Fluctuations in Atmospheric Boundary Layer Plumes

Pollution and short-term health considerations require the accurate prediction of airborne contaminant transport in cities. Even when a stationary source emits tracer gases continuously, the resulting plume fluctuates vigorously in the turbulence that results from air passing over any typical landscape. Computing this flow properly requires large-eddy simulations that resolve the vortices shed from buildings, trees, and terrain because these coherent effects govern the ``turbulent'' dispersion of pollutants, tracer gases, and potentially toxic agents. This paper uses long-time, high-resolution detailed studies of one urban configuration, computed with 5-meter spatial resolution and sub-second temporal resolution, to quantify the deviations of passive tracer plumes from steady state. Even when concentration values at a point are averaged over long times, as an accumulating sensor might do, the range of probable values spans orders of magnitude. At a 5-km scale, averaging tracer concentrations for as long as an hour still leaves likely sampling fluctuations of plus or minus a factor of ten from the long-time average.