Investigating Dispersion of Buoyant Emissions from Low Level Sources in Urban Areas: Water Channel Modeling

The air quality impact of buoyant emissions from low level sources such as distributed generator stations (DGs) in urban areas for source receptor distances of tens and hundreds of meters is not very well understood and currently used models such as USEPA recommended dispersion model AERMOD/PRIME are not able to reproduce ground level concentration associated with these sources accurately. Therefore, a comprehensive laboratory study to understand the dispersion of pollutants released from DGs in urban areas has been conducted. Complex urban geometries have been modeled inside the water channel and ground level concentrations associated with a buoyant release from a low level source inside the modeled urban area have been measured at several downstream distances. In addition to ground level concentration, plume visualization has been done where the averaged plume behavior were investigated under different urban geometries.

Results have shown that the ground level concentrations are highly sensitive to the height of the buildings upstream of the DG. Laboratory measurements have shown that upstream buildings can produce low velocity as well as high turbulent region near the stack which allows for higher plume rise as well as more intense vertical mixing. Thus, the presence of buildings results in effects that counteract each other in changing the ground level concentrations.

AERMOD performance has also been evaluated with results from these measurements, and it has been shown that AERMOD has difficulties predicting concentrations near the stack in the presence of upstream buildings. In order to solve this problem, modifications have been made to AERMOD in treatment of dispersion in the region close to the stack where the effects of upstream buildings on dispersion were accounted for using the measured turbulence and wind speed at downstream distances up to 10 building heights. Results from this modification show significantly improved agreement with the laboratory data.

The effect of different urban geometries on plume rise and dispersion of buoyant emissions from low level sources and the performance of currently used dispersion models in predicting the air quality impact of the emission from these sources will be presented.