Spatial and Temporal Scales of Atmospheric Processes

During transport through the atmosphere, all except the most inert substances are likely to participate in some form of chemical reaction. This process can transform a chemical from its original state, the physical (gas, liquid, or solid) and chemical form in which it first enters the atmosphere, to another state that may have either similar or very different characteristics. Transformation products can differ from their parent substance in their chemical properties, toxicity, and other characteristics. These products may be removed from the atmosphere in a manner very different from that of their precursors. For example, when a substance that was originally emitted as a gas is transformed into a particle, the overall removal is usually hastened since particles often tend to be removed from the air more rapidly than gases. 

Once emitted, species are converted at various rates into substances generally characterized by higher chemical oxidation states than their parent substances. Frequently this oxidative transformation is accompanied by an increase in polarity (and hence water solubility) or other physical and chemical changes from the precursor molecule. An example is the conversion of sulfur dioxide (S02) into sulfuric acid (H2SO4). Sulfur dioxide is moderately water soluble, but its oxidation product, sulfuric acid, is so water soluble that even single molecules of sulfuric acid in air immediately become associated with water molecules. The demise of one substance through a chemical transformation can become another species in situ source. In general, then, a species emitted into the air can be transformed by a chemical process to a product that may have markedly different physico-chemical properties and a unique fate of its own. 

FIGURE 1.4 Spatial and temporal scales of variability for atmospheric constituents. 

Microscale. Phenomena occurring on scales of the order of 0-100 m, such as the meandering and dispersion of a chimney plume and the complicated flow regime in the wake of a large building. 

Mesoscale. Phenomena occurring on scales of tens to hundreds of kilometers, such as land-sea breezes, mountain-valley winds, and migratory high- and low-pressure fronts. 

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Spatial and Temporal Scales of Atmospheric Processes 40 Appendix 1 Derivation of the Geostrophic Wind Speed 43 References 47... 

Transport occurs at a variety of spatial and temporal scales. In atmospheric models, it is customary to distinguish between motions that can be resolved on the numerical grid of the model (often called large-scale ) and sub-grid scale processes (such as... 

The advent of fast computers and the availability of detailed data on the occurrence of certain chemical species have made it possible to construct meaningful cycle models with a much smaller and faster spatial and temporal resolution. These spatial and time scales correspond to those in weather forecast models, i.e. down to 100 km and 1 h. Transport processes (e.g., for CO2 and sulfur compounds) in the oceans and atmosphere can be explicitly described in such models. These are often referred to as "tracer transport models." This type of model will also be discussed briefly in this chapter. 

Temporal and Spatial Scales of Atmospheric Mixing Processes that Control Chemical Concentrations near the Earth s Surface... 

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