The evolution of photochemical smog in a power plant plume

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter. Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the aft ernoon of 16 July 1995 as part of the Southern Oxidants Study-Nashville Mid dle Tennessee Ozone Study. Twelve cross-wind air sampling traverses were ma de at six distance groups from 35 to 116 km from the source. During the sam pling period the winds were from the west-northwest and the plume drifted t owards the city of Nashville TN. Ten of the traverses were made upwind of t he city, where the power plant plume was isolated, and two traverses downwi nd of the city when the plumes were possibly mixed. The results revealed th at even six hours after the release, excess ozone production was limited to the edges of the plume. Only when the plume was sufficiently dispersed, bu t still upwind of Nashville, was excess ozone (up to 109 ppbv, 50-60 ppbv a bove background levels) produced in the center of the plume. The concentrat ions image of the plume and a Lagrangian particle model suggests that porti ons of the power plant plume mixed with the urban plume. The mixed urban po wer plant plume began to regenerate O-3 that peaked at 120 ppbv at a short distance (15-25 km) downwind of Nashville. Ozone productivity (the ratio of excess O-3 to NOy and NOz) in the isolated plume was significantly lower c ompared with that found in the city plume. The production of nitrate, a cha in termination product, was significantly higher in the power plant plume c ompared to the mixed plume, indicating shorter chain length of the photoche mical smog chain reaction mechanism. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.