al, 2000). Fossil fuel emissions, water consumption, food consumption, and waste production are often concentrated in urban areas, which are characterized by high rates of CO2 emissions, transpiration, and evaporation (Douglas, 1983; Decker et al, 2000). The impacts of urban land-atmosphere exchange on the global atmosphere and biogeochemistry will become increasingly important, as human population growth is expected to be concentrated largely in and around cities (United Nations, 2000). Improving our understanding of the dynamics of material flow through urban regions and the interactions between urbanization and land-atmosphere exchange is critical if we are to consider current and future human alterations to the global carbon and hydrologic cycles.

Isotopes are a natural fit to integrative, atmospheric studies over cities because of the differences in isotopic composition between trace gases resulting from human activities and plant and soil processes. These signatures can be used to identify individual components of trace gas sources and assess the influence of urban pollutants on plants and soils. Identifying the sources and influences of urban trace gases is a critical first step toward understanding urban ecosystem function. Unlike land-atmosphere exchange in natural ecosystems, urban land-atmosphere exchange is strongly influenced by human activities and behavior as well as natural processes such as plant and soil gas exchange (Grimm et al, 2000; Pickett et al., 2001). In order to gain a complete understanding of factors influencing emissions of greenhouse gases and pollutants that have both an anthropogenic and a plant/soil component, we must separate these contributions and evaluate the social, institutional, and environmental factors that influence their temporal and spatial distribution.

Here we discuss previous applications of isotopic sampling of urban air and urban plant biomass for partitioning trace gas emissions and improving our understanding of urban land-atmosphere interactions. We will focus on carbon dioxide and the carbon isotope composition of plant material, although similar methods may be applied to other isotopes and constituents of urban ecosystems.

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