In the polar winter tropopause region (T≅200 K, p=200 hPa) over northern Europe, the occurrence of a haze-mode aerosol in the diameter range 0.75 μm10 μm where found in regions with Swv>0.57 (Sice≥1.17). Hence, a range of Swv values existed between 0.5 and 0.57, where the haze mode was present outside of cirrus clouds. The ratio of number densities of haze-mode particles to Aitken-mode particles reached ~1.25% inside and <0.3% outside of ice-supersaturated regions. In descending air parcels a continuous decrease in the number density of haze-mode particles was observed. This suggests a deliquescence transition in rising air parcels and evaporation of liquid haze droplets without recrystallization in sinking air parcels. Consideration of the possible chemical composition, phase, and growth timescales of the haze-mode particles leads to the conclusion that they are likely to contain ammoniated sulfates in the liquid phase. On the basis of measured particle sizes and calculated extinction values, the observed haze mode may be a candidate for subvisible cirrus cloud particles. ¿ 2000 American Geophysical Union