City winds can concentrate pollutants

Wind patterns in cities can concentrate pollutants like exhaust particles and diesel fumes in specific regions in an urban environment, U.S. researchers say. Seemingly chaotic and often powerful wind patterns known as urban flows, created by cities' concrete canyons, isolated greenery and congested traffic, can accumulate pollutant particles in specific areas of the urban environment rather than scatter them randomly, the scientists report in the American Institute of Physics journal Physics of Fluids. Using a new mathematical formula to simulate the long-term random motion of pollutant particles, researchers were able to verify the phenomenon. "The unexpected finding is coherent patterns in fluid flows were thought to have no real analog in nature," Wenbo Tang of Arizona State University said. "In previous studies, the existence of these patterns in fluid flows was only verified with idealized 'theoretical' flows. It was not known if such structures were robust enough to manifest in the environment." Researchers said their findings could be used to generate maps of well and poorly mixed regions and highlight urban areas most susceptible to high concentrations of pollutants.