MICROPHYSICAL AND OPTICAL-FEATURES OF POLLUTED COOLING-TOWER CLOUDS

In November 1993 an airborne field study was performed in order to inv estigate the microphysical and radiative properties of cooling tower w ater clouds initiated by water vapour emissions and polluted by the ex haust from coal-fired power plants. The number-median diameter of the droplet size distributions of these artificial clouds was in the range of 13 mu m. The concentration of smaller droplets (diameters d(D) < 1 0 mu m) increased with height and horizontal distance from the cooling towers. Close to the cooling towers, bimodal spectra were found with a second mode at 19 mu m. The Liquid water content (LWC) ranged betwee n 2 and 5 g/m(3) and effective droplet radii (R-e) between 6 and 9 mu m were measured. LWC and R-e decreased with altitude, whereas the drop let concentration (N-D) remained approximately constant (about 2000 cm (-3)). An enrichment of interstitial aerosol particles with particle d iameters (d(p)) smaller 0.2 mu m compared to the power plant plume in the vicinity of the clouds was observed. Particle activation for d(p) > 0.3 mu m was evident, especially in cooling tower clouds further apa rt and separated from their sources. Furthermore, radiation measuremen ts were performed, which revealed differences in the vertical profiles of downwelling solar and UV radiation flux densities inside the cloud s. The effective droplet radius R-e was parameterized in terms of LWC and N-D using equations known from literature. The close agreement bet ween measured and parameterized R-e indicates a similar coupling of R- e, LWC and N-D as in natural clouds. By means of Mie calculations, vol ume scattering coefficients and asymmetry factors are derived for both the cloud droplets and the aerosol particles. For the cloud droplets, the optical parameters were described by parameterizations from the l iterature. The results show, that the link between radiative and micro physical properties of natural clouds is not changed by the extreme po llution of the artificial clouds. (C) 1997 Elsevier Science B.V.