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ABSTRACT. The formation of particles during cooling of a synthetic ue gas with vapors of sodium and potassium species is studied in a laboratory tubular reactor with laminar ow. It is shown to agree well with a theoretical model for the process. The kinetics of homogeneous nucleation of the pure chloride vapors is described by the classical nucleation theory, adapted to include the participation of stable dimer as well as monomer vapor molecules. The Tolman equation is used to describe the curvature-dependence of the surface tension of small nuclei. The values of the Tolman parameter for NaCl and KCl are determined from the measurements. The homogeneous nucleation of the pure chlorides is suppressed by even relatively small concentrations of foreign seed particles and is therefore unlikely to contribute to the creation of new particles in real ue gases.The addition of SO 2 to the chloride vapor feed, in the presence of oxygen and water vapor, increases the number concentration of ef uent particles signi cantly and affects their composition to include sulphate in addition to chloride. The sulphate content is independent of the peak temperatures of the ue gas but increases with increasing content of oxygen and SO 2 . The study proves that the alkali sulphates are formed by the sulphation of vapor phase rather than solid, alkali chloride. The sulphate vapors are formed in high supersaturation and show a pronounced tendency towards homogeneous nucleation, which is identi ed as the likely source of the submicron particles formed in alkali rich ue gases.

From routine hourly observations reported during 5 years at one site in Denmark, empirical relations for net radiation over green grass are found. These relations give an estimate of the net radiation for the measuring site from the geographical position, local time, representative surface albedo, measured global radiation and/or total cloud cover in oktas. Cloud types are taken into account, if the reported clouds are mainly cirrus forms. This is a result of a classification of the net radiation according to cloud cover and type, i.e. a total of 90 classes. m e r e n t relations are found for different cloud covers,The derived procedure is used on another site in Denmark with another representative surface albedo. From 112 years of data, consistency is found for net radiation measurements at the two sites, allowing extrapolation of the derived net radiation procedure to other sites in Denmark. Data from the Wangara experiment (Clarke, 1971), covering 40 days, showed a similar refation for clear sky conditions, but revealed another general dependency of net radiation upon cloud cover than that found in Denmark. Therefore, relations of the kind found in this study are related to the weather and climate of the measuring site. Comparisons ktween net radiation estimated from the models and measurements from different sites is r* = 0.9.

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