Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sa32b02m&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SA32B-02
Other
0305 Aerosols And Particles (0345, 4801), 0370 Volcanic Effects (8409)
Scientific paper
Noctilucent ("night-luminous") clouds (NLC), or as seen from space, Polar Mesospheric Clouds (PMC), are typically 1 to 2 km thick and located at altitudes of 80 to 85 km, where the temperature is near 150K. NLC generally occur between 50 degrees latitude to the pole from May to August in the Northern Hemisphere, with occasional sightings at lower latitudes. An extraordinary low-latitude sighting occurred on June 21, 1999 at 41oN. Direct evidence that PMC are composed of water ice was recently reported from satellite observations made in the near infrared. The formation of ice clouds in the upper atmosphere has been studied extensively as a result of the role of Polar Stratospheric Clouds (PSC) in polar ozone depletion. There exists ample evidence that preexisting stratospheric liquid sulfate aerosol plays an important role in the formation of solid PSC particles. Until recent laboratory measurements showed otherwise, however, it was believed that photolysis of sulfuric acid in the upper stratosphere would prevent the formation of such aerosol in the mesosphere. We present here calculations from a microphysical atmospheric model which point to sulfate from volcanic and non-volcanic sources alike as the origin of nuclei on which PMC and NLC form. Current theories have relied on meteor 'smoke' particles arising from meteor ablation and recondensation to explain the nucleation of NLC/PMC ice particles. Our calculated sizes and concentrations of high latitude summer mesosphere sulfate aerosol particles are comparable to or exceed those expected of the meteor source. The model shows that large volcanic eruptions will add significantly to this particle population, several years following the injection. The record of the number of NLC sightings in response to large volcanic eruptions is contradictory. However, microphysical models show that injections of particles may result in positive, negative or neutral response in the visual brightness of NLC, depending on sulfur, water vapor, and particulate injections, which have not been observed in the past A related radar phenomenon, Polar Mesopheric Summertime Echoes (PMSE), involves attachment of free electrons to condensates near the mesopause. Theory predicts that such particles would have to be smaller than 10 nm and number in the thousands per cm3. Up to now they were believed to be composed of water-ice. However these intense radar echoes have been detected as far south as 52N, where the mesopause is normally too warm for water-ice saturation, but in the range of sulfuric-acid condensation. We calculate mesospheric sulfate aerosol concentrations of up to 12000 /cm3, the vast majority of which are smaller than 10 nm. The concentration, size, and seasonal and latitudinal distribution of sulfate aerosol corresponds well to PMSE theory and observations.
Mills Michael J.
Solomon Sean
Thomas GS
Toon Brian O.
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