EVALUATION OF BACK TRAJECTORIES ASSOCIATED WITH OZONE TRANSPORT DURING THE 1993 NORTH-ATLANTIC REGIONAL EXPERIMENT

Back trajectories traditionally have been used to identify possible so urces of pollutants measured at a receptor location; however, they neg lect turbulent diffusion processes that affect pollutant transport. Af ter evaluating the performance of a coupled mesoscale and Lagrangian p article dispersion model with observations taken during the 1993 North ern Atlantic Regional Experiment (NARE), a series of forward and back trajectories are analyzed to determine the errors associated with the back trajectory calculations. The forward trajectories are based on th e mean and turbulent wind components and their initial positions corre spond to several urban areas in eastern North America. The back trajec tories employ only the mean wind components and their initial position s correspond to the final positions of the forward trajectories within a sampling volume near Yarmouth, Nova Scotia. Large differences betwe en the forward and back trajectory positions occurred since the back t rajectories did not include the irreversible turbulent diffusion proce sses. The back trajectories had no mechanism to identify surface sourc e regions, with the consequence that the back trajectories would eithe r pass over the ''true'' source region, or miss it altogether as a res ult of locating the particle at an incorrect horizontal position. A ho rizontal displacement error of +/- 100-400 km and a vertical displacem ent error of +/- 0.2-1 km occurred after 2 days of transport. These er rors leveled off between 2 and 3 days, but grew larger again after 3 d ays of transport. The model results suggest that the use of back traje ctories by themselves may be an inappropriate approach for estimating the Source location of a plume emitted near the surface. Copyright (C) 1996 Elsevier Science Ltd