9. PLUME-IN-GRID PROCESS

Substantial anthropogenic emissions of nitrogen oxides

(NOx) and/or sulfur oxides (SOx) are released from

individual point sources into plumes whose horizontal

dimension remains considerably smaller than the typical size

of regional photochemical model grid cells (e.g. 20-40 km)

for some time after release. The plume-in-grid (PinG)

approach in CMAQ, which is based on a Lagrangian

reference frame, was specifically designed to address the

need to more realistically resolve the spatial scale of plumes

emanating from isolated, major elevated point source

emitters (MEPSEs) within an Eulerian regional grid

framework. Since the traditional Eulerian grid modeling

approach has been to instantly mix point source emissions

into an entire grid cell volume, it bypasses the diffusionlimited,

chemical evolution phase occurring in sub-grid

scale plumes during their transport downwind, which may

strongly impact the photochemically-formed species

concentrations in regional model grid cells. The chemical

mixture in fresh, point source plumes, particularly of fossilfuel

power plants, can be characterized to be in a high NOx /

low VOC regime, while the ambient environment

surrounding a plume is often in the opposite chemical

regime. The over dilution and mixing of high NOx point

source emissions into large grid cells can prematurely

initiate rapid ozone production. However, the PinG

technique overcomes this undesirable feature since it

provides spatial resolution within a plume and also simulates

the gradual horizontal growth experienced by real-world

plumes.

 

 

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