Airshed

Airshed a geographic area that, because of topography, meteorology, and/or climate, is frequently affected by the same air mass. 

Urban Airshed Model (UAM) is a du-ee-dimensional grid based photochemical sinuudtion model for urban scale domains. 

Airshed response High, rapid Moderate, rq)id Moderate, rapid None... 

Depending on the form of atmospheric Hg, the associated airshed can vary from global to local scales. 

Variations of these indicators over time ean oeenr dne to changes in emissions, but they can also be due to meteorological variability and to changes in the measurement method employed. Signal-to-noise ratio is a eritioal issue for all of the indicators identified above dne to the global natnre of the airshed for atmospherie Hg and the regional-to-loeal nature of proposed emission rednctions. The development and use of numerieal model simulations of atmospherie Hg emissions, transport, transformations, and deposition should be used to measure pertinent indieators and to better understand the fundamental atmospherie proeesses that affeet Hg dynamics. 

This book has identified the most useful indicators of environmental changes in mercury contamination in 4 compartments of the environment 1) airsheds and watersheds, 2) water and sediment, 3) aquatic organisms (with emphasis on freshwater ecosystems), and 4) wildlife that live in freshwater, terrestrial, and/or coastal ecosystems. The indicators identified in this book are wide-ranging and involve measurements made at several different scales of time and space. The authors believe that these indicators will provide the best information to policymakers, as well as other stakeholders, as to whether environmental concentrations are changing (A indicators) and what the reasons for those changes might be (B indicators). 

Area sources of either a selected chemical or a precursor present a common problem for modeling. In particular, the rich and complex patterns of hydrocarbon emissions from general urban and industrial sources either include or might produce through atmospheric photochemical reactions some of the species on the analysis list. The treatment of such species in photochemical airshed modeling is difficult (8, 9). The effort required for any one such exercise is substantial, and the effort required for a comprehensive analysis of all urban regions relevant to this program would be prohibitive. 

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