Spatial Scale

Atmospheric concentrations depend on emission or formation rates, loss rates, and mixing, which in turn depend on atmospheric conditions and local geography. Local pollution episodes generally occur with low horizontal wind speeds, as is often the case when a high-pressure ridge dominates the synoptic-scale weather. Inhibited vertical mixing also contributes to high surface concentrations. A strong temperature inversion (temperature increasing rapidly with elevation) at low altitude leads to a shallow PBL and prevents transport of pollutants to the free troposphere. Local concentrations are generally highest when ground-level inversions are strongest. A variety of processes, including subsidence, radiation, and advection, can cause inversions. A detailed discussion is beyond the scope of this report. Local orographic conditions, such as lying in a valley, can exacerbate inversions. Long-lived chemicals such as CH4 and N2O can have large-scale (global) effects, but their local concentrations are not usually a problem.

The complexities of the various kinds of air emissions and the temporal and spatial scales of their distribution make their direct measurement at the individual AFO level impractical other than in a research setting. Relatively straightforward methods for measuring emission rates by measuring airflow rates and the concentrations of emitted substances are often not available. Flow rates and pollutant concentrations may be available for some types of confined animal housing but usually not for emissions from soils.

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