Theoretical and experimental studies on the effects of eddy turbulence on open tube passive samplers

Air moving into the open end of a passive sampler affects the pollutant mass transfer by generating eddies leading to a reduction in the molecular diffusion length and a consequent overestimation of the pollutant concentration. This decrease is found to be experimentaly in the range of 10-40 % depending on the wind speed at the site. Assuming that the eddy turbulence is isotropic and the horizontal wind velocity u can be time averaged, an exponential equation has been derived connecting the decrease of molecular diffusion length 1 with u through the surface roughness length ZQ, the average friction velocity u*, the von Karman constant k and a propotinality constant M for a given pollutant.Using the independent laboratory wind tunnel experiments of Hargreaves^ on measurements of N(>? concentrations trapped by Palmes tubes at various wind speeds, the constant M was evaluated to be 1.20 x 10"̂ for NO2Laboratory experiments conducted using a laminar flow vessel and Palmes tubes exposed to high concentrations of NC>2 ( 700-1200 ppb) for one day periods at constant temperature of 27 °C and the relative humidity in the range 50-60% , with the winds speeds of 1-8 m sec" 1 show values agreeing with the theoretical values within 2-5% The field N(>> concentrations measured by Gair and Penkett 11 also compare well with the theoretical values except for the wind speed of 1 m sec"*. The average experimental overestimation is 22% while the theoretical one is 25%