The effects of wind speed and turbulence on the performance of diffusion tube samplers

This paper describes and presents the results of a study to determine the effects of wind speed and turbulence on the performance of diffusion tube samplers, for measuring nitrogen dioxide in the outdoor environment. Samplers of varying length were exposed on the University of East Anglia roof in order to determine the relative reductions in the length of diffusion for the different sampler lengths. Shorter diffusion tube samplers would suffer a greater percentage reduction in the length of diffusion, as a result of turbulence, compared to longer samplers. A total of 17 exposures were carried out. Samplers exposed on the university roof indicated that the calculated nitrogen dioxide concentration for the shorter samplers was significantly higher than for the longer samplers. However, repeating the exposure in a laboratory, where turbulence is expected to be low, showed very little variance in the calculated nitrogen dioxide concentrations for each tube length. For the outdoor exposures, a reduction in the length of diffusion for the standard length (7.1 cm diffusion tube sampler was estimated to be between 7 and 38%. Data corrected, using an iteration programme to calculate the reduction in the diffusion length for each exposure, showed very little variance in the corrected nitrogen dioxide concentration for each tube length, thereby indicating that the calculated reduction in the diffusion length was constant throughout all tube lengths. The relationship between the reduction in the diffusion length and wind speed was found to be highly variable.