Meteoric Aerosols in the Middle Atmosphere
Sammanfattning: This thesis concerns the fate of the meteoric smoke in the Middle Atmosphere, and its effect on ice phenomena such as noctilucent clouds (NLC) and polar stratospheric clouds (PSC). The potential role of NLC as tracer for mesospheric processes and variability, and as a tool for monitoring this remote and inaccessible region, has generated substantial interest within the scientific community. The nucleation of ice in such a dry environment is not trivial. Supersaturation is considered too low for homogeneous nucleation. Hence, pre-existing condensation nuclei are deemed necessary, with smoke particles having long been considered the most likely candidate. Here we show that the atmospheric circulation transports meteoric smoke particles away from the polar region before they coagulate large enough to efficiently act as ice condensation nuclei. We also show that the charging of meteoric smoke, in combination with deviations from the mean thermal state, may solve this dilemma by significantly altering the ice nucleation properties of smoke. Thus, while it is highly questionable whether neutral smoke can provide sufficient amounts of condensation nuclei for ice formation at the polar summer mesopause, charged meteoric smoke proves to be a promising candidate to explain mesospheric ice phenomena as we observe them. We further show that the bulk of the meteoric material is transported to the Arctic winter stratosphere, yielding significantly higher concentrations of meteoric smoke in the region of PSC nucleation than has previously been believed. Our new predictions of meteoric smoke in this region may thus shed new light on open questions relating to PSC nucleation.
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