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episodes of high concentrations of ozone. EKMA also simulates ozone formation only along a single trajectory, not providing any regionwide information about the effects of controls.

The number of analyses carried out using grid-based air quality models is limited but growing (see Chapter 10). Urban scale models, such as the urban airshed model (UAM) and the CIT model, have been applied to a number of cities in the United States and elsewhere. The Regional Oxidant Model (ROM) has been applied recently to the northeastern United States and to urban areas in that region.

Figure 11-1
Ozone isopleth diagram for three cities (A, B, and C) that have the same peak 1-hour 
ozone concentrations (Cp). The VOC/NOx ratios differ: a low ratio (c), a high ratio (B), 
and a medium ratio (A). Isopleths = lines of constant 1-hour peak ozone.

Ekma-Based Studies

EKMA is used to generate ozone isopleth diagrams for cities, and EPA