In situ observations of aerosol properties above ice saturation in the polar tropopause region

Abstract

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 μm <D < 10 μm was frequently observed during airborne in situ measurements of aerosol properties. In ascending air parcels the haze mode was detected as soon as a water saturation ratio Swν = 0.5 (Sice ≅ 1.0) was exceeded. Larger cirrus particles with size D > 10 μm where found in regions with Swν > 0.57 (Sice ≥ 1.17). Hence, a range of Swν 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.