Reduced meteoric smoke particle density at the summer pole - implications for mesospheric ice particle nucleation

Details Reduced meteoric smoke particle density at the summer pole - implications for mesospheric ice particle nucleation Reduced meteoric smoke particle density at the summer pole - implications for mesospheric ice particle nucleation

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsa21a0272m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SA21A-0272

Physics

0305 Aerosols And Particles (0345, 4801, 4906), 0340 Middle Atmosphere: Composition And Chemistry, 0341 Middle Atmosphere: Constituent Transport And Chemistry (3334), 2442 Meteor-Trail Physics, 2447 Modeling And Forecasting

Scientific paper

Noctilucent clouds (NLC) and polar mesospheric summer echoes (PMSE) are phenomena that occur in the summertime polar regions due to the presence of ice particles around the mesopause. That ice particles are able to form in a region with such low water vapor concentration as the mesopause is noteworthy. Even though the summer mesopause is the coldest region on earth, temperatures are generally not low enough for homogeneous nucleation to occur, which necessitates the presence of pre-existing condensation nuclei. The nature of these nuclei has long puzzled the scientific community and many candidates have been suggested, such as particles of meteoric origin, ion clusters, sodium bi-carbonate, sulfate aerosols and soot particles. Out of these the so called "smoke particles", i. e. particles re-condensed from ablated meteoritic material, have long been considered the most likely. Generally, it has been believed that these particles exist in numbers of the order of thousands per cubic centimeter at the mesopause. This belief is based on 1-dimensional studies of meteoric material. A recent 2-dimensional model study, which includes the atmospheric circulation from summer to winter pole however, suggests much lower number densities at the summer mesopause. We here investigate the implications of low number densities for the formation of ice particles. We find that even though resulting ice particle distribution may produce typical NLC brightness, the number density of ice particles is not consistent with what is expected for NLC and PMSE. In particular, it is much lower than the ice particle concentration (>1000 cm-3) typically expected to explain the "electron bite-outs" that are frequently observed in the vicinity of PMSE's. We therefore re-examine the assumptions and parameters that determine the smoke distribution. We show that even though the number of condensation nuclei at the polar summer mesopause can be increased within the uncertainties, the results in most scenarios remain insufficient. We show that charged particles, perhaps in combination with significant deviations from the mean mesospheric state, may be necessary for condensation of ice particles in the polar summer mesosphere.

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