Experimental investigation of averaging time effects on building influenced atmospheric dispersion under different meteorological stability conditions

Field experiments were carried out to measure concentration and concentration fluctuation distributions on the surfaces of a complex shaped building under different stability conditions and to investigate the dependence of these distributions on the averaging time. Meteorological conditions varied from neutral to unstable. The measurements were conducted for two different building orientations to the mean wind direction. Periods of 30 min were selected based on the mean wind direction being normal to either the long or the short building wall within ±10 degrees. For shorter averaging times, the selected concentrations corresponded to the 1, 3, 5, 10 or 15 min periods in which each detector was indicating the highest average within this 30 min period. Averaging time seems to influence more the values of these maximum mean concentrations under unstable conditions than under neutral conditions on all the building walls, while the influence of averaging time is weaker in the case of the wind impinging on the short building wall under neutral conditions. Averaging time seems to have a stronger influence on concentration fluctuation intensity under unstable conditions, while the influence of averaging time on concentration fluctuation intensity is similar for both building orientations investigated. A power law function was found to describe well the dependence of the concentration on averaging time for all examined cases. The absolute values of the power law exponent p measured on the building surface are often higher than the values presented in the literature for open terrain.