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- ItemCharging of mesospheric aerosol particles: The role of photodetachment and photoionization from meteoric smoke and ice particles(Göttingen : Copernicus, 2009) Rapp, M.Time constants for photodetachment, photoemission, and electron capture are considered for two classes of mesospheric aerosol particles, i.e., meteor smoke particles (MSPs) and pure water ice particles. Assuming that MSPs consist of metal oxides like Fe2O3 or SiO, we find that during daytime conditions photodetachment by solar photons is up to 4 orders of magnitude faster than electron attachment such that MSPs cannot be negatively charged in the presence of sunlight. Rather, even photoemission can compete with electron capture unless the electron density becomes very large (≫1000 cm-3) such that MSPs should either be positively charged or neutral in the case of large electron densities. For pure water ice particles, however, both photodetachment and photoemission are negligible due to the wavelength characteristics of its absorption cross section and because the flux of solar photons has already dropped significantly at such short wavelengths. This means that water ice particles should normally be negatively charged. Hence, our results can readily explain the repeated observation of the coexistence of positive and negative aerosol particles in the polar summer mesopause, i.e., small MSPs should be positively charged and ice particles should be negatively charged. These results have further important implications for our understanding of the nucleation of mesospheric ice particles as well as for the interpretation of incoherent scatter radar observations of MSPs. © 2009 Author(s).
- ItemMass analysis of charged aerosol particles in NLC and PMSE during the ECOMA/MASS campaign(München : European Geopyhsical Union, 2009) Robertson, S.; Horányi, M.; Knappmiller, S.; Sternovsky, Z.; Holzworth, R.; Shimogawa, M.; Friedrich, M.; Torkar, K.; Gumbel, J.; Megner, L.; Baumgarten, G.; Latteck, R.; Rapp, M.; Hoppe, U.-P.; Hervig, M.E.MASS (Mesospheric Aerosol Sampling Spectrometer) is a multichannel mass spectrometer for charged aerosol particles, which was flown from the Andøya Rocket Range, Norway, through NLC and PMSE on 3 August 2007 and through PMSE on 6 August 2007. The eight-channel analyzers provided for the first time simultaneous measurements of the charge density residing on aerosol particles in four mass ranges, corresponding to ice particles with radii <0.5 nm (including ions), 0.5–1 nm, 1–2 nm, and >3 nm (approximately). Positive and negative particles were recorded on separate channels. Faraday rotation measurements provided electron density and a means of checking charge density measurements made by the spectrometer. Additional complementary measurements were made by rocket-borne dust impact detectors, electric field booms, a photometer and ground-based radar and lidar. The MASS data from the first flight showed negative charge number densities of 1500–3000 cm−3 for particles with radii >3 nm from 83–88 km approximately coincident with PMSE observed by the ALWIN radar and NLC observed by the ALOMAR lidar. For particles in the 1–2 nm range, number densities of positive and negative charge were similar in magnitude (~2000 cm−3) and for smaller particles, 0.5–1 nm in radius, positive charge was dominant. The occurrence of positive charge on the aerosol particles of the smallest size and predominately negative charge on the particles of largest size suggests that nucleation occurs on positive condensation nuclei and is followed by collection of negative charge during subsequent growth to larger size. Faraday rotation measurements show a bite-out in electron density that increases the time for positive aerosol particles to be neutralized and charged negatively. The larger particles (>3 nm) are observed throughout the NLC region, 83–88 km, and the smaller particles are observed primarily at the high end of the range, 86–88 km. The second flight into PMSE alone at 84–88 km, found only small number densities (~500 cm−3) of particles >3 nm in a narrow altitude range, 86.5–87.5 km. Both positive (~2000 cm−3) and negative (~4500 cm−3) particles with radii 1–2 nm were detected from 85–87.5 km.